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MG MGB Technical - Engine stumbles when hot

My cay runs great, been rolling road tuned, new engine, electronic dizzy, 1977 MGB UK spec with HI4F carbs and a performance(?) manifold.

Idles well until on a hot day (yesterday was a hot day a last) I have to sit in some traffic, the idle starts to drop and becomes very erratic. It will eventually stall if the accelerator isn't pressed a little. The temp gauge is showing half-way. Anybody else had this problem and more importantly solved it ?

When I got home I drove the car into the garage, left it about 10 mins. Went to start it again and it just turned over without any hint of wanting to fire. Plugs were sparking and bone dry. The carbs were too hot to hold. I know it's possibly a vapour lock! But why doesn't the fuel pump just push the vapour (gas) out of the carb like it does with air when just filling the carb ?

Very frustrated about this. Wait so long for a nice day to have the top down and the car doesn't like nice days. Well I'm starting not to like the car then.

Thanks in advance, Charlie.



Charles Goozee

Forgot to mention. As soon as the car cooled down, it started and ran fine.

Charlie.
Charles Goozee

Charles,

I don't think it's a vapour lock, more that the heat is vapourizing the fuel in the carbs/fuel lines. This is basically starving the engine of fuel hence you drop in idle & then the engine dies. When you try & start the car the fuel is then vapourized before if can reach the carb, hence it won't fire. However i may be slighty confused, but i don't think vapour lock & fuel vapourising due to heat is the same thing, can anyone clear that up for me?

Check the condition of your heat shield? & is the cooling fan working properly?

Good luck

Scott

Scott Ralph

Scott,
Thanks for your response.

The heat shield is in place, and the cooling fan is working. I don't think this is actually an engine over-heating problem as such. I'm assuming the above two problems are related (they may not be though).
While the car wouldn't restart, I turned the ignition on and heard one or two clicks of the fuel pump, as if the carbs were already primed with fuel.

I know some of you guys in the States must drive your cars reguarly in higher temps. than experienced here yesterday (around 80F). No idle or restart problems ?

Charlie.
Charles Goozee

Charles,

My theory may be a bit shaky, but here's what I understand about vapor lock:

It tends to be a symptom only found on a 'puller' fuel pump. That is, one which is attached to the engine and has the long line to the tank. Since the pump is attached to the motor, it's in the position of highest possible temperature. At extreme heat, some vapor in the fuel line expands and gathers to an 'air bubble'. Air doesn't pump very efficiently through the diaphragm pump - as the diaphragm actuates, it only stretches the air bubble against the resistance in the line (fuel running up hill out of the tank). The bubble doesn't actually get pulled through, nor does any fuel. As things cool, the air bubble retracts, and the problem recedes.

Not so much an issue with our pumps, which are far from the motor and not subject to underhood temperatures.

Fuel vaporizing would be when the float bowls get hot and some of the fuel in the bowl vaporizes. This vapor would escape via the bowl vents and the fuel replaced by the pump as long as the car's running.

As to your problem, I share it. When the ambient temperature gets to about 90 F, my idle slowly drops to about 600. I had asked the list about a cool-air box, but then decided that the car probably worked as designed before, so customizing is not called for. I have a feeling the bi-metallic springs in my HIF carburetors are old and getting weak.

To be more specific, I don't think they move the jet as much as they used to. So when I warm up the car in the garage to tune the carbs, the springs are still in cool-running position. As the carbs encounter stop and go driving in summer heat, the springs finally move the jets, the mixture leans, and the car idles poorly.

This entire post is conjecture. As always, I'll be happy to be corrected. Learning takes a lifetime and all that.


Best wishes,

Matt K.
Matt Kulka

Charles,

I think that Matt may be on to something here. My experience is with a 79 MGB, detoxed and with 1 3/4 inch SUs fitted. I lived in Louisiana, where summer is 85 to 95 degrees, and never had the problem. However, when I moved to England, with the car, I had to change carb needles to reduce the CO emissions in order to meet the MoT requirement. This meant going from rich to lean. Every hot summer day thereafter, I encountered just what you described. Restoring the needles I had originally selected (there is a good SU book with tables of data, allowing you to select just the mixture/rpm characteristic you want) eliminated it.

For simplicity, I would try just richening the idle mixture first to see if it helps: a little extra fuel evaporation will lower temperatures and may pull you back into smooth running.

Best wishes,

Kevin Kelleher
Kevin Kelleher

I have the same problem with HS4s. What I think is happening (and let me preface by saying that I have no idea what I'm talking about) is this: The needles and jet adjustment given a certain air-fuel mixture under a given set of conditions. We know that charge density drops as underhood temperatures increase -- that's why cold-air boxes help make power. So as the underhood temp rises, the fuel and the air both expand (and become less dense). Air and fuel expand at different rates. Thus, the mixture changes as the air and fuel expand -- if they expanded at the same rate the ratios would remain constant but the absolute amount (mass) of each would become less. If I'm not mistaken, this is the problem HIFs were designed to address... I compensate by setting the idle up a bit.
Rob Edwards

Found this on an old article in the archives. I wouldn't be suprised if this is my real problem.

The substitution of the OE cast manifold for a tubular LCB one has rarely given more than 1 to 2 bhp on many a car I have run up on the rolling road. There is also the issue of the thinner material increasing the emmitted noise levels and far more important if you have temps above 25 degrees C is one of much greater heat radiation which will affect the fuel temperatures in the carbs. Fuel vapourisation is not a new thing to this BBS.

As I really don't like the noise produced by the SS manifold and exhaust, I think I'll convert back to something more original.

Charlie.
Charles Goozee

This thread was discussed between 18/06/2002 and 19/06/2002

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