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MG MGB GT V8 Factory Originals Technical - Fuel Overheating

Can people tell me how they have overcome fuel vapourisation?

I have a 2500 V8 with weber 500, after i have run for about 30 minutes i can feel the car holding back bit, if i floor it it goes good, i am pretty sure it is fuel vapourisation.

The fuel filter is next to the carb, i have ceramic coated the headers, it is a set of huggers so does not go through the wings.

Is there some way of putting a heat shield around the carb?

Thanks in advance

GLG Lavis

If your fuel filter is located in a hot environment, then it's acting as a "fuel warmer". I'd start by moving your filter.

One thing I found was that gasoline with 10 percent ethanol made my problems with "percolation" and subsequent rough idle much worse than when I ran regular gasoline.

In the past, with a worn out 400cfm carb that was giving me problems, I used a plastic ("phenolic") spacer between manifold and carb. It seemed to help... but tinkering with float adjustment helped much more. Switching to a newer, better-condition Edelbrock (Weber) 500 made the problem a memory.

Some people have fabbed-up simple, flat, sheetmetal heat shields that mount between carb and manifold... Easily done, but I'm skeptical that they're necessary if the other solutions are tried.


holes in the inner fenders will help to get the hot air out. I use RV8 style exhaust manifolds and this helps a lot to both, lower under bonnet temperature and power.

On my car, i have a Filterking presure regulater/filter unit installed. The large volume/capacity of the filter also keeps vapourisation off the carburator.

Another change i made was to fit a pair of Mitsumi fuel pumps in the boot and that helped to keep the presure at the right level, even under heavy load.

Before you start to invest into your fuel systen, check your carburator first. If it is a Holly 390, you should do anything to have the floats set to the right hight. Changing the primary jets to the .51 size and replacing the secondary metering plate with a metering block with .53 jets was a further step i made. The spring for the diaphragm of the secondaries should also be checked, as it is calibrated for 1.5 ton vehecle wight at the factory but lighter springs from the Holley trick kit helps as do the different cams for the excellerator pump, that also come wit the kit.

I also thought to have this problems with my carb and read about the holley range in a book i bought at an US car specialist. Then it took me one weekend to try (not a straight forward job with a Holley) and after different tests and details changed, there are no problems left as it was before, just out of the box and into the GTV8.

Due to the very time consuming procedure to make the Holley work in a MG V8, i can agree to all the fellows that go for the Edelbrock/Weber Carb, making setup easier and cheaper, not talking about the waste of time with the Holly compared to the Edelbrock.

If I would do another conversion in the future, I would not try a Holley again.


If you really have vapor locking problems and have tried what Curtis suggested.
Then the real fix is to install a fuel system with a return line.
Install a 9-14 lb fuel pump, install a fuel regulator from Holley. this regulator will have 3 or 4 ports, adjust to 5-7 lb on one of the ports install a return fuel line, run this line to the tank or large filter in the rear (optional) this keeps the fuel flowing and it's a common practice with high hp vehicles.

Second is to install a fuel cooler, install the cooler in a location that will not keep the fuel to cold or to hot, it has to be able to maintain normal fuel temps. Similar to what some Porsches use (track use)
This cooler can be install in the engine compartament.
I use the cooler and the fuel is just right during 115-120, in the winter I by pass the cooler.
Bill Guzman


Not sure you have a fuel vaporisation problem, my friends V8 roadster has the same set up as yours and does not have any problems.

What rods and jets are you using in the primary chokes, did you re jet it from the out of the box spec? if not this may be causing the hesitation you are describing.

Kevin Jackson

Hi Kevin

I was away when the carb was set up, it was done by a specialist and was informed that the jets were changed (weber 500) i am going back over to the rolling road next weekend so i will see if it is just a bit of tuning required and find out what jets were put in.

I cannot find the paperwork at the moment to say if it was done.

will keep you posted.

Thanks all

GLG Lavis


i took my car back for a rolling road tune-up yesterday, after all the tests i was told that the car was good, also that using a weber 500 means you cannot get the set-up perfect due to the way it is set up?

i was told that i can maybe retard the ignition a touch which will lose some power.

It was explained that with the weber 500 there will be a slight problem when you reach the upper limit of the primary but it is fine when it goes to the secondary, this lines up with the slight hesitation the car has at certain throttle setting, when i put my foot down it goes away (onto secondary).

Does this sound correct?

It is a bit frightening when you see your car being tested,the car is really trashed. the bhp was not to bad though, 220 at fly, 172 at the rear wheels.

It was commented that it was one of the best he has tested, so cannot be to bad, only comment was the brakes could be better (took it for a road test).

Hopefully this is it for a while, just need to get rid of some bay temperature.

GLG Lavis


had this sort of hesitation too, althoug my car is fitted with a Holley 390.
After changing the spring for the vac secondaries it became a little better. changing the cam of the excellerating pump and the shooters, it was cured.

I do not know upon the construction details of the Weber 500, but trying to set up comparable mimics there might help.



Did they confirm that they had changed the primary rods and jets to the ones I specified, ie. rods 1441 and jets 1421. From what you said in your last post it doesn't sound like they are very familiar with the Edelbock/Weber carb.

It sounds like your engine is running fine at wide open throttle when it's running on all 4 barrels, but still may be too rich on the primaries and this may be causing partial plug fouling once the engine is up to full running temperature, and when you put your foot down it hesitates and then clears it's throat.

If they confirm that the correct rods and jets have been fitted you could try adjusting the pump calibration, it's an external adjustment, very simple and explained on page 14 of the carb manual.


Kevin Jackson


Forgot to comment on your rolling road operators comment re the design of the carb, which is not correct. The Edelbrock/Weber is a very good carb and much easier to calibrate than the Holly once set up properly, it should provide excellent service.

Re ignition timing it's unlike ly that you would need th retard the ignition unless it already over advanced.

A decent SD1 engine with Stage 1 heads a fast road cam 9.75/1 compression and your carb will need 10 degrees of advance at 1000RPM and idealy 34 degrees by 3500RPM with the vacuum advance temporarily diconnected for testing purposes, it's unlikely with a standard distributor that you will get the 34 degees by 3500RPM unless it's been recurved, most standard distributors do not give full advance till much later in the rev range.

Can you confirm the spec of the engine and what distributor you are using.


Kevin Jackson

Engine hesitation ussaully is blame to the carb and yes it can be. Ign can be at fault and cause the same simptoms as the carb would.
Holley carbs are a bit more sensitive to ign problems.

Ign timing is different for each engine. Headers, cam, manifold, compression etc does change the ign timing and advance of the timing process, springs weights and vacuum canister.

Before doing any carb settings and mods, the ign should be done first and get as close as possible with the ign, then do the carb.

If you have a cam with more duration then the vacuum advance should be come in faster (adjsutable vacuum pot) If you have more compression than stock at least 1 or 2 points then retard the overall advance curve with stiffer springs.
Then adjsut the initial timing to specs as starting point, advance 1-3 degrees and road test. Also if you have more compressions and headeers then run a colder plug one step down should be sufficiant for the street.

To find the initial timing on your particular engine is best done with a vacuum gauge.

Bill Guzman

Thanks everyone,

I will confirm the distributor at the weekend. i have it locked away in a garage at the moment.

GLG Lavis

This thread was discussed between 30/04/2007 and 17/05/2007

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