Soot on no.3 plug epic!
Turbo Mini Forums

Hi chaps, my first post on here but unfortunately I don't think it's an easy fix.... I've been tearing my hair out with this problem for the last week.

Engine is a 998 A+ engine, with stage 1 kit and otherwise standard apart from a wasted spark system of my own design. Car has been running amazingly well up until last weekend, when problems began. Car began to run quite rough and lacked the power it had previously shown.

On return from a run, I switched back to old distributor setup to eliminate my home built circuitry and problem still evident. Removing plugs showed 1,2 and 4 to be a great colour but number 3 very sooty and obviously having difficulty firing properly (although the car does still drive fairly well so I don't think the engine has lost power on that cylinder totally).

So I tried the easy fixes, new plugs, HT leads and coil. Still sooted on no.3. So I swapped the location of plugs and leads to no avail. Struggling for ideas, I did a compression test, all four cylinders showing 160-170 psi so no problems there. Couldn't think of anything else however, so I took the head off anyway. The head looks fine, valves ok too. Gasket was not blown at all and was only just over a year old anyway.

Rather than risk putting the old head back on and find the same problem, I put a recondition head back on, new valves all lapped in fine etc....

The problem is still there however! There are no leaks on inlet/exhaust manifold (in any case, number 3 cylinder would be paired with other cylinders and they work fine). I have put a dial guage on the valves and cylinder number three gets the same amount of lift as the other cylinders. The end of the pushrods look fine and obviously I have been adjusting tappets so they have spot-on clearances throughout the head replacement process.

The car uses little to no oil (the engine is 1989 metro and has only covered approx 35k), certainly no blue smoke etc which would suggest a valve guide / piston ring problem. The bores all looked fine when I had the head off too. In any case, the spark plug is sooted not oiley, so I don't think the problem is down to oil.

I have run compressed air into the cylinders to try a leak-down test too. Cylinders two and three seemed to leak pressure at the same rate yet number two always fires and three does not.

I appear to have compression, fuel and a spark yet number three cylinder just wont have it. Does anybody have any tips at all as I have totally run out of ideas. To make matters worse, the car is due to be on the rolling road during the week!

Thanks guys

Was the compression test done with the engine hot or cold?

Hi Tom, all compression tests have been done with the engine 'warm' ie. the engine has been at running temperature probably quarter of an hour before.

Just another thought, is it unheard of for the timing chain to jump on an a-series engine?

Thanks

I'd try doing the compression test again, cold, and then as hot as possible to compare differences. It seems you have ruled out the cyl head by swapping it so in my opinion you need to look at the gasket and also the piston.
I have never known a timing chain jump. Especially on the A+ engine with rubber tensioner.

If the chain jumped, it would affect all cylinders though.

It does sound like you have most aspects covered....weird.

You say inlet and exhaust valve on that cylinder are defo the same as others ?
Same with valve clearances ?

Have you tried loose clearances to ensure there is defo no chance of them being held slightly open ?

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Hi, I have checked compressions cold now, all fine again, as with warm readings.

All valves are identical, the new head has all new valves and freshly cut and lapped seats, very frustrating!

I'm now taking the whole engine out to replace it with another 998 A+. Unfortunately I don't know if this engine is even a runner, but I might as well risk it!

Once thing is for sure though, I will strip this engine down to the last nut and bolt to find this problem, I need to know now just for interests sake!

Thanks for the help

check to see if you have proper valve lift on no 3 .
regards robert

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Hi Robert, I have put a dial gauge on both inlet and exhaust valves for cylinders 3 and 4. Both show approx .295" for exhaust and .310" for inlet, which I assume is correct (they are both the same at any rate).

Hi,
A sooty exhaust valve is usually because it's not sealing fully, but as you've changed the head already, maybe a sticking cam follower or something else obscure ???
Cheers,
Gavin :)

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Hi Gavin, I think it must be something like this, I have the old engine out now - once the new one is in the cam follower covers will be coming off, hopefully to reveal the secrets. The strange thing is the valve train all operates fine when rolling the car in gear though.

ok then bent pushrod ,or broken valve spring ,um cracked head ,um valve guide oil seal off guide ,um any good ?

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Hi, pushrods all fine (I thought it may be a bent pushrod too!). I've had all the valve springs out, and they are ok. The new head has just had new valve guides put in and I put new oil seals on before putting the head on the block.

Still no further on in terms of diagnostics on the old engine but new engine is in and should be ready to go soon, fingers crossed it will be ok!

This is getting crazy! New engine is in, thankfully it seems to be a really great unit , even has a nice clutch. Just been for a short test drive and I have pulled the plugs, three great ones and a sooty number 3! So after dinner it's new carb time, though why that should make a difference when it's paired with number 4 is beyond me

what else than the carb has been moved over to the new unit from the old one?

Problem solved!! Seems like it at any rate... the two things I swapped over from the old engine were carb and fuel pump which I thought seemed less risky than using items fresh from the scrapyard. So I swapped these two items onto the car and now it seems to be running fine again, no soot on any plugs and much improved idle!

I assume that either the fuel pressure was too high or the float valve was no longer holding pressure properly, but I have no idea why it was only number three cylinder that was affected. Must be something to do with fuel puddling in the manifold / airflow etc, vizard would probably know.

Thanks again for all the help

well done !!

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM