2.5 Carb engine - funny running!

[Jonny-Jimbo]

Hi All,

I have taken the week off work to do some work on the cars - unfortunately they have all fallen apart on me!! Some bits I can deal with, others have left me stumped.

On Monday I stripped the cooling system down as it developed a leak from the pump. I managed to put it back together, and started the engine (a Mk2 2.5 running on SU HS type carbs), to check for leaks. As it is it's still pissing coolant out everywhere, but I think the lower pipe may have failed.

Anyway, during the initial running the car ran quite roughly, although this is nothing new; since I bought the car it has always been rough when running on choke from cold, and after warming up it picks up nicely.

The engine ran quite badly, before I turned it off. I was then going to move the car to check the front suspension top mounts etc. The car wouldn't start - not even a cough or splutter. It turned over fine. I thought I may have flooded the engine with the choke, so I left it for a long time (2 hrs or so) and tried again, and it ran, but badly. I then clicked the key off, let the engine stop, and then tried again - absolutely nothing in terms of firing.

I did the usual jobs like checking spark (Yes it is there, it is strong and at the right time), and then checked for fuel after the pump but before the carbs. There was fuel, and at a reasonably strong pulsing intervals.

I tried again (just in case), but still nothing in terms of firing. I went to a specialist to get some parts and ask for advise - it wasn't like anything else they've really seen. I picked up a can of Start Ya Bastard (Easy Start) on the way home, and tried that. Still nothing.

I then got someone else to turn the key whilst I operated the carbs by hand - if I lifted the air valves (the sliding pistons) by hand the engine would start to fire, but badly, and then the engine speed would pick up to match the airflow, and would run. But still quite badly. if I let go of the air valves, they would return to the bridge and the engine would die - the engine would then refuse to start again.

This led me to believe that there was a lack of compression, and therefore no 'suck' to go with the squeeze, so no bang and no blow. I checked the valve gaps, and some were a bit close, but nothing major. I reset them all (engine cold, to 0.010"). Tried again, and still nothing.

I then got worried there was a lack of compression, so I left it at that.

This afternoon I tried again, but with compression tester in hand. I tested the compressions, and cylinders 4 and 5 were a little below average, with 1 and 2 showing the best.

I refitted all the plugs and turned the key - the engine ran, and ran quite well! But not brilliantly. I shut it off, and then surprise, it wouldn't restart!! Still turned over, but wouldn't fire.

I then stripped all the plugs out, cleaned and gapped them, then turned the engine over 10 seconds or so BEFORE refitting the plugs. I then put the plugs in, checked the torque on each, refitted the leads etc and then the engine fired up perfectly!! After a few seconds it started popping and farting before completely dieing.

I then stripped the plugs out again, had a cup of tea, turned the engine over again and had another cup of tea. I then turned the engine over again, refitted the plugs and leads etc, and off she went first turn of the key. I kept my foot on the throttle a little bit longer, and she ran, but quite badly. At the top of the rev range there was a very strong back fire every 2 to 3 seconds. if I held the engine at partial throttle it ran well, and if I turned the ignition key off and back on again before the engine completely stopped turning it would pick up again with only a minor 'bang' from the unburned fuel. i did this a few times, and each time it was fine.

I then turned the ignition off, with my foot hard on the throttle, as the engine often runs on. The engine came to a halt as expected. I then waited a few seconds, and turned the key again, it went back to it's old ways. It turned over fine, but wouldn't fire.

In short - WHAT THE HELL IS GOING ON?????

It seems the only way I can get the car to run is by pulling the plugs out and cranking the engine a bit.

So main questions;

1) What is going on?
2) Ay suggestions
3) What (roughly) should the compression be on a Mk2 2.5 engine on carbs (it has been rebuilt, worked fine and did RBRR last year with no issues)

Any help that leads to the arrest of these big gremlins will be much appreciated!!

PS, I do have a video of it's long running period complete with loud and noticeable bangs at high revs.

[Richard B]

I would expect about 175psi compression , anything higher is better, anything lower than 150psi is worrying. By the book, there should be no more than 10% variance between cylinders.



Ignition timing?

Cam timing?

[Royboy66]

J -J. How old is the fuel ??

[piman]

Hello Jonny,

I would be looking at a fuel  blockage of some sort, possibly in the float chamber jets or in the base blocing the flexible pipe to the jets? If you can get an old spray bottle, fill it with petrol and spray into the intake when you crank the engine, if it fires quickly it is a fuel supply problem. If not and it's on points try a different condenser.
I don't know that this 'how old is the petrol' is that relevant as I regularly start one of my cars which has been off the road for well over two years, without putting any fuel in the tank. I know it's often said but I don't wholly believe that is a big problem?

Alec

[Davemate]

I would try a new condenser, unless you parked it up months ago I would be very surprised if the carbs would "bugger up" that quickly.
Sounds electrical to me, but I have been wrong before  ;D

[Alex]

It's got electronic ignition......I'd try new plugs as you may have soaked them in fuel...

[Tim Bancroft]

Is the dizzy clamp bolt tight?

[Nick Jones]

Sounds like flooding to me (with possible after effects to plugs).  It's possible one or both of the jets is sticking down after you put the choke in?  Whip a plug out after it's failed to start and see if it's wet or dry.  After that, go through the ignition system as suggested.

Nick

[TedTaylor]

Is your electronic ignition system playing up?  .......... not unheard of.

[Alex]

I think Nicks right, that's why I mentioned the plugs ;-)

[Glenn]

Agree with above, lots of petrol can kill plugs very rapidly (think NGKs are particularly bad).

The other thing I was wondering Jonny is when you say stripped the cooling system down, did you take the water pump off the block and if so did you take the manifolds off to get at the water pipe?  I've had a similar sounding running problem which was caused by air leaks around the inlet manifold due to reassembling with the old gasket.  Just a thought.

[iggy]

just a thought is the choke staying on a bit after you have pushed it back in, this happened to my car bought another cable :)

[Jonny-Jimbo]

Hi all,

Thanks for all your suggestions. I checked over the carbs and links etc and all seemed to be shutting off properly.

In the end I swapped over my entire ignition system from my Vitesse, and changed each piece back one by one (literally one by one, not complete systems!)

In the end the fix was annoyingly simple. HT lead failure - plug four was sootier than a chimney sweep and the rhythmic misfire made me think one cylinder wasn't firing. It now seems to run well.

However compression is very very low, average of 110PSI hot on most, with 4 and 5 down to 96psi.

Are the thoughts that the head gasket is likely fair? The coolant seems good and clear enough though. Valve stem seals maybe?

[nang]

Try pulling the plugs and put a bit of oil down each bore and do another compression test. If they come up ,probably rings. If it's still the same probably needs a valve grind.
Tony.