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MG MGB GT V8 Factory Originals Technical - Poor starting when hot.
My V8 is extremely reluctant to start when hot. I know that the under bonnet temperature is often regarded as the culprit. I don't understand the mechanism, though. When I turn on the ignition, the fuel pump (Facet) starts and (I'd have thought) rapidly replenishes the fuel boiled out of the float chambers (Edelbrock 500). I covered the fuel pipe with aluminium foil where it runs near (about a foot away) the header on the car's left hand side - didn't make any difference.
Can anyone explain what is happening?
|Nick, I can't explain your phenomenon theoretically, but I do know this: before you turn on the ignition, give the gas pedal two smart pumps and you will be off to the races (I have essentially the same setup as you do; this has proven to work over many years!)|
|I am not familiar with these carbs, but if they use an accelerator pump as Darrens comment indicates then adding more fuel to a hot engine may be a little more complicated.|
The carb float chamber is full when the engine is stopped. Whilst it has been running there is a large volume of air passed through the carb and this keeps it's temperature much lower than the engine under it. Once stopped a condition known as heat soak occurs with engine heat now rising through the carb. This heats the fuel and expands it.
As the one way valve on the fuel inlet into the float chamber will not allow reverse flow the fule has to go somehwere. It is usually foreced through the main jet and into the inlet manifold where it will in time evaporate, but leave a very strong vapour residue. If you try and restart the engine during this period of fuel being forced into the manifold you are trying to start a 'flooded engine' and so adding fuel by pumping the accelerator will only add to the problem.
What you need here is a gentle opening of the throttle to between half and fully open and then hold it there whilst the engine is cranked. During this period the open throttle will allow large volumes of air to flow and dilute the over rich mixture to a combustable level. The engine usually starts with a slightly uneven beat then quickly 'clears it's throat' and revs normally.
The other side of the coin is where the expanded fule has had time to fully evaporate and dissipate out of the induction system, this will take some time and not be a hot engine condition as the engine will have cooled down. As you rightly point out the pump will replenish the carb and starting will be normal.
Shielding the fuel feed pipe is a good start, but of limited effect. The only practical way of reducing the problem would be to try and insulate the carb from the engine and the best way of doing this is a sandwich plate of insulating material along with longer mounting studs to suit. This is a common factor with Webers and anyone with experience with the Weber equipped MG Maestro's of 1983/4 will know exactly what I mean.
The other guys are probably right however an other angle to think of if their ideas don't work is that electrical commponents suffer from excessive heat. With the increase is heat the coil, leads, spark plugs and starter motor have trouble operating. The increase in heat causes an increase in resistance and a drop in the cranking power of the starter motor and smaller spark at the plug.
|The few times I've had this problem was due to no fuel flow.I have a clear fuel filter just before the carb., that's how I know there was none. I "fixed" the problem by opening the fuel tank cap.The cap is supposed to be vented, it says so in three different languages.I got a bit paranoiac after the second time it happened & started opening the cap before each hot start.It seems to be normal now though. The car is a '71 CB, holley carb. & pump.I've also found in the past on other cars, that if the ign. is too far advanced, then this can also affect hot starting. HTHs Barrie E|
|I have the same problem as you describe Nick.I'm sure I have a heat soak problem,been chasing it for a year.Have tried different insulators,float levels,etc,and am still chasing it.One thing you might try if you have your fuel pump on a manual switch is to run the car with the pump off before you shut it down.This will lower the level in the bowls and possibly prevent the gas from flooding into the manifold.Not a fix but might indicate if flooding is indeed your problem.|
|Try wrapping the fuel line in a rubber heater hose.Slit the hose and begin covering down by the sub frame rails. Secure this with pull tight plastic fasteners and then wrap in aluminum foil. This worked in St. Louis Missouri (temps sometimes above 100 f) on a fuel injection V-8 conversion. Best of luck.|
Great explanation. I'm sure you're right, as I had already discovered that it would start more easily if I hold the throttle open. I don't have room for an insulating spacer as the air filter assembly already (just) touches the bonnet. I'm not that worried about it, now that I understand what's going on.
This thread was discussed between 27/02/2001 and 08/03/2001
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