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MG Midget and Sprite Technical - Petrol Vapourisation

Anyone else suffer with this?

My 1275 midget suffers on really hot days, if it gets stuck in traffic. After about 10 minutes it will start misfiring and eventually will stop. 30 minutes cooling down will fix the problem, until the time.

I have an electic fan fitted, with the standard fan removed. The carb heat shield is in place and everything else is pretty standard.

The engine cooling is fine. On a hot day it will run on the normal mark on the temperature gauge and running at more than 25mph the fan will not need to run.

I have heard of several fixes that involve small fans for the carbs and insulation for various parts of the fuel system, but does anyone know where the vapourisation is actually occurring and how to fix it?

Mike
M Parris

Mike,

Are you sure it's fuel related, I had issues at times with the same symptoms and it was on one occasion a coil that broke down when hot and on another occasion one new set of points wore rapidly and resulted in virtually no points gap so it closed when the engine was hot. The coil issue was fixed with a new coil, the problem one was about a year old so I had discounted it as a possible cause. The points problem was fixed with re-gapping the points, the problem points were only a month or 2 old so I had discounted those as a cause as I had never had a previous set of points wear much between yearly checks, this one new set closed almost completely in a month or 2.
David Billington

This is a very common problem especially when the engine driven fan is not used. Underhood temperature soars due to a lack of air flow. The fuel is boiling in the carburettor float bowls, the vapor causes the mixture to go lean. Some ways to help: put the fan back on, blow air directly to the carbs, use a fuel line cooler, insulate as much of the fuel system as possibe, ventilate the engine compartment.
Mike G

I've had something similar recently. At present I have put it down to the Lumenition Optronic system being susceptible to heat (seems ok when back on normal points). Check temperature of fuel bowls when it stops. Of course the stock answer is always 'condenser' so change that too. They are not immune to temperature effects.

Steve
Steve Church

I've had yet another plug break down this weekend..fires fine at low loads but after 10 mins running, under load (ie low speed, top gear, accelerate) it drops a cylinder. About the third time it's happened..what is it with the quality of plugs these days? Either I'm unlucky, there's something about my engine that eats them or there is a high proportion of poor ones about! Anyway, the reason I mention this is that the symptoms of a plug failing are easily confused with fuel vaporising..they don't just stop working but generate erratic missfires!
Robin Cohen

what brand are they Robin? - most people seem happy with NGK for quality.
David Smith

First lot were 'cheap as you like' four for a couple of quid from an autojumble, so no suprise. Last ones were NGK, I've also had this happen a few times with the Caterham, exactly the same. High speed/load missfires, endless chasing of red herrings and it's a duff plug!
Robin Cohen

that's worrying, sounds like another company whose quality is going gently southwards...
David Smith

I had this problem on my tuned 1500 after i removed the original cooling fan for that extra couple of HP! My first attempt at a cure was to fit a cooling wrap to the manifold.This worked great, a lot less heat and a big reduction in noise. Two weeks later the head gasket went,,just where the two exhaust ports are. Signs of a lot of heat around the area, I suppose the heat has to go somewhere. A new headgasket and replacing the fan helped the problem but i also added some extra ventilation for the engine bay (see photo) and judging by the amount of heat that comes out of them they are very effective. A m I right in thinking that unleaded petrol makes the engine run hotter or dous it boil at a lower temperature?

Cheers Carl


C Bintcliffe

Another way of venting heat quite successfuly is by removing the gear lever gaitors and top metal cover (on the earlier cars). The amount of heat which comes out the top of the transmision tunnel is unbelievable. It also allows more flow around the gearbox. Very useful for keeping the interior warm in winter but not so much fun in the summer.
My other very effective system for removal of underbonnet heat is increadibly bad panel fitting leaving nice big gaps for the heat to escape.
G Lazarus

Or maybe Carl will make the wing louvre apertures we are all keenly awaiting?
Daniel Thirteen-Twelve

Thanks for the responses.

I am using a Lumenition electronic ignition setup, so points and condenser are not an issue. I am pretty sure the ignition setup is not the problem.

Not sure I want to go down the grills and gear lever modification route!

Has anyone else tried insulation? Is the copper fuel pipe that runs near the exhaust a good candidate for some insulation?


Mike
M Parris

Mike,
It's a good candidate but my bets are that it wouldn't cure the problem. A lot of the problem is the manifold heating up the float bowl. Even with a heat shield the residual temperature can really create some problems.

I had this BADLY on my original twin carbs and I'd be stuttering my way out of London traffic everytime without fail. With my HIF44 I get no such problems.




*awaits Carl's post surprising us with an order form for the louvres*
Rich Amos (1330cc Blaze Red '72)

I would avoid removing any kind of barrier between engine bay and cockpit, you might find yourself gently falling asleep at the wheel next time your out and about.
mike

I have been having a similar problem with my car also. I took off the ZS and put SU's on this spring. The car runs OK but when it gets hot fuel will spray out the bowl vent hole when I accelerate. I can see this after I go for a short drive to warm the car up and then open the hood and run the throttle by hand. I've tried insulating the bowls with some exhaust wrap but this didn't help either. At this point, I'm almost ready to go back to the ZS but I really don't want to.
Tim Michnay

Mike have you tried putting the original fan back on yet? This seems to be one of the main causes of this problem.
Tim Do you have the originan fan on your car? The fuel is litterally boiling in the float bowl which messes up the mixture and allows the float to drop hence overflowing fuel from the carb.
It may be worth running a duct from the front of the car directing cold air at the float bowl although this will only work while you are moving it may help.

I am feeling a bit of pressure to get some of the vents made! I had better get on with it!!

Carl
C Bintcliffe

Hi Carl,

No I have not put the original fan back as at tick over the electric fan pushes a lot more air than the engine driven one does.

I had the problem before fitting the electric fan.



Mike
Mike Parris

Carl,
Putting the original fan back on does help. The reason I initially took it off and replaced it with an electric on is that it is so noisy(US spec 1500). Also my clutch is on the way out and it was cheaper to put an electric fan in than to replace the clutch.

What I stil don't quite see is why the fuel would only overflow when revving the engine or under acceleration. For now, though, I'll just live with the noise from the fan.
Tim Michnay

>>> What I still don't quite see is why the fuel would only overflow when revving the engine or under acceleration. <<<

Yeah, that's curious. Normally, overflow seems to happen when your float valve (read, "Grose Jet") fails to seal well enough to keep the fuel level down during times of low demand, i.e., idle. Been there.

-:G:-
Gryf Ketcherside

This thread was discussed between 08/06/2008 and 12/06/2008

MG Midget and Sprite Technical index

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