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MG Midget and Sprite Technical - Poor engine response

The car has been going swimmingly well, especially since I had the distributor (59D4 with Powerspark module) serviced and recurved last year. Very responsive, better torque. Generally nice to drive. Then about a month ago it began to feel odd. It was as if the ignition was retarded, but I had not changed it. Starting not as good, very reluctant to pick up revs especially when cold. Torque much poorer. It felt like ignition was the problem. I fitted a new Viper VCS coil, which seemed to make it a bit better but did not cure it. I've checked the mechanical advance twice, and made sure it's properly operating and lubricated. But that was all done by H and H Ignitions anyway. New plugs went in yesterday - NGK BPR6ES. No difference. I am now looking at the carb, which is an HIF44. With the air cleaner off I can see that the piston is freely moving and not sticking, and the correct dashpot oil is to the right level. On a cold start up with the engine idling, when I open the throttle the piston barely moves and lots of vapour spits out of the carb mouth. I have just taken off the vacuum chamber and can't find anything wrong anywhere.

After a highway run the plug colour is exactly correct and shows that the timing is correct. Fuel consumption is poor, rarely much over 30 mpg. I rang Powerspark for advice, and they said the module will either work or not work, there is nothing in between. It is just a switching device and doesn't control the timing, which is exactly what I expected them to say. I have reset the timing on the strobe until I am blue in the face and still can't find the problem. I think as I have probably exhausted all the ignition possibilities, the problem must lie in fuelling, but I can't visualise what to look for.

I thought about putting the distributor back on points, but when I had it modified they changed the base plate and it won't accept points any more. But having spoken to Powerspark I don't think that's the problem anyway.
Les Rose

Induction manifold air leak maybe?
GuyW

which type of plug wires are you running--
William Revit

Guy, the manifold gasket is new, but I will check.

Willy, the plug leads are silicone and also new.
Les Rose

Les. The first thing you need to do is to perform a series of standardized tests, record the results, and post them so that other people can see actual, measurable facts rather than general feelings. Colin Campbell's splendid book, "The Sports Car Engine" is a standard reference work that should be on the shelves of any serious hobbyist. In the book, Campbell notes a test series that he used, over a number of years, to fully test out the engines of the cars he was working on. After that, he delves into understanding and diagnosing various problems that the initial testing seems to have discovered.

The problem that you describe in this post might be the ignition system (dizzy, coil, leads), or it might be an electrical problem before the ignition system reducing the voltage flow into the ignition system. Without doing a through inspection, in a systematic manner, it is not possible to make any decisions. Replacing parts, on the possible chance that one part may be the problem, is not a good idea as we all have seen new parts that did not perform as well as 40 year old parts. Actual tesing, and documentation of the results, is the only method of trying to define and diagnose the situation that actually works.

While you have defined the problem as being ignition related, the problem has an equal chance of being fuel related (low fuel delivery rate, bad carbie settings, etc.) and a slight chance of being neither electrical nor fuel. (E.G.--A slipping clutch gives some of the symptoms of loss of power.)

Do not know what local libraries are like in the UK, not having been there is over 40 years. In the US, every library I have dealt with has an "inter-library loan" service. If they do not have the book you are interested in at their library, they will order it in from some other library that does have it and check it out to you for some period of time. At the very least, photo-copying the section of Cambell's book dealing with engine testing will be a very good thing to have.

The other Les
Les Bengtson

Les--silicone leads,When you say new, how new, new new or new last year new--- I take it that the silicone is the outer casing---are these resistor leads or solid core.
I notice you're running resistor plugs. --- If you have both resistor leads and resistor plugs together then just maybe the combo is too much for the coil to overcome the combined resistance---specially when your new wiz-bang coil improved things.
I'd suggest you measure the resistance of your plug leads and coil lead to see what you have there, make sure one or more haven't gone open circuit or high resistance, and if they are in fact resistor leads and in good condition maybe try a set of BP6ES non resistor plugs.

willy
William Revit

Is there any point in resistor leads if you don't have a radio? I had assumed not, so use solid copper on both my cars. Am I missing something and that's not a pun!
GuyW

Les, did you resolve your fuel pump issue? I'm just speculating if there is a possible, if unlikely connection between the two issues you're facing?
Jeremy MkIII

As you say Guy, no radio no need for supressor leads or caps. But *some* electronic ignitions suffer from rfi too, so they need supressor leads/caps.

I use stranded copper ht leads too.
anamnesis

A friend was having exactly the same problem as you. After changing vitually all of the ignition components with no improvement we tried the rotor arm from my car. Problem solved immediately.


Jan T
J Targosz

Thanks for the ideas chaps. Yes I have a radio. I only use it for recorded music but I still got interference, hence the resistor plugs. The leads are quite new - replaced with dizzy cap after this problem arose. I measured the resistance of the leads - 5.3 kohm. They are the correct ones for electronic ignition, according to Powerspark. Copper leads should not be used. I always used carbon string leads before, with no problems.

I replaced the manifold gasket, although I could not see anything wrong with the old one.

Jeremy - I think the fuel pump issue is OK now.

Whatever the multitude of possible problems you good folk have mentioned, the fact remains that I did not change anything for a while before this problem arose. There is no temporal association with any changes I have made to the engine. I was so pleased with the improvement after the distributor was reworked, that I dared not make any more changes. Then suddenly something weird happened of its own accord. I really need to get it on the rolling road but I'll have to drive a long way to get to somebody who knows what they're doing.
Les Rose

Have you done a wet & dry compression test?

And, slightly random but is the vacuum advance tube intact, properly fitted and not leaking?
And, does your HIF have the oil breather pipe connection. Is that all intact?

My thinking is you mention that the carb piston isn't lifting. If it's not physically sticking, then it suggests the engine is not pulling enough vacuum (so try compression test) or that there is air leaking in (vacuum advance pipe or crankcase breather pipe splits/ leaks?)
If the piston doesn't lift, then the jet needle doesnt allow sufficient fuel in and the engine is strangled for air and fuel.
GuyW

About 160/170 miles I'd reckon. Worth it though as Peter knows what he's doing.
Jeremy MkIII

Thanks for the recommendation Jeremy. I would be happy to help Les but I think he would need to bring spare everythings up and work under our guidance sorting things and trying things. My time is best spent fine tuning. As I have loads of work to get on with an early start of say 8ish gives Les till 5pm to sort it for £120 plus any bits he doesn't have in the boot!

Les Bengtson, great book, probably my favourite, my copy has my flow bench calcs written in it as I was trying to learn my craft, I bought a copy for my son Simon to learn from now he works with me. He loves the book too!

Peter
Peter Burgess Tuning

Les, you say, --

" On a cold start up with the engine idling, when I open the throttle the piston barely moves and lots of vapour spits out of the carb mouth. "

I too think air leak/weak mixture. Or maybe a leaking intake valve?

But I have a couple of questions and thoughts too.

Q. Is what you call vapour, really unspent air/fuel, or is it actually a backfire of partially combusted fuel through the carb'?

Q. Although I too think air leak or weak mixture, I don't quite know why now; I must have forgotten. How exactly, does a weak mixture or an air leak, cause spitting back through the carb' mouth? To spit anything back into the carb', there must be a pressure wave behind it. How does an air leak or weak mixture cause that pressure wave?





anamnesis

I have done a dry compression test. 160 psi on all four. Will do a wet test when I get a moment.

Vacuum advance pipe is in perfect condition.

Breather uses ported vacuum, ie to carb not manifold.

When I said vapour I really meant mist. By definition vapour is invisible.

I'd love to get to Peter Burgess but a three hour drive to get there for 8am, then a three hour drive back in the evening, is not something I relish at my age.

A leaking valve would make it run rough, but it's very smooth.

As I have said, I have run the car with this spec for nearly 20 years, except for the distributor reworking which was done last year.

Les Rose

Les, check centrifugal advance not jammed and the magnetic breaker isn't loose and retarding ignition
Peter Burgess Tuning

The reason I was getting drawn to leaking manifold, or air getting in via ported engine breather or vacuum advance pipe was that I thought any of those might hijack the suction that should be lifting the piston. Why isn't the piston lifting?

A fairly common error which also has that effect is when someone puts the carb/ air filter gasket on the wrong way and blocks off the balance orifice. But that cannot be the case here as you must have had the air filters off to see what the piston was doing.
GuyW

Les, stay over and enjoy the delights of Alfreton!!
Jeremy MkIII

Ok, mist then. That suggests unburnt air/fuel mixture doesn't it? If it was a backfire due to timing, it wouldn't be a mist would it? It would at least be 'smoke', or even worse, flame. I've had that.

Anyway, just to get my head around it as my old brain is getting confused. lol.

Timing out. If it's too late/retarded, it must ignite the charge in the exhaust. But if the spark is too late, isn't the inlet valve shut? So how does it spit back through the carb' ?

Answering my own question.

Is it due to the overlap of valve timing? Exhaust valve opens, partly burnt charge continues to burn in exhaust port because of late spark, overlapping inlet valve starts to open, and the air/fuel mixture drawn in, is ignited by flame in exhaust? Is that right?

And even if it's not the cause here, can anyone explain just for my peace of mind, why a weak mixture and or an air leak, causes spitting back through the carb. It seems to be accepted wisdom that it does. But how EXACTLY?
.
anamnesis

I have done the compression test:

Cylinder 1: 170 dry, 190 wet
Cylinder 2: 170 dry, 190 wet
Cylinder 3: 155 dry, 175 wet
Cylinder 4: 155 dry, 175 wet

Engine warm, throttle wired open.

Here is a video of the carb showing piston movement and mist spitting out:

https://www.youtube.com/shorts/8Dga4yc0DD8

The timing at 1200 rpm is 7º btdc, vacuum advance disconnected and sealed.
Les Rose

All up 20 wet, but difference between 12 and 34 remains at 15. Leaky valve or hg. Either way head off to look? Pistons lifting enough, my money is on valves.

When did you last check valve clearances?

Or maybe ring blowby 34, down into crankcase, and up into inlet manifold via engine breather?

anamnesis

Do you spun the engine a standard number if rotations?
I would expect less of a difference than that between wet and dry tests. 20, to me indicates poor ring / bore fit.
The lower compression on 3 & 4 could be valves or possibly leakage between cylinders across the head gasket.

I dont know if any of this explains your progressive loss of performance, though I think 175 is fairly low.
GuyW

I don’t think I get high figures when mine was last tested- being even is more important and totally depends on what has been done to the head volume.

The workshop manual quotes for 1275 HC engines (8.8) a cranking pressure of 120lb/sq.in @ 350 rpm.

Well I’ll see what happens tomorrow when it goes on the r/r if it has issues ! I hope not !

R.
richard b

I note that the timing is listed at 7 deg BTDC at 1200 rpm. Is this correct? If so, there is a possible cause of the problem. Original practice, before inexpensive strobe type timing lights were commonly available, was to dial in 10 degree of advance when doing static timing. This, commonly, would give about 15 degrees at somewhere between 1200 and 1500 rpm as the mechanical advance was kicking in. I do not have the timing curve for the 1275 engine to hand, but doubt very much that only 7 degrees of advance at 1200 rpm is sufficient. Thus, the timing is significantly retarded and will result in significantly (and measurable) loss of performance. Overheating is, also, something I have observed when the timing was overly retarded. Might be worth checking out.

Les
Les Bengtson

Les---watched your vid. --my money is on a headgasket on it's way out between 3-4
You can see in the vid that the piston is pulsing whereas it should rise smoothly
Your low compressions on 3-4 back that up.
What happens is that 3-4 follow each other in the firing order so when No.3 reaches tdc. and fires, No.4 is at bdc with it's inlet valve still open----When No.3 fires part of the combustion blows past the leaking gasket and out through No.4's open valve into the inlet manifold.
To prove it you can take No.3 pluglead off and that 'should' stop it firing back into the manifold, but it'll be running on 3cyls so might be hard to see any difference.
If you do run it with a lead off make sure the end of the lead can spark to earth or you'll kill your rotor button.
Also, I agree with Les, 7deg. isn't really very much.
With the timing at normal 10 static you should be somewhere around 12'ish at idle with a strobe.

On another point, which was what I was going to mention next until seeing the vid.
With the HIF, if you've got him off, just check that the float hasn't partially sunk(fuel in it)--I've had these just partly sunk and give the faults you were mentioning earlier, hard starting, down on power, poor fuel economy and spitting haze from the carb mouth---bit hard to identify in their early stage of sinking as they have to be sunk a fair way before starting to show fuel out the floatbowl vent.

But I'm now convinced you have a headgasket issue, head off,--and as good a time as any to check that float.
If you were running your dist. at 7deg idle with the strobe which is probably about 5deg off the mark, what did you have at full advance--just out of interest.
William Revit

Willie, Great minds ?
<<The lower compression on 3 & 4 could be valves or possibly leakage between cylinders across the head gasket.>>
Though your follow up explanation is somewhat more detailed!
We shall see!
GuyW

Wa shal Guy, we shal.
William Revit

Yep great minds. Lol.

As I said, "All up 20 wet, but difference between 12 and 34 remains at 15. Leaky valve or hg. Either way head off to look?" 😄

Great explanation of the exhaust route to the carb Willy. I was trying to get my head around that.


anamnesis

Anam
I had to read "12" and "34" a couple of times before the penny dropped. D'oh!

😮
Bill Bretherton

Oops. Sorry Bill, I should have used commas there. 🙂

Or, I'm just really bad at subtraction. 🤣🤣.
anamnesis

Hi Willy

"If you do run it with a lead off make sure the end of the lead can spark to earth or you'll kill your rotor button. "

Do you mean the whole rotor arm, or the end segment, or the centre where it contacts the carbon in the distributor?

Not heard of that happening before, certainly never happened to me anyway. Worse case, it just arcs somewhere else doesn't it?
anamnesis

Les, you were right I did not have enough advance. I just tweaked it up a bit and it is running much better. The problem is that with the PowerSpark unit I can't time it statically. The manufacturer says this is because it has variable dwell and you have to time it on the strobe. I managed to do this before I had the distributor recurved, and all seemed to be well. I just bunged it back in with the same timing as I had before (I have marks on the distributor to make sure it always goes back in the same place). I am baffled as to what has changed in the mean time, as that timing no longer works, so I have had to go back to the advance point I had with the old distributor curve. I'm starting to lose the plot here because the new curve has less total advance and more at low to mid range. That's the reverse of Vizard's recommendation for this spec of engine. I think maybe I got away with it until the coil started to flag.

The compression situation is even more interesting. Some of you will remember that this is a new engine. I had persistent gasket failures between three and four that went on for years, despite repeated head skimming. So I bit the bullet and rebuilt the spare engine, skimming both the head and block face again. There is nothing wrong with the castings now. I keep trying different gaskets. It is a composition one at the moment, but I have two copper ones in stock. It looks as if I will need to try one of these and see what that does to the compressions. However I did another test today just to be sure, and cylinders 3 and 4 do come up with a shot of oil, so I'm a bit worried about ring bedding. I made sure the engine did ten revolutions on the starter each time.

Just to be clear, I have adjusted the valve clearances until I'm blue in the face. Valves were lapped in again when I rebuilt the engine last year. Guides are bronze and correctly honed.

Thanks again for all your interesting comments chaps.
Les Rose

Les, is it still spitting back through the carb' mouth, after re- timing/advancing?

In any event it won't run properly if the hg is blowing between 3,4, -- even if the timing is dead right.

If you've ruled out leaky valves, that leaves the hg. The back, 3/4, can run hotter anyway on these engines. If you've been that far retarded, the extra heat might account for blown hg again.

Head off to see what's going on. Was the head crack tested? And sump off so you can pull out 3 and 4 for a look at the rings.

anamnesis

Les--it's quite normal for the compressions to rise with a shot of oil in there, just depends on the size of the shot---it's not a very big combustion chamber so doesn't take a lot to change the volume of it
IF you do pull the head off it'll be interesting to see some pics of block/head/gasket
Does your car have a heater, or is the heater outlet on the head blocked off.--might be a coolant circulation issue-

An--You answered your own question with-it just arcs somewhere else doesn't it--yep down the side of the rotor or through it, once there's a track burnt on it, it's the start of the end--
William Revit

Thanks Willy, I'll ground mine from now on too. Always worth preventing trouble, esp' if there's no effort involved.
anamnesis

No I didn't get the head crack tested. I didn't have any reason to at the time. It is a problem with these heads, I have had cracks with two of them.

Yes I have a heater, turned on at the moment. With the new aluminium radiator it runs very cool, especially in recent cold weather.

I haven't checked the piston lift and mist spitting yet, but will do so in the next couple of days. I'll make sure I have a set of piston rings in stock before I go any further.
Les Rose

What a saga. I feel for you Les.

Whilst not a difficult jobs in themselves, having to do them repeatedly with fairly short intervals, gets a bit frustrating to say the least, when all you really want to be doing is driving it. At least the weather is warming up. Slog on. 👍

anamnesis

Yes Anam, all jobs are simple on such simple cars, but a lot of the repetition is because of my ineptitude! I should have dug an inspection pit when I had the garage built. It's crawling underneath which I don't relish these days, and the axle stands get in the way. Maybe I should get some ramps.

Now I have the timing about right, a plug test shows much weaker mixture, so I have reset that. I should have checked the carb spitting while I was at it, and now the rain is apocalyptic, so that will have to wait.
Les Rose

Probably not something you've done wrong per se Les. It could well just be the consequences of modifying the engine from standard spec's, and not yet found the best tuning/settings for it.

I don't relish crawling under mine either these days, and as you say the axle stands get in the way. I use drive up ramps when I need better access, they are definitely worth having. I've got an old set that give good height.

Pissing down here today too. Oh to be in my 20's again. Rain or shine, hot or cold, indoors or out; I could get under my Sprite anywhere, anytime, any conditions. 😅😅.



anamnesis

A neighbour ditched his old drive up ramps as IIRC the new Morgan was very low and instead bought a hydraulic axle lift which you drive on then jack up the axle and lock at height. It lifts higher than the ramps would but I doubt he'll be using it much more due to age, maybe I should make a bid for it.

This type of thing https://pelatools.com/vehicle-lifts-jacks-and-stands/vehicle-ramps/hydraulic-wheel-ramps-1500-kg/ .
David Billington

They look good. Hmmmm. Might investigate a set of those myself. 😁
anamnesis

They look good and I have considered similar before, but felt the use they would get didnt justify the outlay as most of the time when its up in the air the wheels need to come off.

Trev
T Mason

Perhaps a tad expensive, but quite versatile I think. Quite easy to have a wheel off.

https://cjautosheywood.co.uk/product/hydraulic-car-ramps-1350kg-capacity-cr012/

https://youtu.be/wqIOiJKCXwQ?feature=shared

Good review in the PDF file.





anamnesis

Or cheaper, these on Amazon.

Quite a nest idea. I've got spare ramps. I wonder how cheaply I could buy a couple of bottle jacks, and alter my spare ramps. 😄.

https://tinyurl.com/4r9wp3t2


anamnesis

I know you can still put an axle stand under to take a wheel off but I always thought that defeated the object a bit.

Trev
T Mason

I've seen a set that go under the centre section of the car then the car can be see/sawed up on either end.
see if I can find a pic. somewhere.
William Revit

Found him--
https://www.ebay.co.uk/itm/124539732739?itmmeta=01HT4YQ9A79GGMEYJHQDDGRG40&hash=item1cff258303:g:
William Revit

Had a good 50 mile run yesterday. Mixture a tad rich according to plugs, so leaned it out a bit. Fuel consumption may be better. Still baffled by the retarded timing, as I have a pointer on the timing cover zeroed by using a dial gauge on the piston crown. Not had the head off yet. May wait until after Drive it Day.
Les Rose

Might be interesting to do another compression test--
Is it still fluffing out the carburettor----
William Revit

Just had another 30 mile run, including an Italian tune-up. Going like stink. Compressions just the same afterwards, and still puffing mist from carb. Head off as soon as I have dealt with paintwork blemishes - show season coming up!
Les Rose

I have now had the head off, and didn't find anything obviously wrong. Cylinder bores are perfect, gasket looks fine. As a precaution I pulled the valves from cylinders 3 and 4. The number 3 exhaust had some marks on the seat, so I ground in all the valves from those cylinders. All back together now, and I just went out to warm it up to retighten the head nuts. Compressions are now:

1 - 165
2 - 160
3 - 155
4 - 150

It's not the most precise of pressure gauges so I think I will settle for this. The figures are slightly lower than last time, reflecting I think that this was after a short run not a long one. I don't think the composition head gasket had anything to do with it, but I've replaced it with a copper one anyway.
Les Rose

Les--could that be the battery gradually spinning it over slower as the tests were taken, it looks like a gradual drop off. It'd be interesting to do No4 first and work forward just to compare----one day
So there wasn't any sign of a blow through between 3-4 or a split in the sealing ring of the gasket in that 3-4 area---?
willy
William Revit

I never thought of that Willy. It's an almost new battery, but it wouldn't be the first time one has failed under guarantee. Yes the gasket was fine, no sign of distress anywhere.

Back to the paintwork now.
Les Rose

"As a precaution I pulled the valves from cylinders 3 and 4. The number 3 exhaust had some marks on the seat, so I ground in all the valves from those cylinders"

Q's.
How did the inlets look?

Didn't you look at the other valve seats from 1 and 2? Did you upend the head and do a petrol leak test in the combustion chambers?

When you do compression tests, are you wide open throttle?

Is it still spitting back through the carb?
anamnesis

Answers for Anam:

The inlets looked perfect. But I ground them anyway.

No I didn't look at the valves from 1 and 2. No I didn't do a petrol leak test. I am a lazy *****.

Yes the throttle was wired fully open for the compression test.

After a 30 mile run today I could still see a barely perceptible mist from the carb mouth on blipping the throttle. I don't think this is anything coming out of the carb, it is most likely condensation caused by suction. It will be interesting to see if it still does this in a heat wave when the air is dry. Right now the atmosphere is very damp.

I think I have got just about everything back to the sweet spot and the car is very nice to drive again. Plug test (see photo) shows mixture a bit lean, so I have enriched it a quarter turn. I'm puzzled by the dark patch on the electrode tip - all the plugs are like that. The timing looks ok.



Les Rose

This thread was discussed between 21/03/2024 and 11/04/2024

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.