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MG Midget and Sprite Technical - Running Fine then a loss of power

Where do I start with this.

My wife's 69 sprite had an issue with water in the coolant so I sourced another bloke and had it rebuilt - bore/balance/cam/head work etc.

Put it all in the car and apart from having to change out the coil, condenser, points, and remembering to put an earth cable on all was well. I left the gearbox in to save the angle and damaging paintwork.

Second run out car gets stuck in first car :(, so engine and gearbox out, use of a gearbox I had lying around and all back together remembering the earth strap and all is well.

Finally resolved the pinking and running on when ignition turned off but have an issue when hitting a dual carraigeway.

It will be running nicely at 60mph but you can then feel it starting to miss and eventually loose all power and not run nicely at all.

If you stop for a few minutes it will run fine for a few minutes but the same happens again.

I'm thinking coil or condensor anyone throw any light on it?

Thanks in advance, lol sorry about the pun there! As I think I might need to retard that slightly.

Shaun
Shaun

Hi Shaun,

When it starts to miss and hesitate, does the rev counter needle go frantic? This would indicate an electrical problem - coil condenser etc as you suspect.

But it could be fuel related - have you checked the fuel filler cap air hole is clear? - do your carbs have a heat shield?

Or it could be valves, if the clearances are too tight the valves may be getting overheated and bind.

So check the rev counter behaviour first. Or remove the filler cap and test again to see if this cures the problem
Guy W

Thanks Guy

Removed the filler cap as that was my initial thought, but the same happened again.

Didn't notice the tacho go crazy but it doesn't stay at a steady rev even when idling.

My approach is going to be.

1. check torque of head nuts as now it has been run they could do with being checked.

2. check the valve clearances, engine builder said .12 inlet and .15 exhaust.

3. check the dwell.

4. swap the condenser and see if I can get a steady idle.

After that I think it's check the voltage regulator to see if the dynamo has fried the coil.

But grateful to any suggestions.
Shaun

The rev counter should be steady, apart from the physical shaking of it from the car vibrations. If there's a fault on the low tension side of the ignition it does make the rev counter needle flick quite distinctly.

Personally, I would set exhaust at 16 thou, although I think it depends on which cam you are using. If that makes no difference, you can always close it down again.
Guy W

What about distributor rotor arm? The newer ones can fail when they get warm. When the drop to ambient they can be ok again. The so-called "red" ones seem to be better made.
As soon as it starts to run rough, are you able to stop and check whether there is sufficient fuel in the carb float chambers? That could be another cause? You should be hearing the pump rattling away as it tries to catch up.
Graeme Williams

in addition to what's already been put, modern made condensers and CB points can be abysmal quality so best to stick to existing working ones and to clean CB rather than replace

as said also rotor arms can be abysmal

as good provider of above is the Distributor Doctor - http://www.distributordoctor.com/

have you checked the CB points gap again

another thought is to pull the choke out as soon as it plays up to see if there's a momentary improvement suggesting a fuel related problem
Nigel Atkins

Another easy test for electrical....

When it does it agian... feel up the coil, is it just normal warm, or is it blazing hot and leave 2nd degree burns on your hands.

If so Bad ground or double grounding

All above points are great places to start

Any over heating by chance ?

Prop


Prop and the Blackhole Midget

Quick check for rotor.
Take off dizzy cap.
Disconnect HT to dizzy cap at cap or use substitute lead.
Hold lead above rotor.
Fire ignition.
No spark across gap (HT - rotor) is good
A spark indicates earthing so new rotor.
Alan
www.masckent.org
Alan Anstead

Update

So checked head torque, valve clearance, taking the dizzy out and checking the points clearance.

Changed the rotor arm for an old one I had.

Found the dizzy not sitting on the clamp plate properly so resolved that.

Checked the dwell 58.5 which is within tolerance as the manual says 60 +-3.

Runs fine on the back roads in traffic, hit the dual carriageway and after about a mile power loss comes back. Tachometer straight as a die no wobble. Pull off the road idle is fine and drive is good get back onto the dual carriageway for the return journey and exactly at the same distance a loss of power but then fine on the back roads in traffic.

Thoughts now surround fuelling problem so going to swap out the fuel pump. Shame I have no fuel line as I would like to change that out at the same time.

As always I welcome any other thoughts.

Thanks for you're thoughts so far
Shaun
Shaun

did here of a new pump being faulty recently - I put I thought it more likely to be the wiring to the pump than the pump but I was wrong (again)
Nigel Atkins

Perhaps I should add the engine has had a +60 overbore, skimmed block which meant the pistons had to have material moved to lower the cr.

Free flow exhaust already in place. The engine builder builds race engines for one of the mini series so he did his base head mods.

I have read in the archives about using a facet instead of the su, do I need to use a regulator with it as I have found a good deal on eBay?

Thanks in advance
Shaun

I think the clue is in the mods you've described. The engine is now likely to be drawing more air/fuel,and as a result 1)The needles will be become progressively weaker the further you go from idle and 2)Without a spring change the carb piston could be maxing before you've reached peak revs. Both of these means it will lean out at 'higher' sustained revs leading to a progressive misfire till you back off to where the air /fuel ratio becomes normalised.
With almost any departure from standard you need to have the fuelling (and ignition) recalibrated to allow the engine make the benefit.
F Pollock

someone will say if I'm wrong but a standard delivery pump is fine for all but the more higher tuned cars, personally I don't like the clattering Facet Qube

no regulator needed with a standard pump

if you need to change the pump see if you can get an electronic original style replacement pump, Q&H or equivalent are good and were about half the price of an SU electronic until Q&H went away but are still two-thirds the price of the SU - http://classicparts4cars.co.uk/mg-midget-electronic-fuel-pump---auf214--mfp171e-133-p.asp

you may need a red spring for the dashpots but the correct needles will be more important - I'm a fan of John Twists recommendation of "90 weight gear oil" in the dashpots, keeps things very smooth

what air filters are you using too, are they clean

you need to get the car set about right and get it used to loosen the engine (and rest of the car) then you'll probably need to adjust items until things settle then see Peter BURGESS for a RR tune-up to really pin things down and get to the optimum

some have engines and set ups that are way above standard that have special requirements but most of us can work within pretty standard parts subject to all being in good condition and correct settings without the need of the pecial requirements
Nigel Atkins

Hi Nigel

Thanks for the input, to true my plans are to run it for 500 miles to 'bed' the engine in before taking to emerald's rolling road in Watton to get it dialled in.

I just have no idea on what needles to get in the meantime so I can get the miles on the engine before hand. A friend has lent me a su pump so going to try that first.

I can get the dashpot springs but where do I start with needles for the su's, should I go to someone like minisport?

Using the big K&N filter on the car at the moment.

PS been thinking about the titan with a HIF44 to make life easier.

all help greatly appreciated.
Shaun
Shaun

As ever, try to eliminate the problem one step at a time, and if that step does not work, return to previous condition so as not to muddy the waters.

From the post, fuel system somewhere sounds a good bet. I have the standard Facet road vesion pump fitted to a slighly modded 1275 and it works fine with no regulator. Bit of a swine to fit though - required a mounting plate.

That said, do not ignore the blinking obvious. Is the current one really goosed? Is the fuel filter blocked, do you have an air leak anywhere in the fuel system or the carbs - do the easy stuff first.
Mark O

Check the vent in your fuel cap. We had a car in recently with very similar symptoms - fine around town but would die after a short-ish time at motorway speeds. We found the vent had been covered over by the use of a different seal. That was enough to slow the delivery of fuel as vacuum built up inside the tank. Removed seal and problem went away. Fully addressed with a new cap.
Paul Walbran

Hi Shaun,

RR
I strongly recommend you go to Peter Burgess for his expert rolling road tuning at excellent value too - check out what he does from his advert on this site

I'm not on commission and your car is not mine so no loss to me where ever you go but just to let you know Peter Burgess modifies A and B heads and builds A and B engines, he's very experienced in setting up classics with carbs particularly SUs, he's multiple times better than a respected Mini race engine specialist I went to on recommendation before I knew of Peter Burgess

the longer journey to his place would do your car the power of good and the extra miles would also help - the additional costs of getting there will be regained in the value and quality of his setting up

I'd put at least 1,000 miles and probably three oil & filter changes with a check/adjust of tappets and other adjustments as required (points, plugs, timing mixture) before the visit

depending on who and how the engine was rebuilt it may not fully open up until around 6k miles or more so personally I'd have another set up then and after just regularly drive (and service) the car and enjoy it - unless the carbs are very worn you shouldn't need to fiddle with them and if they're worn what's the point of a RR tune up

needles
I'd seek advice from whoever is going to do the RR set up, prepare and give them full details of your set up

dashpot spring
IF you get red springs hold on to the blue ones as the red one may not be required - I swapped from red to blue and didn't need to swap back - this is assuming your wife's car is to be a road and not track car

HIF44 and Titan
unless your present carbs are very worn (and then they can be refurbed or replaced) once they are set up you can leave them alone unless they need adjustment at servicing because of other worn parts and components like dissy, points, etc. - I've got a fully electronic 123 dissy (Peter has a few left) so I don't have to do some of the regular service fiddling or the macho roadside fiddling I just do the rest of the servicing - there is a bit of set up fiddling with the HIF44 and Titan as you'll see if you follow the threads on their installation as Mark can tell you :)

diagnosis
as Mark has put to resolve problems do a step by step logical diagnosis, moving only one step at a time based on the results of the previous step, including RTfM, (and as said) check the bleeding obvious and recheck rather skip passed even if you've already checked a couple of time before don't dismiss any steps

notes
I think you're too mechanically advanced for my non-technical, non-mechanical, general notes but they might still be of some use to you, if you want to see just email me and I'll send them to you
Nigel Atkins

I got to agree....

check your electrical grounding for corrosion pump 1st that its crisp and clean

check the fuel pump points ( yepp, just like dissy) for corrsion, dirt and wear.

Check the diapram is working okay and not leaking

Check that there is not a fuel filter at the pump from a previous owner

Use a clear fuel filter ... and run the engine at a sustained speed for 5 minutes like on.the high way and look for bubbles in the clear fuel filter

Fuel pipes and hoses.... the very fact that you would mention that, raises suspsion....it cant hurt

If you buy a faucet pump or any pump... get one with no more then 3 psi or you will need a regulator....anything above 3 psi and the fuel will blow past the needle valves in the carb and not allow the engine to run.

I think the ideal fuel pump pressure is 1.5 psi

Do you have the carb heat shild in place

Basically what we are talking about is a sustained constant driving speed of little peddle action that is causing this problem

Prop
Prop and the Blackhole Midget

The rear brakes are working okay... not slightly dragging by chance

You have 20/50w motor oil in the transmission, correct ??? And NOT gear oil

Prop
Prop and the Blackhole Midget

shaun,
K N filters require richer needles. im sure that will cure your problem. i had the same problem. have a word with greg at sussex classic parts. tell him what needles you have and im sure he will advise.
regards bob.
bob taylor

if the car came with the long plate K&N filter then it *should* have had the needles changed then if that was necessary but if you done further work to the car then the standard needles if they remained or even the changed needles for the K&N may no longer be the best suited

it may also have stub stacks fitted, the cheap small ones I've found make very little, if any, difference other than giving a nice sound to the induction

at the back of the Driver's Handbook for your car it will tell you the (3 letters) f the standard needles and possibly for rich and weak needles too so you can compare with what's fitted
Nigel Atkins

Next installment.

Pulled the fuel pump today and found a Moprod electronic one sitting there, removed it took the pump side apart to look at the gauze filters and found none.

No filters in the fuel line and cracked fuel hoses. No leaks though so will be replacing those and adding a filter.

I guess the best place for the filter is the engine bay so you can check it easily.

Found a set of H6 needles in my spares box which I remembering ordering in preperation of my Midget before going the K route so will use them on the sprite.

Going to try the needles before trying a different fuel pump. One step at a time....

thanks for all the sanity checks.


Shaun

Shaun

OK, understood. So is your pump delivering what it should. Check it out by breaking the line near the carbs, turn on ignition, noting the volume of fuel pumped into a container, noting the time taken, and comparing to the desired rate...
Mark O

Shaun,
<<No filters in the fuel line and cracked fuel hoses>>

I think you said you would be replacing the fuel lines. But worth noting that cracks in the fuel line upstream of the pump (i.e. between tank and pump) may not leak fuel out, but can allow air to be drawn in. The rubber of the hose wall works like a valve, sealing against leakage out, but when the pump is working hard it creates suction that allows air to be drawn in and fed through to the carbs. So fuel delivery drops, just when its needed most, at prolonged higher speeds.

Could be your problem?
Guy W

I'm currently having some rough running, and occasional stalling problems with my MKII Midget when the engine bay temperature rises (in hot weather). I cannot yet workout where the problem lies: fuel or electrical. I've swapped most HT components to eliminate them from my inquiries and also ruled out the LT connections to/from the coil.

I'm interested in Shaun's comment in the third posting: "...check the voltage regulator to see if the dynamo has fried the coil"

Can someone explain how the regulator can affect the coil as I cannot see how that can happen?

Also, how hot should the coil get in normal use - just warm to touch or hand hot? I know it shouldn't be too hot to touch. I have had this happen and replaced a pretty new coil however, if the regulator can damage the coil in some way I'll need to test it as this could be my culprit without me knowing.
PN Sellen

Guy - you echo my thoughts, shame i'd run out of fuel hose so had to order some. Changing out the needles, found it still had AN so the H6 should make an inprovement.

PN - it was during a google search I found a reference to the dynamo providing too much voltage @ 3,000 rpm + if the voltage regulator was playing up. This in turn would fry the coil. This at first glance was an option for my issue and if the fuel hose and richer needles does not sort is my next port of call.

Regards
Shaun
Shaun

Thanks Shaun, that'll help me in my investigations.

Philip
PN Sellen

I think guy nailed it

If there cracks in the fuel line, id replace regardless to performance

Im a bit concerned about using an H6 needle... did I misunderstand and you have a single SU H6 carb thats powering the engine ??

Prop
Prop and the Blackhole Midget



PN...

you said it only does poorly when the weather is hot....

Thats a classic case of fuel vaporization....very common with the midget in the heat of summer...make sure you still have the heat shield in place at the carbs...this is a common problem, they get removed thinking there not important and whola

Next ... look for issues that make the engine run hotter or leaner... what your trying to source is "what is making the fuel at the carbs vaporize from a liquid to a gas before it reaches the cly head

check your dissy timing isnt to advanced
Minor intake manifold vacume leak, carbs set to lean, wrong heat range of spark plugs and of course a missing heat shield.... are the most common areas for over heating the carbs and vaporizing the fuel

Worst case cure... wrap the exhauste manifold and front partial of the exhuste pipe with that ceramic exhauste wrap clothe... do it only during the hottest part of the summer ... the ""RUMORED"" risk Is the wrap creates condensation and will rust out the exhaust system....its 50/50 on the debate for or agianst

Prop
Prop and the Blackhole Midget

Shaun,
If you are thinking of changing the SU needles or springs, this site might be of help:

http://www.winsu.co.uk/what.html

Guy

PS,Its a spreadsheet programme that you download. I have used it without problems and it got a clean bill of health from my AVG scan.
Guy W

All fixed now.

Replaced the fuel hoses

Tank to pump, pump to copper that runs to the engine bay.

Changed the needles as well, when I first switched on to check for leaks all was good.

Second time I switched on the float chamber was'nt stopping fuel and just pumping out of the carb.

Cured that and had a great test drive with no signs of the power loss.

Just got to retard the timing slightly as pinking slightly and adjust the carb as slight hesitation when initial acceleration occasionally.

Even sounds better at idle, nice and smooth.

Thanks for everyone's help, can't believe something as simple as a cracked hose would give so much grieve!!!
Shaun

Well done Shaun. Its great when one finally tracks down a problem like that.

Hesitation when accelerating could be lack of dashpot damper oil. But otherwise checking the timing will maybe sort it out.
Guy W

Shaun,
well done, this seems problem to be coming up a lot just recently, (which means I should have though of it, but didn't, good job Guy did) yours might have been old hose but I've had trouble with 1/4" fuel hose and other modern rubbish rubber for 7+ years now

I swapped to Goodyear modern rubber 6mm fuel hose and it's a better fit to the carbs and hasn't leaked

for the dashpot oil I'm using "90 weight gear oil" (not as thick as you might think) as recommended by John Twist and it makes things very smooth

as I use my car all year round I'll be interested to see how it fares in winter when very cold but but JT was very definite about its use so I expect it to be fine
Nigel Atkins

Thank you Nigel, but I think Mark was the first to come up with it this time. Mark posted: <<do you have an air leak anywhere in the fuel system or the carbs?>>

I was just responding to Shaun when he said the pipes were cracked, but not leaking - a classic situation that allows air in, although no fuel appears to drip out so it is assumed they are ok.
Guy W

arh right, sorry Mark

I've seen a few threads recently on fuel problems due to rubber fuel hose, hopefully mine are all in the past but who knows according to some this (potential) 10% ethanol fuel will destroy my carbs and rob me of my good looks - reminds me of when unleaded would caused your engine to fall apart instantly
Nigel Atkins

So...next installment just when you think it's safe to go out!

Changed the needles back from H6 to AN and the problem has returned. Dipped the clutch when it happened and the engine stalled.

I assumed it was the fuel pump so swapped that out, engine pulled stronger but when on a dual carriageway after about 3/4 mile loss of power .

So the only difference now is jets. After that it's a box of matches time........or rebuild carbs time, I have a feeling one of the float chambers is playing up so going to replace the valves as a matter of course after changing the jets back to h6.

If anyone has any bright idea's they will be gratefully received.

It was nice to drive it back to back with my K Midget though, chalk and cheese come to mind.
Shaun

Chaps

I make no claims on originality! If I can reciprocate all assistance given to me in the past, that will suffice. To return to Shaun's problem - if you idle after loss of power, does it behave itself again? If so, then yes, float height may be an issue, blocked main jet(s), or blocked fuel filter would be my first ports of call.
Mark O

Missed that last message, Shaun. Mark may be right about fuel shortage. But if the problem only returned when you swapped the H6 for AN needles, why not just swap the H6 needles back in again? If you do this - and make NO other changes - then it should confirm that the needle selection is the cause. Or at least the trigger to the problem.

I don't know if an AN needle gives a weaker mix than the H6 at higher revs, but if it does then it could be that running on weak mixture for a few minutes is causing the valves to overheat and stick a bit which would give the loss of power. It feels almost like a rev limiter coming on with a diesel engine.
Guy W

Update, it runs well.

So I put the H6 needles back in and all is well again. Just some fine tuning required now to cure the dieseling when turning off but that can soon be cured. Just a minor pinking issue to resolve as well but that can be done using the vernier adjustment on the dizzy.

I have to say I have learnt a lot about carbs, ignition and timing sorting this out. However it has made me more confident in resolving the issue should it happen again.

Thanks for all the help and advice, wherever possible I will share it with others.

Thanks
Shaun
Shaun

Needle comparison.


F Pollock

This thread was discussed between 26/08/2013 and 17/09/2013

MG Midget and Sprite Technical index

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