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MG MGA - Stalling when hot

My first post to the board so hello everyone!

I have a 1622cc in a Mark 1 body. It has electronic ignition, a lead-free head and a 5 speed box.

The problem I have is that when the car gets hot (say after a 20 minute run at 60mph) it stalls when I pull up at a junction and I have to wait about 20 mins for it to cool before I can start it. Runs fine when hot as long as I dont stop! The problem has got worse over the past 2 years.

The problem has got worse and wrapping the exhaust manifold did not help. I have a heat sheild between the manifold and the float chambers.

The dynamic timing is set to 20 degrees at 800rpm.

I use premium grade unleaded fuel.

The engine is not overheating according to the temperature gauge.

Its at the stage where I am driving it very little now since stalling at a junction with traffic behind you takes away the enjoyment somewhat!

If anyone has any ideas I would be really grateful to hear them. Maybe I should be looking at the coil/ignition rather than assuming vapour lock is the problem?

Thanks.

Steve.

SP Mitchell

A couple of things I would do first

1 I would set the timing correctly and that means rev the engine to 4.5K RPM with the vacuum disconnected and set the timing to about 32 Degrees BTDC. This ensures that it is set perfectly IRRESPECTIVE of which distributor is fitted.

Then set the carbs with the engine hot so that it will idle when hot.

One step at a time finds all problems eventually IMO.
Robert (Bob) Midget Turbo

Steve,
The first thing you need to do is find out if it is a fuel related or ignition related issue. Take a spare spark plug with you when you go for a drive. When it is hot, pull over in a convient spot and see if it stalls. If it does, pull a plug wire off and put it on your spare spark plug.Hold it against the engine to ground it and crank the engine over with the starter to see if you get spark across the gap of the plug. If you get a good spark, then the problem is probably a fuel issue. Take the tops off of your float bowls and see what the fuel level is. (It is a good idea to check the fuel level after starting the engine and then shutting it off while it is still cool enough to restart just to see where it normally is.) If the fuel level is different after stalling when hot then it's a fuel issue. Now you can proceed to trouble shoot the actual cause of stalling. Good luck and let us know what you find.
Ed Bell

Thanks very much for your comments Bob and Ed.

I checked the timing this morning and it is slightly out - was 36 degrees BTDC at 4500rpm. I was short of time today and will adjust it tomorrow. I had the vacuum pipe pulled off as Bob suggested - I didnt plug the end of it so if I should have done that to get an accurate reading could you let me know?

The carbs run fine when hot until they die - you dont get any warning - so adjusting them when hot may not yield much. But it did occur to me that maybe they are generally running lean so I will have the mixture checked by our local garage with the engine hot.

Tomorrow I will check the levels of fuel in the float chambers when the engine is running normally and make a note of that.

Once the timing and mixture are set I will move onto Ed's further suggestions which seem very sensible. I will need to make sure my wife is with me when it next stalls so I can check the spark while she turns the engine over - she will no doubt look forward to that!

Thanks again.

Steve.
SP Mitchell

"I will need to make sure my wife is with me when it next stalls so I can check the spark while she turns the engine over - she will no doubt look forward to that!"

One of the advantages of the pull starter on an MGA is that you don't need a helper to turn the engine over and you don't need a remote starter switch. The starter switch works quite nicely right under the hood(bonnet). Checking for spark is so easy it would be the first thing I would do.

Jeff Schultz

Steve,
I am somewhat of a novice compared to most on this forum but am intrigued with your problem, especially as I had something similar.
I was wondering why it only happens when you pull up at a junction. Maybe because you no longer have air stream through the grill? In my case when I first had my car, it stalled at junctions but only occasionaly failed to restart straight way. In the end I put it down to a combination of a faulty battery earth connection and worn carbs (causing a low idle/stalling at junction, & bad earth preventing restart).
Does it just cut out, or splutter a bit first as I am guessing that if it is vapour lock it would splutter. And if you have a heat shield in place, why should there be vapour lock?
The others on the forum will no doubt comment, but I would guess your timing would need to be worse than that to create the problem?

I have read somewhere this sort of problem is more likely to be a faulty coil failing when hot. And do you have points or an electronic ignition, maybe it is just the condenser failing when it gets hot?

No doubt Ed's spark plug suggestion will give you the right clues. Be really interested to hear how you get on.
Graham M V

fuel pump? Or coil?
Mike Razor

I had Exactly the same problem on my 1947 TC turned out to be the coil.
To test I rigged up a spare coil on the bulkhead and when it next happened I switched the wires over to the spare coil- The car then started straight away !!!
Paul
P D Camp

Thanks for the additional suggestions.

Jeff- thanks, my wife will be relieved!

Graham - I think it happens when I pull up at a junction becuase the engine bay temperature is high and the revs are allowed to fall below some critical level due to me allowing the car to drop to tick-over speed. It doesnt often splutter. As I pull up at the junction and take my foot of the pedal, the revs fall, but just keep falling past tick-over rate until the engine dies. Sometimes it will not quite die until I press the accelerator pedal to move off when the lights go green and then it just drops dead. I know it sounds like vapourisation but strange thing is it never did this for the first year in which I had the car. I have had electronic ignition on the car since I bought it - I friend who used to work for Smiths did suggest that it may be that getting too hot.

It seems to be one of those mysteries that could be one or more of many things. Reading other threads it seems it could be:

1. Fuel pump delivering too little. My pump, located at the rear of the car ticks continuously but always has and I have not noticed a change in speed of the pump.

2. Blockage in the fuel line filter. Mine is OK.

3. Fuel lines under the bonnet not braided. Mine were braided before this problem arose and got changed for plain rubber due to a split in the orginial line but is brading likely to really solve this? I doubt it somehow.

4. Blockage in filters at the carb/fuel line connections (I didnt know there was a filter in the banjo connectors so learnt sonething there).

5. Float chambers incorrectly aligned reducing fuel supply.

6. Floats/needles not working properly.

7. Vapouristion at the jets due to high engine bay temperature. I dont really see why this should be happening now. I use 98 octane fuel and the heat sheild is fitted. The air hose from front grille to the carbs is missing so I could reinstate that.

8. Jets not set properly so giving wrong mixture. Plugs look OK and the Colourtune looked OK (I know its only a basic check but then its only a basic engine). I will have that checked at the local garage to see if the mixture changes much when the engine gets hot.

9. Faulty coil. The guy who sold me the car gave me a brand new spare so I can check that in due course. Yeah, I know... why did he happen to have a brand new spare?? But the car ran for several months before the problem arose.

10. Over-heated electronic ignition. No idea how I check that. The system in the car is by a company called Newtronic and it comprises a scanning disc which replaces the rotor arm.

11. Incorrect timing. Its not far out (and is bang on 20 degrees at tickover which seems to be the recommended basic setting) but I will tweek it to give 32 degress at 4000rpm with vacuum hose removed and plugged as this seems to be the consensus to give the best overall setting.

Once the mixture and timing are verified correct I will, when it next stalls, push it onto the footway (while other drivers look on with the usual pity) and then do Ed's check with the spark plug to see if the ignition system is the problem, or if it can be eliminated leaving the fuel system as the source.



Steve.




SP Mitchell

Steve

My pet hobby horse is the rear filter box being fitted upside down. It brings weird symptoms into play, although I am not sure about cutting-out being one of them. Essentially, fitting the box upside down causes a marked lag in the rear carb piston operation. The engine effectively only then works on the front carb feedng all cylinders. A very common problem.

Have a look at what I am talking about on my website: http://www.mgaroadster.co.uk/odds_and_sods.htm second set of photos down the page.

Steve
Steve Gyles

Steve

Just noticed your comment about blocking the vacuum pipe during dynamic testing. I am of the opinion that blocking it makes no difference as both sides of the vacuum diaphragm will be open to the same ambient pressure. Any change to that pressure caused by fan speed will be equally felt by both sides of the diaphragm, cancelling one another out. I could even venture to suggest that blocking the hole could cause a pressure differential during fan speed alteration although it would be so slight to be negligible.

Steve
Steve Gyles

Hi Steve M

Yep your timing figure at 36 degrees is not a million miles away but try it at 32 by all means.

Naturally the spark plug test is an excellent idea as is to have a new coil ready. Hopefully that will give more info about the problem

Steve G is right about the filters being fitted upside down, this can certainly cause the engine to stumble on opening the throttle from tickover and is a good one to check.

When setting the timing as I described it is not necessary to block the vacuum as we just need the mechanical advance to be at maximum when setting the timing, mixture irregularities are irrelevent.

Steve G what fan/fan speed are you talking about?
Bob Turbo Midget England

Thanks Steve,

I checked the filter back plates and they are on correctly so I can see the holes into the carbs which I checked and are clear. That would have been a nice fix - my life with any of my MG's has rarely been so easy though.

Did once get a fantastic result from a forum on VW Polo though. ABS light was permantently on and although the brakes seemed to work fine, VW garage said diagnostic showed it needed a new ABS pump. Cost of pump was over £600.00 and with labour and VAT I was expecting a cost of £1000.00 plus. Advice from Polo forum was to change a fuse which sat on top of the battery and which cracks due to temperature variations in the engine bay. Fuse cost about £2.50 and fitting took 5 mins. Bingo - ABS light went off. Reckon if I never get another quick fix in my life then I am still fortunate to have had that one!

Steve.
SP Mitchell

Another good way to check the coil is to see if it is too hot to touch. If so it is bad. Although, the car should sputter and run like it is fuel starved when the coil begins to breakdown. I realize that the coil has nothing to do with the fuel but it is a similar symptom. Also, check all your fuse brackets. I have a TD that would be running fine until it got hot and would just stop dead as if someone had pulled the plug. Fought with trying to find it for ever and one day I checked the fuse holder and it seems to have lost its ability to hold the fuse. Squeezed it down some and never had that problem again.
Mike Razor

Let's just remember that 90% of apparent fuel problems in these scenarios end up being electrical.

Bob, the only reason I was referring to fan speed is that it produces dynamic pressure which is how airspeed is measured through pitot tubes. You have one hole on the pitot tube facing directly into the airflow. The pressure felt down this hole is the ambient air pressure + the dynamic pressure of the air blast. On the side of the pitot tube is another hole. This measures only the ambient pressure. Subtract one from the other and you get pure dynamic pressure and that's what pilots read on their airspeed indicator.

The relevance to this thread is that airflow through the engine compartment could in theory have some effect on the diaphragm, but only likely if the disconnected pipe was pointing directly forward - not normally the case in the standard dizzy set-up. In any case, the flow would be quite disrupted around the dynamo etc forward of the diaphragm, so its effect would be minimal.

Steve
Steve Gyles

Steve, no need to explain Pitot tubes to me nor venturis? I have been an Instrumentation Engineer all my life (I am now 58) so well educated on flow measurement. My goodness I even understand air speed and lift and the difference in supersonic wing design. :) (yes I am bragging!!)

Anyway back to the point Steve not sure we could produce any pressure differences near the dissy due to fan draught! however if the hole for the diaphragm was pointing directly at the fan it would have no effect whatsoever as it is a vacuum device not a pressure device so to have any effect we would need a huge air flow at 90 degrees to the diaphragm unit. If that were to happen perhaps we could see a small change. Interesting concept however! :)

By the way I was sat in the pilots seat of a Vulcan today, that was very special!!
Robert (Bob) Midget Turbo

Good call Bob. Brain failure this end, in it being a vacuum diaphragm, i.e. working in the opposite direction.

I sympathise with you having to sit in a Vulcan cockpit! I always thought the Vulcan looked good from the outside but never had any desire to sit inside. Of that period I enjoyed 6 years in the single seat Lightning F2A and F6 - never had to make small talk with co-pilots, navigators, air electrics Officers, nor air engineers!!

Steve
Steve Gyles

Yep 5 seats in a cockpit the size of a toilet! 18 hrs I think the flights would last?

What I don't understand however Steve is why the Victor is not placed in such high esteem. It was a far superior aeroplane wasn't it?
Robert (Bob) Midget Turbo

Steve (M)
I am off to the bookies to put a fiver on it being the coil! Bearing in mind you have it in stock, that would be a good result and easy fix for you
Graham M V

Steve, you can only really eliminate fuel supply as the problem by physically looking inside the float chambers.
The next time the problem happens, immediately switch off the ignition, take the top off the float chambers, first the rear and then the front one.
Usually if there is a fuel supply issue the rear float chamber fills first and the front float chamber gets what is left.
So if you find that the front chamber isnt full then fuel flow is the problem.

You can check the fuel flow to the carbs by removing the fuel pipe from the float chamber, put the end into a fuel can or large measuring jug and see how much volume per minute is pumped through.
(check on Barneys website "mgaguru" to find the correct figure for volume per minute )
If fuel flow is ok through the pipe, then check any filters on the float chambers and also the float valves.

I had a similar problem which proved to be some clear silicon sealant that had found its way into a glass bodied fuel filter and which gradually worked its way over the outlet pipe after a few miles and blocked it. The silicone was invisible when immersed the petrol.
As soon as the pump was switched off, the silicone fell away from the exit pipe and the fault disappeared.
It took me 3 months to find the problem and I just hope you manage to fix it quicker than I did.
Thinking it was an electrical problem, I changed the ign coil twice, points and condenser twice, plugs twice and I even soldered the points connections to see if that was the issue.

You will definitely get there but I think it may be well worth checking the fuel first.

Colyn


Colyn Firth

Bob
is the Vulcan bomber you were sitting in the one they have gotten back to flying condition again and if so, where is it based?
I would love to see it again as I recall wandering around a Vulcan bomb bay as a 14 year old air cadet and being amazed by the size of it.
(sorry its not on subject Steve)
Colyn
Colyn Firth


Thanks Colyn,

You may well be right. But I will stick to "Plan A" for now and check that there is a spark there when it stalls. I'm not an expert unlike some others here and my simple brain will get confused if I switch approach now.

A week's work calls and I will give it a fast run next Saturday.

Have a good week - thank you (and everyone else) for the advice.

Steve.
SP Mitchell

I just finished an 11 yr. restoration and have the same problem! I WAS LEANING TOWARD the slop in the throttle shaft. Aluminum carb expands faster than steel shaft,taking in more air. But I have not tried a new coil. GOOD LUCK,,HAPPY MOTORING JOHN
John Gordon

Hi Colyn

Unfortunately not, It is a static display aeroplane at Newark air museum.
Robert (Bob) Midget Turbo

Colyn

I think the sole remaining flying Vulcan is based at RAF Lyneham.

The CAA will not give the Lightning a flying licence (too dangerous to fly!!). Two are now based at Bruntingthorpe in Leicestershire. They are allowed to do fast taxying to nose wheel lift off. See here: http://www.lightningpilots.com/

Sorry for off topic post.

Steve

PS. See if you can spot me in the 11 Sqn photos
Steve Gyles

Hi Steve is that you in the pink trousers




Gordon
g c pugh

Spotted you Steve = good one of the dog too !
Cam Cunningham

This thread was discussed between 24/07/2010 and 28/07/2010

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