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MG Midget and Sprite Technical - Lack of Revs on Hills

I’m sure there was a thread on this recently but I can’t find it so sorry for starting a fresh one.

MkIV Sprite bored +.030”, LCB manifold but otherwise standard AFIK. Bought just over a year ago as a car to enjoy and try our hand at hillclimbing so it’s far from concours but basically sound. Early gremlins have been overcome so now it runs well on the road, though no long runs risked yet!

It’s a different story on the hill and this weekend was typical. My son and I share the car and he has always run first. With the engine well warmed up it has run cleanly for most of his run but then starts ‘dragging’ (his description) towards the end. When it’s my turn a few minutes later, I can feel right from the start that it’s not going to give full revs. It goes to about 4000 then won’t go any more. On Sunday, after a good first half run it just wouldn’t go on the main straight and finally cut out so I had to pull off. On my second run, having managed to cool it down a bit more, it ran Ok-ish for ¾ of the hill, then just wouldn’t pull - 3000rpm max. Lifting off completely does not bring it back. Frustratingly, the power often comes back on the straight and level after the finish line, but not so this time, continued lumpy and cut out. Started fine after a few minutes cooling down and revved freely to 6000 rpm with no load.

This problem only shows up on the hill when giving it maximum welly. With little or no load on the engine, it revs freely on the level and when stationery. The problem only seems to happen when the engine is hot, though at its hottest the gauge only goes just beyond N when waiting in the queue for the second run, otherwise it stays just below N and it is not losing coolant.

Things done to date:
Red rotor arm (cured hot cutting out before we even started on the hills)
New Intermotor sports coil (before the rotor arm)
New condenser (before the coil!)
Facet fast road fuel pump fitted after first outing when we thought the problem was poor fuel flow.
New fuel hoses – tank to pump, pump to pipe, pipe to carbs, not renewed the pipe between the carbs yet.
Another new condenser because of a definite failure in August.

This weekend – tried another new condenser plus the old Lucas sports coil. (cf Is This Normal? in midget/Sprite General!)
Changed carburettor oil from thin SU oil for 20W50 as per handbook
New plugs
New low tension connectors and coil to distributor lead.

Not done yet:
Replace plug leads and distributor cap – the cap looks clean and unmarked
Replace points, though the gap’s been checked and they’ve been cleaned and look OK.
Replace main fuel pipe.

Could the heat be affecting something in the distributor? It’s a 23D4 with no vacuum advance. Timing not checked recently but set at 22 degrees BTDC as per Haynes less than 1000 miles ago.

So far I have left the carburettors well alone but, now that the season has ended, I finally lifted the dashpots. Both springs were elongated in the middle – could stretched springs be the problem?
The piston in the rear carburettor seems more sluggish than the other and didn’t lift as high when revved, but the engine seemed to be revving pretty freely. Could a sticky piston make it run badly and cut out?
On another thread it suggested that for a +.030” overbore different red carb springs were required. Silly question – how can I tell if it’s a red spring? Is it red all over or does it just have a paint blob? No sign of any red on either of my springs.

Could it be down to wrong needles? Current needles are, I believe, standard AN needles (MGOC part no1478) fitted about 4000 miles ago by a PO.

I really can’t tell whether it’s a fuel or ignition problem. There are clearly several things on the list to try but any comments or suggestions for things to investigate, check or change would be most welcome.

I really had hoped to end the season on a PB but instead I failed to even reach my target time, so very frustrated. At least my son managed to set three new PBs between on his practice and event runs so one of us is happy!

Colin Mee

Colin,
you do seem to have tried almost all possibilities! But one that I don't think you mention is the possibility of the valves binding as the engine gets fully hot. This may happen if you are using a "performance" camshaft and when the engine is at high revs and under load. At high revs the valves remain closed for too short a time to dissipate heat through the valve seats, overheat and bind in their guides. The solution is simple; open up the tappet clearances a bit. Camshaft suppliers typically say the valves should be set to 16 thou, rather than the handbook normal figure of 12 thou.
Guy W

still have standard fuel tank? how much fuel was in it? could it be the old 'cracked pickup pipe inside the tank' issue, could it?
David Smith

You said the timing is set by way of haynes manual at 22 degrees btdc...but at what rpm?

You also mentioned you havent changed spark plugs yet... it could be the gap is set to far apart or its the wrong temperature heat range as in to hot

I like guys thought also... the valve lash is set to close, noisy valves are happy valves

Prop

Prop and the Blackhole Midget

I'm with Guy- valve clearances. Especially if you're using 1.5 ratio rockers, they need to be wider still.
Growler

Hi

Thanks for the comments.

Guy. Growler - I've not had the engine out and opened up yet so I don't know which cam shaft I've got, but the tappets are set at 12 thou so that sounds like a good one to start with - and it shouldn't cost anything! It's still on standard pressed steel rockers. It

Prop - The timing was done at 1200rpm and new plugs went in before our final runs on Sunday - the runs were better than the morning ones but the revving problem was still there.

Dave - Yes, it's a standard tank. We were running on a full tank at the start of the day - got caught out on a previous meeting when we got an air locked fuel pump due to fuel surge on a low tank which had only done 75 miles from full! I make that less than 15mpg when having fun!

I don't think that fuel pick up or tank ventilation is the problem as we opened it to top up with fuel a minute before my last run just in case even though the gauge was showing over 2/3 full and the lack of pull was evident right from the start.

Thanks for all the advice - I'll report back on progress.
Colin Mee

Further thoughts.

The fact that the engine starts and revs freely again after only a few minutes of cooling down would seem to back up the binding valves theory. Opening the tappets up by a few thou will only extend the heat dissipation time by micro seconds but if that's sufficient when the engine's running fast then just a few minutes with the engine off would allow quite significant cooling to take place.

Any comments on whether the coils on the carb springs should be the same for their full length or 'stretched' in the middle?

Cheers

Colin
Colin Mee

Just my two pennyworth.

I had similar issues on a couple of occasions with my racer, i.e. runs fine in the paddock etc. but wouldn't pull under load when hot.

1) There was a bad batch of valve guides a few years ago. They were marginally too small and when they heated up the hole closed up enough to restrict the valve stem. Took a while to find but solved by reaming marginally larger. Keith Calver could tell you the source and when they were about.

2) A broken wire to the fuel pump making and breaking intermittently. again fine in the paddock, but at race speeds the wire moved and cut off the pump. when you slowed down or back in the paddock it was fine again. In practice it was very difficult to tell if it was a fuel or an ignition issue. It cost 3 race DNFs before we found it. I now have fuel pressure gauge so I can tell ....

3) Poor earths to an Ignitor (pertronix) can cause serious misfires under load after a few laps. (I know that is not the issue here but it might help someone. Again KC found this one)



Mark Lister

I dont think its carb (piston/spring) related because that would be a consistant problem meaning it would show up in the sons 1st run also

Just so im clear based on what you have said about timing...you timed the ignition to 22 degrees btdc at 1200 rpm ???

If that the case.... id say thats your problem, your way to advanced reduce the ignition timing to 10 degrees btdc at 1000 rpm,...

Prop
Prop and the Blackhole Midget

Or if the dissy curve isnt matching the egine very well

Try 26-28 degrees at 3200-3600 rpm

Btw colin,

Thank you for using a timming gun, that seems to be a rarity ....its fairly common for people to say... "oh I just listened to it, or set it staticly" ....hahaha, so consider your self an odd duck, in a good way
Prop and the Blackhole Midget

Crap...scatch my above spec, thats wrong

Should be 28-32 degrees at 3600 - 4000 rpm

Prop
Prop and the Blackhole Midget

Prop - happy to consider myself an Odd Duck - where I originate from in Leicestershire, 'Me Duck' was a term of endearment!

The timing gun was bought specially for the Sprite as my old HT one was too dim, so any excuse to use it again would be welcome and I'll consider trying the faster engine speed.

Mark - Checking the old receipts, a PO had 4 exhaust guides and valves fitted when the rebore and crank grind were done back in 1992 about 30,000 miles ago. The work was done by a Bournemouth garage but the hand-written invoice doesn't give the source, make or part number of the parts used. Could my guides be from the bad batch you mention? Surely they would have bedded in by now, unless all the POs were more gentle with it than me!

I will be checking the wiring for the fuel pump, some of it has been replaced previously but I might just put in a complete new wire to be sure - I'll probably rewire the fuel gauge sender at the same time!

By the way - this problem has really only shown when we were sharing the car at the same event. On weekends when my son did the Saturday and I did the Sunday it never really showed itself which, to me, suggests it really is heat related. We also each had a driving school day in it. I did 10 runs on my day with at least half an hour between runs with no hint of revving problems, only worsening starting for me due to a dodgy condensor that screwed up our next event a few days later!
Colin Mee

1992 - 4 guides and new exhaust valves was very likely to upgrade the head to lead-free. And the leadfree valve guides would be bronze (IIRC?) rather than cast iron and don't conduct the heat away as well. All adds to my theory about overheating and binding valves.
Guy W

I've finally managed to make YouTube co-operate so I thought you might like to see this. It's the last run on my driving school day when the car ran perfectly on the track. There was no timing on the day but I reckon this was about 84 seconds. At the start of the year we thought we should be doing it in about 80. I've got my PB down to 77.86 so I'm feeling quite pleased with myself.

http://youtu.be/4AMDS7DhAtE

Hope the link works!

Unfortunately there's no video of any of the problem runs as the camera was playing up too!
Colin Mee

Colin,
"me duck" is "Norfamtun" too

the dashpot springs are evenly wound except for the very ends of each spring which are much tighter (and I've a feeling that they might be directional but you'd have to check on a carb strip down vid (possibly(?) John Twist))

the colour is daubed on the springs (I gave away two red springs if you're desperate I can see if they're unused still)

whether you need a red spring is a different matter, if you read up in a SU manual it will help explain

Rod and I have been following John Twist's advice and running on "90 weight gear oil" (not as thick as you might think it is) with good success, very smooth acceleration

the carb needles needed can relate to the air filters/exhaust used but as with carb springs only a rolling road set up (after servicing, setting tappets) would really set you up fully for sport (or road) use - Peter Burgess is your man

as there are poor quality CB points and condensers about I wouldn't swap them unless required and then get them from somewhere like the Distributor Doctor - http://www.distributordoctor.com/

as for good quality well made HT leads that actual can (slightly) improve performance I can thoroughly recommend 'performanceleads' - http://www.performanceleads.co.uk/

Nigel Atkins

Guy, Bronze is a much better conductor than cast iron! C I is used because it's cheap and the free graphite acts as a lubricant.
Colin, Fitting new valve guides always involves reaming them after pressing, so assuming the right size reamer or one which wasn't worn, was used, running clearance should be OK. The other variable is the stem diameter of the valves................check them, you can just about do it with a vernier, valves in place.
You mention one SU piston being sluggish. Jet might not be centred, It could be binding when hot due to slight twisting of the dash pot or "muck". Take the dashpots off, clean the inside and the piston AND the pot seat on the body. The "pot" should "ring" on. With dampers removed lift the pistons and allow to fall together, they should both hit the bridge at the same time, if they don't something's up, clean and reseat until they do. Even swap the pots over to get even fall.
Piston springs..........uneven coils OK, uneven strength isn't, though not disasterous. Checking springs is easy with electronic kitchen scales and a piece of folded card.
Is the carb heatsheild in tact. Assuming you are using a tubular header, they radiate a lot of heat to your float chambers and fuel lines. Vapour locks can cause your issues. Happens more often with modern, more volatile fuels.
Allan Reeling

Allan,

I thought that yesterday due to the copper present so I looked it up. It may be better or worse than cast iron just depends on the CI and which bronze is used, see http://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html
David Billington

when checking and cleaning dashpots you could also check they're matched, see - 41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA


for dashpot cover and piston all inside surfaces need to be dry (see exception detailed), cleaned "using a petrol-moistened cloth, clean the inside bore of suction chamber and the two diameters of the piston. Lightly oil the piston rod (part) only"

see also at 1:50 here - http://www.youtube.com/watch?v=v9QI3NlvwiY&feature=plcp
Nigel Atkins

Allan, as I typed that I thought "this doesn't sound right"! But there is sometimes an issue of heat dissipation once new guides have been installed for lead-free conversions. Thinking about it, it may be because the guides are reamed to a greater clearance for lead-free so there is less direct contact between valve stem and valve guide, reducing the conduction of heat away from the valve. So I may be muddling things up there.


That said, I still suspect that the loss of power at high revs and under high load could be down to valves binding a bit due to them overheating. It certainly seems to fit the symptoms, although as we all know, that same symptoms can have different causes! Anyway the cure, by opening up the rocker clearances, is easy enough to try. If it doesn't cure the problem, then its back to the drawing board! But at least it doesn't cost anything and can be readily done in the paddock between runs.
Guy W

ETA: it's an alternative dashpot spring that has a smaller diameter end, that also has a washer
Nigel Atkins

Very interesting theories about the valve guides. I have my doubts about a lead-free conversion as there is no mention of the seat inserts that I thought were required. Incidentally, As I don't know whether the head has been converted I have been dosing with 'Substi-plomb' as a precaution at each refill.

I'll be trying the tappet adjustment first but, if that doesn't solve I'll get the vernier out. I have been thinking of having the engine out over the winter to get at the clutch and tidy the engine bay up a bit so that might be the time to have the head off for closer inspection.

Thanks for the advice on carburettor checking and servicing. I have only had a very quick look at them so far but I will certainly try the cleaning, checking and swapping as suggested. I'm reasonably familiar with the HS2 because my Minor has one but it's been off the road for too long and I'm a bit out of practice at servicing it.

The heat shield is still in place so I don't think vapourisation is the problem as the fuel line is fairly well away from it and enters the engine bay in front of and 'upwind' of the manifold. How long would it normally take a system to recover from vapourisation?

When I think I've ironed out all the main faults I'll make contact with Pete Burgess to look at setting it up with air filters and the correct needles and springs.
Colin Mee

Maybe you are right Colin. It is your car so you should know! But for just the exhaust valves and guides to be replaced in 1992 and not be done as a lead-free upgrade at that time would seem to be a bit illogical to me. But if seat inserts were fitted, then they would have needed grinding. As I gather you have service invoice records for the car you would expect there to be one for perhaps £100 worth of work if they were done.
Guy W

Hi Guy

I agree, with the amount of work being done it would have been sensible to include the inserts but the invoice suggests they only fitted valves and guides - see attached - prices today just aren't what they used to be!

Colin


Colin Mee

Allan,

I thought that yesterday due to the copper present so I looked it up. It may be better or worse than cast iron just depends on the CI and which bronze is used, see http://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html
David Billington

Colin,
just to be clear I wasn't suggesting swapping round just to checked that hasn't already happened

there are pdf downloads of the 1970 edition of the SU tuning book, this is the first I found (but not the best example) - http://www.spridgetguru.com/TuningSU_Crbrttrs.html

some more JT vids to give more info, pick out the ones relevant to you -

44 Secrets of the SU Carburettor Part 1 of 2 - http://www.youtube.com/watch?v=7GRAcqDySog&feature=plcp

43 Secrets of the SU Part 2 of 2 - http://www.youtube.com/watch?v=60Bj_2cZQnc&feature=plcp

29 SU H-Type Carburetters - http://www.youtube.com/watch?v=GHNlT5yHDxk&feature=plcp

41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA

(more to follow)
Nigel Atkins

147 MG Carburetter Tuning - http://www.youtube.com/watch?v=4nvGLgO6pj0&feature=plcp

35 Tuning HIF Carburetters - http://www.youtube.com/watch?v=ASeMfXfjNpw&feature=plcp

Needle and seat - http://www.youtube.com/watch?v=pVX_JClyeDQ&list=FLr4Udxf9e9Cq7fxH4V23MWQ&feature=mh_lolz

Setting the Float Height - http://www.youtube.com/watch?v=82YNx-RkGNI&list=UU40j4KqUJPMVv4FQ29ro-xQ&index=11
Nigel Atkins

Colin

I've been laid up with the Noro virus for the past few days so only just catching up. Two thoughts:
1) I'd put money on valve clearance being an issue
2) you mention you are using original pressed steel rockers. So was I until old age and wear and tear (the rockers not me) caught up and an adjuster stripped its thread so that #3 exhaust remained shut. It was not a total disaster but that happening while running caused all sorts of engine chaos and led to a top-end rebuild. If I were you I'd junk those rockers and fit some forged ones. I used Minispares 1.5 forged rockers but Peter May can also supply forged 1.3 rockers.

Just two quick thoughts. Now I must get back to the business of projectile vomiting!!
Chris H (1970 Midget 1275)

Chris - Sorry to hear you've been struck down - thanks for taking the time to reply.

I have checked the rockers visually but I'll inspect them more carefully now. I don't have any records suggesting that they've been changed but they didn't have the deep wear grooves that I've seen on other engines. I will be adding forged ones to the shopping list which gets longer every day!

Nigel - Thanks for all those links, they look really useful and they've given me some useful info already - added to my favourites list for future reference, especially the Tuning PDF.

Re-swapping pots - that's something I would only do for testing and comparison, I always do my best to make sure everything, including nuts, bolts and washers, goes back where it came from, unless it's obvious that someone had done it wrong in the past.

Colin
Colin Mee

David,
Very interesting data. One assumes the 4% - 5% carbon is doing the conducting but find it hard to believe that a small amount of tin inhibits conductivity of bronze to that degree, but that is the nature of alloys!
Colin, if there is a overheating/binding issue with valves it's more likely the exhaust, since the inlets are cooled to an extent by the incoming mixture.
Allan Reeling

Hi Colin
It wasn't shaft wear that was the problem - it was the end of each steel rocker arm where it was pressed around and threaded for the tappet adjustment screw. One of mine just popped up 1/4 inches giving a massive clearance. I don't think these rockers are good to 6000rpm when fresh and when worn are a failure waiting to happen.
Chris H (1970 Midget 1275)

For the pressed steel rockers, I used the tip from Vizard's book. That is, weld the seam together with a wire feed welder. I've run them well over 6000 RPM.

Charley
C R Huff

Thanks for that one Charley - a lot cheaper than the forged rockers I was looking at earlier.

The problem I've been having has only shown up on the track and not in normal daily driving. I'm only doing sprint type events where I'm only hitting 6000 revs for very brief periods on runs lasting less than a minute and a half so the welding option may well be sufficient.

When I'm sure it's a valve problem and I've got a solution I'll take a close look at the upgrade options that won't require a full rebuild or lots of money!

Chris - I haven't found any noticeable shaft wear, it was wear on the pads of the rockers that I was thinking about as that can lead to incorrect tappet gaps - though I suspect any wear here would work in my favour and give a wider gap.

I managed to take the Sprite for a test run today on the bypass where it's easier to keep an eye on the rev counter than it is on the track! With the engine at normal working temperature it revved freely into the red section with no load (foot on clutch). Under hard acceleration it went easily up to 5000 rpm then went 'flat' and wouldn't pull any more. I did this a number of times and with the flat point dropping to 4500 rpm.

SWMBO's birthday today and much extension building required to keep her contented so only time for a brief test once the day's work was done!

Planning to re-set the tappets to 16thou tomorrow and do the same run again. I'll report back.

Colin
Colin Mee

Did you re-adjust your timing from the 22 degrees at 1000 rpm yet....to 10 degrees

I still think 22 degrees @ 1000 rpm is way to advanced

prop
Prop and the Blackhole Midget

Colin, I think this will be a red herring, but I notice you say that the rear carb piston is sluggish to lift. What air filters are you using? Any chance you have got a blocked breather port on the air filter flange on the rear carb?
Aside from this I still think the valves may need opening up a bit. Worth a try and at worst it may just make it run a bit more noisily !
Have fun with it today !
Guy W

Was thinking about your rev problem yesterday as I was working on a friends car that had a similar problem but was fuel starved at 3000 rpm. One of the carb float bowl lines was crushed over preventing full flow. The carbs were stock SUs. The car wouldrun fine but would just cut off at 3000 and wouldnt go more. Something to look at anyway. Good luck!
Steven Devine

VALVE FLOAT??
Allan Reeling

..THAT ONLY HAPPENS SOMETIMES AND GOES AWAY WHEN THE ENGINE COOLS DOWN??
Growler

Valve Float - is that the same as Valve Bounce and how do I detect it / either / both?

Isn't 5000rpm a bit low for the valves to be boucing?

I don't understand why it only happens when there's a load on the engine and not when blipping the throttle when stationary. Is it because under load the mixture's changing and making it run hotter?

Sadly didn't manage a test yesterday as the blighter wouldn't start when I needed the garage space for cutting 8 x 4 sheets. No time to tinker so had to push it out and back in to come back to later in the week. Tappets still waiting to be re-set.

I've always assumed I've got a standard cam but, if, as Guy suggested, it's a performance cam, is there a way of finding out without pulling and opening the engine?
Would a dial gauge on the pushrod end of the rocker give me a reliable reading and if so, what lift would a standard cam give? Or is the performance all in the profile with no additional lift?

Colin
Colin Mee

Just assuming it was rev related, but somewhat clutching at straws. As to when in the rev range. probably dependant on the state of the springs and the lift/profile of the cam. As to symptoms, one would assume sudden loss of power as the compression goes! You don't need much power to simply rev the engine, under load, up a hill is a different matter. Performance cams are more than just the amount of lift, lift might be pretty standard but duration and timing different. If it was totally rev related it could be that the cam is mis-timed. But re-reading it doesn't exactly fit all your symptoms.
Allan Reeling

My 945cc could not rev when hot. It had plenty power when cold or on very cold days(-12 oC). Peter Burgess found the problem: The lead free seats (NOT!! done by Peter) where loose and the head was cracked.

Flip


Flip Brühl

I wonder if the stretched carb springs are a clue. If the piston is reaching full lift too early and you continue to run at WOT the engine will lean out, misfire and refuse to go any faster till you back off, or apply less load. A PO may have stretched the springs in an attempt to 'recalibrate' needle lift.
F Pollock

Good grief flip,

Thats not a crack, thats a scale model of the grand canyon.

I cant belive that thing actually ran
Prop and the Blackhole Midget

To spec a cam out,

you would need to install a degree wheel on the crankshaft

Set up a DTI on the 1st pushrod at tdc

Then get graph paper

And plot on the graph paper ever 3-5 degrees the the amount of lift being registered on the DTI

Then do the next 3 valves the same way, that will give you a graph image of what the cam looks like

It might be interesting to put a DTI on the top of the piston and plot it also in a differant color next to the cam.

Prop
Prop and the Blackhole Midget

At the weekend I finally took time out from extension building to re-set the tappets @ .016 as Guy and others suggested. I was rather surprised when I opened the rocker cover to find forged rockers there - I could have sworn they were pressed steel the last time I looked! Would these be standard on a '67 HAN9 Sprite?

With the tappets reset, I went for a run. With it warmed up nicely I cruised the bypass one way @ 60ish absolutely fine. Round the roundabout and heading back the other way it was hard on the throttle - straight up to 6000 and cruising well - for about two minutes, then power began to drop, stuttering, no misfiring, just missing cylinders. Staggered home on 2 or 3 cylinders until it died as I turned into our lane. Freewheeled till it stopped, waited 2 mins and then started fine. Revving freely when back in the garage.

I popped into the garage tonight to get something and couldn't resist starting it in the dark to see if the distributor was arcing - sure enough, pretty blue lights - but coming from the coil. It was arcing between the neck of the coil and both of the spare low tension terminals.

It's back on the original coil at the moment, but the revving problem has occurred with a new one as well.

There's a new set of HT leads and distributor cap waiting to go on at the weekend - is there anything I need to know about fitting side entry leads? I've only ever previously used leads with plugs both ends!

On another thread about similar a problem with similar symptoms there was much discussion about the possibility of too high a voltage going to the coil. Not that adept at using a multimeter - where should I take the readings from?

I'm itching to get the Sprite running properly so I can begin to enjoy using it properly - I haven't dared venture more than 20 miles from home since I had it - so any ideas or comments would be very welcome.

Colin
Colin Mee

What suspense! Oh well the tappets thing was only ever a suggestion worth trying because it was cheap and easy to do. I must admit I was quite surprised to find so many others agreed with the suggestion although the symptoms did fit.

If it is arcing between the king lead and the coil terminals it suggests that this is the least resistance path. Which means that the way the spark is supposed to be going is at a very high resistance. Plugs, rotor arm, leads? I thought you had changed all of these.
Guy W

Well thats interesting....

The good news is your in the win column with a gold star....id say you have zeroed in on the primary Issue....its electrical, and in the ignition system


My guess is its going to be something stupid...id look at the little sharp set screws that penetrate the spark plug wire look for loose and corrosion

Check your wire connections "" bullet connectors and connections" for corrosion

Check your the back of your tachometer...for loose and corroded connections

Check your wiring harness for accidential pinches and wayward screws breaking into the wiring, and look for wiring that is shaffing agianst sharp metal edges

The next time your out on a 20 mile run and it does this agian ... grab hold of the coil...is it super hot, that would leave 2nd degree burns or is the coil just warm

Congratz....id say your about there...at least now you know what road to persue

Keep us posted on what you find
Prop and the Blackhole Midget

mark up the leads and dissy for correct order - even the experienced make mistakes (mainly because they don't recheck)

for red rotor arms there are other places but I'd recommend you go here to be sure - the Distributor Doctor - http://www.distributordoctor.com/

as for good quality well made HT leads that actual can (slightly) improve performance I can thoroughly recommend 'performanceleads' - http://www.performanceleads.co.uk/

once you've replaced poor quality parts with good quality you go directly to the good quality next time to save hassle and be certain of the quality of the parts to rule them out of the equation
Nigel Atkins

Nice job Colin! You must feel a great satisfaction
in finding the issue. Lots of good suggestions here
by many involved... Nigel suggestin of the red rotor
is a good one to. Guy and prop are always helpful when your pulling your hair out! The members here really have quite a bit of knowledge to share out here!

: -)
Steven Devine

"It was arcing between the neck of the coil and both of the spare low tension terminals."

Is the coil clean, free from oil laden with crud? That will provide a current path.

Is the lead pushed fully home into the coil? If not just what you describe can happen.

Is there a hairline crack in the neck of the coil, or is the main HT lead perished or covered in oil, where it enters the coil?
Lawrence Slater

All good questions Lawrence, although Colin does say it has occurred with different coils fitted. Maybe they are all (or both?) damaged or dirty but Colin seems too attentive to allow that.
Guy W

Thanks for the compliment Guy. While you certainly wouldn't want to eat anything that had been in there, I do try to keep the engine bay as clean as I can but there is still a lot of work to do - and lots of MOT welder's underseal crud still to be removed from wires, but not the ignition ones.

I have tried to stay strictly to the 'change one thing at a time' ruling when trying to trace faults like this but I'm feeling pretty foolish now, as I have had a set of HT leads waiting to go in for months. When I first got the car I definitely had ignition problems and I replaced the condensor, points and coil before the red rotor arm cured the problem, so I stopped there. The revving problem has always seemed to me to be a fueling / induction problem and I was sure the wider valve settings would cure it.

Before I try the new HT leads, I thought you might like to see a couple of photos. Not the easiest thing to photograph but I hope you get the idea. Adding to the feeling of foolishness is the realisation that the HT lead to the coil doesn't match the ones to the plugs (which are all numbered, so no mix ups there) and suggests that a PO had problems in this department! Although I have it pushed on as far as it will go, I'm sure it could be a better fit. The arcing is between the coil's neck and the 'spare', uninsulated LT terminals. I tried insulating them with bits of PVC tube last night but that didn't help. There are no obvious signs of cracks or damage on the coil - this is the new one that has been on for about a year)

Hoping the new leads do the job - I'll report back.

Colin


Colin Mee

I doubt that adding bits of insulating tape is going to help. If its arc as it is, its because the HT lead has too much resistance. I would strongly suspect that old blue bit of HT cable, or the end fittings. There is likely to be a break inside the insulation or at the end connections. Or its a silicon anti-interference cable where the core is breaking down at high revs.
Guy W

I'll tell you what I think I can see there.

1). The connection of the low tension lead to the points is sparking. That could be because it isn't tight enough, and as the points close to make the circuit to energise the coil, the current jumps the very small gap and creates the same spark you see at the points.

2). The connection of the HT lead into the neck of the coil is sparking. That too could be because it isn't tight enough, and as the points open and the coil discharges, not only is the current jumping the gap at the plugs, but it's jumping across the poor connection at the coil.

I can't see arcing between the neck of the coil and the low tension spade connector. If it were doing that it would be very visible. Much more like a lightning strike.
Lawrence Slater

looking at the HT coil lead on the coil it doesn't appear to be perhaps fully seated (I might be wrong of course) but I'd check two things

a) peel back the boot and check the lead connection to the terminal is in good condition and secure and that the terminal is as tight fit into the coil

b) I found a HT lead boot that was such a tight fit over the coil terminal that I needed to 'burb' the boot whilst fitting it over the terminal otherwise when I pushed on the lead it would cause the HT lead terminal to be pressure out of the coil terminal - hard to imagine I know but it happened

whilst I was at it I'd check the other (dissy) end of the coil HT lead as above too

whether that particular coil HT lead, it's run to the dissy and right angle connector at the coil is the optimum for your car I can't see but I'd keep it and the other leads clipped with (spacer) combs so that they're not touching other components - I realised that the HT leads and other wires might be a bit unruly because of you trying to find this fault and/or taking the photo
Nigel Atkins

Colin, I just realised what the black rectangle below the photo is!
;-)
Guy W

Nigel. I must congratulate you. :)

"a) peel back the boot and check the lead connection to the terminal is in good condition and secure and that the terminal is as tight fit into the coil

b) I found a HT lead boot that was such a tight fit over the coil terminal that I needed to 'burb' the boot whilst fitting it over the terminal otherwise when I pushed on the lead it would cause the HT lead terminal to be pressure out of the coil terminal - hard to imagine I know but it happened"

Apart from not knowing what the word "burb" means, the rest of it is highly likely. I too have had the same experience of an ht lead being ejected from the neck of the coil, because the boot isn't fully seated. I suspect by "burb" you mean to fold or pull the boot back, to allow the HT lead to be pushed fully into the coil, and then fold/push the boot over it.
Lawrence Slater

Lawrence - Nigel and I must be of the same generation - When Tupperware first came out they told you that you had to 'burp' their boxes by closing the lid then lifting the corner and squeezing to get excess air out before closing it to get an air-tight seal.

And Nigel - I think you might right as the boot for HT lead does tend to work its way off a bit.

I'll make sure that all the new boots are properly burped when they go on.

Colin
Colin Mee

Ah, burp, not "burb". Yup now I get it. I'm of the tupperware generation too, but confess I've never heard of burping the stuff. I've still got tupperware beakers in use today. They must be 50+ years old as they are from my childhood. It's amazing how long that stuff lasted. According to ebay, it's collectable now.
Lawrence Slater

I know exactly what I mean but this keyboard is Chinese/American and mangles my words - NSA conspiracy against me I expect

for the younger I have, 'you may need to 'burp' the ends if the terminal boots are tight (ask your mum or grandma about 'burping' Tupperware to get the idea)'

Colin,
I'd cut the leads to length to neatly fit and avoid all other items but do allow for timing movement of the dissy cap, a bit of movement of the cap can alter the layout of the leads, remember you can always cut off more later but you can't lengthen the leads once they cut

when fitting the leads to the cap just make sure the lead is fully seated back and if the leads are thin that they sit in the cap terminals centrally so that the cap screws hit the inner core well and the outer sheaf holds the lead securely - just takes a little bit of time and effort on each connection - if the leads are a tight fit then it should be self centring if the cap is well made

nothing wrong with the cheap (£10/12?) 7mm silicone leads but with the 'performance' leads you can see and feel the improvement in things like the fit of the end terminal on plugs (and cap) and then you have the durability and probable longevity let alone the proven actual performance improvement (by a very sceptical Peter Burgess, thread in the Archive)

for others - http://www.performanceleads.co.uk/
Nigel Atkins

I always have to "spread" the clip of the lead that fits into the coil, as its always to loose for my taste

And I also have experianced the need to burp the leed boot in order to hold the clip into the coil

Try a little electric dieletic grease on the clip that fits into the coil...that will help reduce electrical resistance

Prop
Prop and the Blackhole Midget

I think I've finally found it - and, just as Prop suggested, it's something really stupid and not confined to hills!

I fitted the new HT leads - no more arcing and instant starting - still no change to the spluttering!

I have stuck strictly to the 'only change one thing at a time' rule but with the weather and extension building that has meant at least a week between runs most times when I have just managed to squeeze a run in after doing enough building work to satisfy SWMBO but before it gets dark.

A colleague suggested vapourisation but the carbs and fuel line have been stone cold each time I've checked them after an episode.

I recently found myself near Ombersley so I thought I'd see if I could find Peter May's place - What a delight, I could have spent ages there. He suggested a vacuum in the tank. The filler cap does breathe but with some resistance so I tried a plastic 'emergency' cap with bags of ventilation - no change.

Convinced it was ignition, and probably the coil, from the way it gradually dropped cylinders, I tried the original Lucas Sports coil - no change.

Another new red rotor arm from the Distributor Doctor - no change.

New condensor from Distributor doctor - no change.

Someone suggested carburettor icing. I've had this before with my Saab on foggy days but I couldn't see how it could have been this on a hot August day. Anyway, I took off the air cleaners and checked the float chambers while I was at it and they were fine with virtually no sediment. The carb pistons were rising and falling nicely together when the engine was revved.

I had a day off today so, with SWMBO not around and between pointing and door hanging I went for a run to see if the carbs would ice up seeing as it was quite cold and damp. After just over 4 miles it began to lose cylinders. I kept going until it finally lost them all and I switched off. Not a trace of ice but I did open up the float chambers and both were completely empty! It has been fuel all along as I had originally thought back in April - why on earth hadn't I done this check before!

Back in the garage, the output at the pump was very poor but when I disconnected the feed to the pump there was only the mearest trickle from a full tank, even after I had pressurised it to push the fuel out.

It looks as though the tank will be coming out over Christmas! Of course, S*d's law is at play as I only filled it on Sunday - and just to rub it in, my NOS Lucas Sports Coil arrived today - so now I have two spare coils to go with about half-a dozen spare condensors!

I'll let you know what I find when I finally get the tank out.

Thanks for all the comments and advice as I've learnt a lot from them and they've given me several lines to investigate for improving performance once the Sprite's running properly.

Feeling rather stupid!

Colin
Colin Mee

I wouldnt drop the tank just yet

Id hook up a temp 2 gal tank in the boot and hook up the fuel pump and take it for a drive... if it fails agian, then the tank is fine, the issue is ither the fuel pump is dying, or you have a bad fuel line thats allowing air into they system...ither a hose clip is not tight or the line is rusted or damaged and its allowing air into the system

Prop
Prop and the Blackhole Midget

or perhaps rubber hoses collapsing in or deteriorated inside?

I've had four or five lots of rubbish 1/4" rubber fuel line, now I use modern 6mm which fitted better to the carbs and has caused no problems
Nigel Atkins

Colin,
Not all that sure where the pick up is in the tank. There's usually an obvious place, the low point. Check that the tank hasn't been "pushed up there by a tyre bay jack or similar.
Allan Reeling

Btw colin

I had similar situation last week

What I did, was used an air chuck on the end of the compressor and just held it into place on the pipe that goes to the fuel tank in the wheel well and shot about 30psi of air into it, that cleared the blockage

I didnt use any more psi then that because there is a filter on the end inside the tank

Prop
Prop and the Blackhole Midget

There's DEFINITELY something not right with the tank - and Prop, your suggestion's the next one to try if I can find a suitable air line!

The pump is fine - I just tried pumping direct from a can into a jar which it filled in less than 30secs whereas it took over 5 minutes to fill the same jar when pumping from the tank.

Nigel - the hoses are all new, front and back, they went in in the Summer. The MGOC braided ones at the front could be better quality but they're completely clean and clear inside.

Allan - the tank's got a pretty clean bottom externally - no dents or bashes anywhere.

This evening I disconnected the pipe at the tank itself and even though it's completely full, the petrol only came out in a dribble. I presume the pick up filter is blocked.

The air line suggestion is worth a try but I think that would be a temporary measure. I really need to take the tank out and take a good look inside. It has obviously been out in the past so it will be interesting to see what a PO might have left behind.

Thanks for the ideas and advice.

Colin

Colin Mee

Colin,
Sod's Law, you checked the electrics so it'll be fuel same as if you start checking one end of the fuel line supply it'll be the other and if you get rid of your spares you'll need them but if you hold on to them they are never used

I only discovered a fuel leak on my MGB tank just after filling the 12 gallons of it

I wonder if you disconnect the tank pipe with the fuel cap removed if that would make any difference, if it doesn't then it probably means there's no vacuum problem with the fuel cap or tank and that the filter or pipe is partially blocked
Nigel Atkins

Nigel

I'm almost certain (famous last words) that it's the pick up that's blocked. I'm still using the temporary filler cap and that doesn't provide any sort of seal that could help create a vacuum.

I'm glad I've held onto all the old Morris spares that I acquired years ago as some of the distributor bits have come in useful this time round. I don't think I'd have found my arcing HT leads for quite some time if I hadn't been trying to solve this problem and I probably wouldn't have discovered the Distributor Doctor either so this little problem has had some benefits.

Colin
Colin Mee

Colin,
I almost put if you still have that temp filler cap but assumed you wouldn't so as always assuming is wrong but getting down to a possible blocked pipe or filter by step by step diagnostics is good

as you say the improvements and knowledge you picked up along the way about on the electrics side is good and will help with setting the car up and running and performance as well as providing a better base to progress from

bear in mind just because a part or component continues to work doesn't mean that it's working at its best or optimum, so service replacement of it before it fails will help to keep the car more consistently at a good level of performance
Nigel Atkins

Anther thought... you could just blow to pipe out, and leave the tank be, if it cloggs up agian, then drop the tank for a proper fix....thats the direction im going

I just think messing with the tank can turn into a walking dead wanna be nightmare that could take up an entire saterday very easily

Prop
Prop and the Blackhole Midget

You could blow the pipe obstruction (if that's where you think the problem is) back into the tank and fit a filter inline between the tank and pump.

I have one there and one just before the carburettor (single HIF44)

Then enjoy the car at last.
Bill1

I'd take the tank out and give it a good clean. There are firms that will steam clean the insides.Its fine to blow it back initially but if there is any crud/rust in there the pipe will only block again in my experience (usually when you are flat out going up a hill!)
Bob Beaumont

Empty the tank, take it off and clean out the problem. You know you're going to have to!!!! If you have a spare fuel pump, rig it up to empty the tank through the filler, more pleasant than suck yuck syphon.
Allan Reeling

Allan, Bob
That's the current plan. I'm sure blowing back / reverse flushing the system won't remove whatever the blockage is. It will just stir it up to return when it's least convenient. I'm sure it was launching the Sprite up hill in anger that stirred up the gunge in the tank in the first place as it had been fine before that first outing.

At least I've got a spare SU pump to empty the tank with - I'll probably find that I didn't need to change it after all!

Once I can peer inside the tank properly I'll know whether cleaning will do it or whether I need a new one. Even at MGOC prices the cost of a new tank is less than the entry fee for a day's hillclimbing and last season, on top of several slow runs, I 'lost' one complete event with a Fail and a very slow run due to the fuel problem. A new tank will just mean that a few go faster goodies will have to wait a bit longer.

Bill, Nigel, I'll definitely be putting a couple of filters in the line. I've already got a Filter King pressure regulator/filter waiting to go in and, from the way the pump behaved yesterday, I just might need the regulator to keep it under control!

Colin
Colin Mee

I would not filter an SU pump....

Why, because david dubois explicitly said not to, I dont recall the reasoning exactly... but when a man with that much indepeth knowledge and experiance says its ... its safe to put it in the bible...im not sure if david.is still around, its been a few years scence ive seen him on the net...but I wish him well in his simi retirement

Now on the electric pumps... I think thats a differant story... my electric came with a built it filter

Prop
Prop and the Blackhole Midget

Something else to consider

You could just leave the tank and continue to blow it out if it does reoccure...its simple and fast....less then 5 minutes

On mine, im thinking of securing a tire shredder valve on a rubber fuel pipe and keep it stored next to pump with a clip .

If the pick up pipe inside the tank becomes clogged agian...ill just clip the rubber fuel hose onto the fuel pipe and use any air compressor with a tire attatchment to reblow the tank, then reclip the fuel line back on and bolt down the wheel and ready agian for the next 15,000 miles

Prop
Prop and the Blackhole Midget

Prop

Thanks for the suggestions. I'm not planning to re-install the SU at present, just use it to drain the tank for inspection.

I think I would only want to use 'blowing the tank' as a temporary 'get me home' measure until the actual cause was cured. I don't want to waste any more timed runs due to it clogging.

I also want to get out and about to explore some Welsh hills but until I know it's cured I'd be worrying constantly that it might clog and cut out on me at an awkward moment. It happened to me years ago in the middle lane of the M5 just as I was level with the cab of the first in a convoy of trucks I was overtaking in my Minor - a very brown trouser moment that I don't want to repeat!

Colin
Colin Mee

Sounds great collin,

Im just projecting my own laziness onto you for only wanting to do the minimums...but you have great reasoning, and workable plan...

Go for it and I wish you the best

Just another thought if its cleaned professionally, id also consider having it sealed inside as well

Prop
Prop and the Blackhole Midget

I think I might have found it!

The Sprite has had to wait as extension building has taken priority but I managed an hour on it this evening.

I emptied the tank the other day using the original SU pump on PVC tube through the filler. Every available can and petrol tank filled - looking forward to seeing how the chain saw and garden tractor go on ESSO Premium! I filtered the last gallon through a paper towel but there was hardly any sediment left.

5 litres took 5 1/2 mins which I think works out at around 12 galls per hour, which is about right, so I probably didn't need the Facet pump after all!

The tank came out really easily this evening. Sloshing the last half gallon round and tipping it out only left a small amount of sediment in a paper towel and the inside looks clear of loose stuff. There was some gunge on the pick up's strainer but not much and it scraped off easily.

Blowing down the inlet showed there was still a restriction and probing with a copper wire drew out a bit of rust staining but nothing solid - it might have loosened something though as, after the next blowing session I had brain fade and sucked! Mouth full of nasty gritty stuff! Perhaps I should have knocked up a Pooter before I started this job.

I don't recommend the technique but it seems to have done the trick as the pick up seems to 'breathe' freely now.

I'll do a bit of flow testing to make sure and then, after a lick of paint, it will be back on the car for a road test.

The plasterer's back on Saturday, which means more extension work for me, so it might be a while before I'm allowed back into the garage!

Looking forward to having a car that runs properly at last so that I can start trying some of the improvements you've been suggesting!

Colin Mee

well pump rate sounds very and the grit is better out than in the tank, or you

if you've not already done so you should check for grit in the pump filter although it seems like it might be clear by the rate, replace disposable filter at carbs if you have one and check the bottom of the float bowls for grit

I'm not sure what you do with the tank, if anything, to prevent rust, damp, condensation, ect., this time of year until you refit and refill it

you may well have found the cause as Sod's Law if you start at the engine end first it'll be at the tank end

let us know how you get on
Nigel Atkins

Nigel

Thanks for prompting me to think about storing the tank while it's empty. I'll leave it in my airy but damp garage for a day or two for the worst of the petrol vapour to go then move it to my dry shed where it's warmer for painting. I'm also looking into getting it properly cleaned inside like Prop suggested if I can find someone to do it in the Shrewsbury / Telford area.

I opened the float chambers up a few weeks ago and was surprised to find them completely clear of crud, so the line filters must have been doing their job but I will check them to make sure they're clear.

Colin
Colin Mee

Now would be the time to hook the pump up to 2 gallon tank and go for a drive and see if the midget tank was the issue

If it does as before, then you have some more work to do...if the car runs like a top...then you have found the offending issue

For 5 minute investment the pay off is huge

Prop
Prop and the Blackhole Midget

AT LAST!

A fine sunny day and SWMBO off at Crufts so I gave up on assembling the staircase and put the tank back in the Sprite and off I went.

With a thoroughly cleaned and freshly painted tank, the Sprite ran almost perfectly. A gentle cruise down the bypass @ 50 then back @ 70 - and round again just to make sure before a really good blat round some country lanes with plenty of full throttle acceleration on the straight bits. Not a hint of the stumbling and loss of revs that I was getting before I cleaned out the fuel pick up pipe.

After about 30 miles there was an slight reluctance to fire cleanly on one cylinder sometimes when accelerating but this seemed to clear itself. I think some of the carburettor setting advice that people provided will help sort this out.

I'll be keeping a very close eye on the fuel filters from now on and probably dropping the tank at least once a year just to see what's happening inside. I decided against using a sealer. I considered just sloshing Kurust round inside to kill any rust but Hammerite told me it hasn't been tested with petrol so they couldn't say what would happen and didn't recommend it.

I now just have to decide whether to keep the tappets at 16 thou or set them back to 12 before watching Nigel's YouTubes and giving the SUs a thorough going over - and then there's and FWB, but let's not go there!

Thanks for all the advice. While the actual cause of my problems was, as Prop predicted, something really stupid, I have learnt quite a lot that I wouldn't have done otherwise and some useful guidance for some of the other jobs still on the to-do list.

Many Thanks

Colin
Colin Mee

good news Colin

get plastic disposable petrol filter and change them at the annual service

don't forget the order - tappets, CB points, plugs, timing carb mixture, best done as part of a full car service
Nigel Atkins

This thread was discussed between 30/09/2013 and 09/03/2014

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