Novel problem - engine dies during engine braking - any assistance welcomed

[marty_ell]

Hi Group

My poor old moggie has never shown anything like this problem before. This one has me beat !

The symptom is that the engine pulls strongly for part to full throttle but seems to misfire at low throttle and during engine braking. When the clutch is fully disengaged the engine splutters and mostly dies completely. Only a quick yank on the choke stands a chance of bringing it back to life, which takes a few seconds. This problem happens both when the engine is hot and cold.

I have been through everything I can think of:
- clean and service the carburettor
- repeat using my spare carburettor
- check fuel pump and filter
- filled up with fresh petrol
- change to spare ignition coil
- revert from electronic ignition to contact breaker
- clean and service the distributor and vacuum advance
- check the timing

The problem remains unchanged. None of my local contacts have any useful suggestions to make.

I would appreciate any tips the group may have - the car is just about driveable, but it is not comfortable to do so and is annoying to impatient other road users.

Cheers, Marty

[simmitc]

The fact that it recovers with choke suggests that the mixture could be a bit lean, so check for air leaks at the manifold. If you had not already chnaged the carb then I would look at the jet, but you have eliminated that by the swap.

[Bill_qaz]

Definitely sounds like air leak on inlet side. Spray carb or brake cleaner or even WD40 around inlet joint and connections with engine idling, servo if you have one. Any leak will suck it in and increase engine speed. Don't do oit with very hot engine and take care as its flammable.

[ManyMinors]

Is your car completely standard? Do you have a brake servo fitted?

[svenedin]

Is your car completely standard? Do you have a brake servo fitted?

I think this is the answer ManyMinors. A leak in the hose to servo or the diaphragm in the servo. When the brakes are operated air leaks into the engine causing it to stall from a weak mixture (hence using the choke as a rescue fix).

Have a look at the spark plugs. If they are pale, this confirms the weak mixture theory.

Also check breather hoses. Stalling under engine braking: the throttle is shut but the engine is still trying to gulp in air and it is doing it through a perished hose or loose hose. This would not apply to cars with breather hose to air filter only those to inlet manifold or carburettor.

As already mentioned check manifold and carburettor to manifold for air leaks. Occasionally, the throttle spindle bushes in the carburettor are so worn that air leaks in there too but OP has already tried another carburettor.

I have had a vacuum pipe split where the jubilee clip was. It was difficult to spot the split.

Stephen

[kevin s]

Might also be worth doing a compression test, could be a head gasket beginning to go between 2 cylinders or valve clearances.

[marty_ell]

Hi All

Thanks muchly for your suggestions.

>>The fact that it recovers with choke suggests that the mixture could be a bit lean,
>>so check for air leaks at the manifold. If you had not already
>>chnaged the carb then I would look at the jet, but you have eliminated that by the swap.
I do run it lean for economy, but the engine has worked fine like this for many years.
I did check the gaskets both sides of the carb and they appeared OK. Since the problem came on over a couple of days I was expecting a sudden breakdown of something, but nothing's obvious.

>>Definitely sounds like air leak on inlet side. Spray carb or brake cleaner or even WD40 around
>>inlet joint and connections with engine idling, servo if you have one. Any leak will suck it
>>in and increase engine speed. Don't do oit with very hot engine and take care as its flammable.
A good idea, I'll try it.

>>Is your car completely standard? Do you have a brake servo fitted?
No brake servo.

>>Have a look at the spark plugs. If they are pale, this confirms the weak mixture theory.
Plugs are a nice light brown colour, but then most of my journeys are quite long at part throttle when the engine is pulling fine.

>>Also check breather hoses. Stalling under engine braking: the throttle is shut but
>>the engine is still trying to gulp in air and it is doing it through a
>>perished hose or loose hose. This would not apply to cars with breather hose
>>to air filter only those to inlet manifold or carburettor.
I have a K&N filter which I service regularly; I'm pretty sure there is ample air available at the carb input.

>>As already mentioned check manifold and carburettor to manifold for air leaks. Occasionally,
>>the throttle spindle bushes in the carburettor are so worn that air leaks in there too
>>but OP has already tried another carburettor.
The current carb has a well fitting spindle while the spare's is loose but there seems no difference in the problem.

>> I have had a vacuum pipe split where the jubilee clip was. It was difficult to spot the split.
Have pulled off the vacuum advance pipe, again no change.

>>Might also be worth doing a compression test, could be a head gasket beginning
>>to go between 2 cylinders or valve clearances.
Good suggestion - have done a test. Compression is low on all cyls (engine is well overdue for a rebuild), particularly no. 2 where I cracked a piston (peterdjon kindly provided a temporary replacement), but no leaks are apparent from the results. Also experience tells me the engine will run on three cylinders, not happily, but obviously running whereas the problem is a complete dying.
Valve clearances, I think, are fine, I set them just a couple of months ago and there is no increase in tappet noise.

I should take a second look, including the items I've serviced. I'm thinking a good look at the gaskets, including the inlet manifold-to-block would be wise.

However I am still unable to figure out why the engine will run OK when starting and at idle, but then die on engine braking. More cogitation required.

Further suggestions welcomed. Eventually I may have to admit defeat and visit a moggie specialist with a fat wallet, for the first time in decades.

Cheers, Marty

[kevin s]

have you tried it with the vac advance removed and plugged? could be it's over advancing on decel, it does sound like a inlet leak though, which perhaps gets worse as things warm up.

[oliver90owner]

However I am still unable to figure out why the engine will run OK when starting and at idle, but then die on engine braking. More cogitation required.

When the throttle plate in the carburettor is closed, the engine is unable to “gulp’ in lots of air even at high speed. This causes the ‘higher’ vacuum pressure (reduced atmospheric pressure) in the inlet manifold from throttle plate onwards. Likewise, the other side of the closed throttle plate means there is no air flow to draw fuel in from the main jet. Does that aid your cogitation process?

[svenedin]

However I am still unable to figure out why the engine will run OK when starting and at idle, but then die on engine braking. More cogitation required.

When the throttle plate in the carburettor is closed, the engine is unable to “gulp’ in lots of air even at high speed. This causes the ‘higher’ vacuum pressure (reduced atmospheric pressure) in the inlet manifold from throttle plate onwards. Likewise, the other side of the closed throttle plate means there is no air flow to draw fuel in from the main jet. Does that aid your cogitation process?

Exactly!

[kevin s]

and when you are slowing down it's even worse, the inertia of the vehicle, engine and driveline drives the engine against a closed throttle making it a very effective vacuum pump.

[svenedin]

and when you are slowing down it's even worse, the inertia of the vehicle, engine and driveline drives the engine against a closed throttle making it a very effective vacuum pump.

So consequently any loose hose, perished hose, bad gasket or loose studs are going to show themselves at this point. To identify a hiding split in a hose it is necessary to take the hose off and bend the hose back and forth along its length. That opens up the split so that you can see it.

A photo of your engine bay would help.

[marty_ell]

Hi All

Huzzah ! Found it !
The problem was at the inlet manifold to block gasket. Naturally the break was right underneath where it was near-impossible to inspect However I guess I shouldn't complain for 54 years of good hard gasket-ing work.

A new gasket has restored normal ops. Now I have the joyous task of replacing my best carb and electronic ignition.

It has not been a wasted effort, I took the opportunity to clean and service and I have already noticed a little more pep at full throttle.

Many thanks for all your useful suggestions, particularly on air leaks; heaven knows how long it would have taken me to figure it out otherwise. Fingers crossed for some trouble-free motoring for the rest of summer

Thanks to all, Marty

[svenedin]

Good news. Well done.

Stephen

[philthehill]

The trouble with the manifold gasket is that over time it dries out and then crumbles. To overcome this problem grease the gasket on both faces before fitting.
The gasket will remain supple and if necessary can be reused.

[oliver90owner]

The easy and simple way to determine if the vacuum pipe to the servo is dodgy is to remove it from the manifold, plug the manifold connection and check if the problem ceases to exist - being aware of course that you may be driving (for a short time ) with no servo assist. An easy definitive check, not like bending the pipe around and back and forth.

Spraying oil around the gaskets usually locates any air leak by causing a temporary change in engine speed and sound. Never use anything that can ignite near a hot exhaust.

[marty_ell]

>>The trouble with the manifold gasket is that over time it dries out and then crumbles. To overcome this problem
>>grease the gasket on both faces before fitting.
>>The gasket will remain supple and if necessary can be reused.

A good tip. I like to use copper grease where parts will run hot, I find it works indefinitely, so I gave the new gasket a coating on both sides. Whether a modern gasket will last as long as the 54 year old original, time (and I suppose an eventual new owner) will tell ....
Ta, Marty