Hi,
Just got a Stag with a Weber (eldebrock) 500 carb and JWR manifold on a RV8, starts great from cold, and if you stop for a minute or so while warm it fires up straight away. Leave it more than a minute and it cranks for 10 seconds before firing up, no one wants that !
Where should I look first ?
I will be servicing shortly as I don't think anything has been touched for years service wise.
Thanks,
Stu
Warm start up problems.. Weber 500
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I suspect your problem is the warm engine heating up the fuel in the carb when it's stood causing vapour in the carb. My MGB does this, and it is a PITA.
The solution is to put an insulating spacer between the carb and manifold, depending on your bonnet clearance. I've considered it on mine, but I don't think I have enough bonnet clearance for anything but a very thin spacer. On your manifold, you'll need a spacer that keeps the primaries and secondaries separate as they are on the manifold itself, if you get me.
The solution is to put an insulating spacer between the carb and manifold, depending on your bonnet clearance. I've considered it on mine, but I don't think I have enough bonnet clearance for anything but a very thin spacer. On your manifold, you'll need a spacer that keeps the primaries and secondaries separate as they are on the manifold itself, if you get me.
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Ian Anderson
- Forum Contributor
- Posts: 2396
- Joined: Sun Nov 19, 2006 9:46 pm
- Location: Edinburgh
Zimbabwe type fix
Ok 40degrees and ethanol mix fuel
Take some cloth and wrap it around your fuel pipe in engine bay between pump and carb,
Take your windscreen washer pipe and feed it into the cloth.
Cable tie together
When you are trying to start a hot engine first squirt the cloth with the fluid which evaporates and cools the fuel going to the carb
Crank it over and it gets cold fuel and starts straight away
Simples!!
Ian
Ok 40degrees and ethanol mix fuel
Take some cloth and wrap it around your fuel pipe in engine bay between pump and carb,
Take your windscreen washer pipe and feed it into the cloth.
Cable tie together
When you are trying to start a hot engine first squirt the cloth with the fluid which evaporates and cools the fuel going to the carb
Crank it over and it gets cold fuel and starts straight away
Simples!!
Ian
Owner of an "On the Road" GT40 Replica by DAX powered by 3.9Hotwre Efi, worked over by DJ Motors. EFi Working but still does some kangaroo at low revs (Damn the speed limits) In to paint shop 18/03/08.
When you next try to start it from warm, turn the ign on and give it a few seconds for the fuel pump to make sure the carb is full.
Give it two pumps of the pedal and wait another 2-3 seconds. Then see if it starts on the key?
Give it two pumps of the pedal and wait another 2-3 seconds. Then see if it starts on the key?
Perry Stephenson
MGB GT + Rover V8
9.62 @ 137.37mph
Now looking for 8 seconds with a SBC engine
The problem is the fuel boiling in the carb like others have said but its not because this results in the bowls being empty, the bowls will fill up in a second or two assuming that you have a decent electric fuel pump.
What happens is that the fuel turns into a vapour and because the vapour is heavier than air it flows as a vapour into the inlet manifold and into any cylinder that happens to have the inlet valve is open, all the air is displaced by the vapour. Then when you try to start the engine it is breathing pure petrol vapour and this won't burn without air. Take a look at the exhaust smoke when the engine does eventually fire up, it will be black and sooty which indicates a rich mixture.
Pumping the throttle just adds even more fuel into the engine which is not what it needs.
The engine takes quite a lot of cranking to get rid of the vapour because it does not get much air during each revolution due to the throttle being virtually shut. A wide open throttle would help but unfortunately opening the throttle will cause the accelerator pump to dump fuel into the manifold.
My theory as backed up by the fact that if you let the engine get stone cold it will fire up quite quickly even though the bowls needed re-filling. This is because the fuel has turned back into a liquid which occupies about 100 times less volume than when it was in vapour form, this reduction in volume allows air back into the manifold. Some of the liquid fuel will help with a cold start, the rest just gets blown out the exhaust!
A quarter inch space will make a difference if it is made from a decent heat insulating material.
What happens is that the fuel turns into a vapour and because the vapour is heavier than air it flows as a vapour into the inlet manifold and into any cylinder that happens to have the inlet valve is open, all the air is displaced by the vapour. Then when you try to start the engine it is breathing pure petrol vapour and this won't burn without air. Take a look at the exhaust smoke when the engine does eventually fire up, it will be black and sooty which indicates a rich mixture.
Pumping the throttle just adds even more fuel into the engine which is not what it needs.
The engine takes quite a lot of cranking to get rid of the vapour because it does not get much air during each revolution due to the throttle being virtually shut. A wide open throttle would help but unfortunately opening the throttle will cause the accelerator pump to dump fuel into the manifold.
My theory as backed up by the fact that if you let the engine get stone cold it will fire up quite quickly even though the bowls needed re-filling. This is because the fuel has turned back into a liquid which occupies about 100 times less volume than when it was in vapour form, this reduction in volume allows air back into the manifold. Some of the liquid fuel will help with a cold start, the rest just gets blown out the exhaust!
A quarter inch space will make a difference if it is made from a decent heat insulating material.
It wont make any difference which way it goes round to be honest. I've done both on a dual plane manifold.
Perry Stephenson
MGB GT + Rover V8
9.62 @ 137.37mph
Now looking for 8 seconds with a SBC engine
It does matter which way round to a small degree if the manifold is a dual port, the lower ports in the split runners are supposed to be fed by the primary side of the carb, the idea is that the secondary side is heated less by the engine because it is insulated from the engine by lower runners.
The carb could be fitted either way round on a dual plane manifold but a dual port is basically two 360 degree manifolds, one on top of the other.
The manifold in the picture looks like a dual port to me but I can not be sure!
The carb could be fitted either way round on a dual plane manifold but a dual port is basically two 360 degree manifolds, one on top of the other.
The manifold in the picture looks like a dual port to me but I can not be sure!
sidecar wrote:It does matter which way round to a small degree if the manifold is a dual port, the lower ports in the split runners are supposed to be fed by the primary side of the carb, the idea is that the secondary side is heated less by the engine because it is insulated from the engine by lower runners.
I've had them both ways round on dual planes and there was no difference in starting cold or hot. Then again I've never had the warm start issue on the 6 MGB V8 conversions that I've worked on or built ?
Most of the cars I've come across with the primaries to the rear have been done that way because the owners have taken the short route with the throttle cable while knowing no better.
Its down to personal preference I guess. But I always mount carbs with primaries to the front on a fresh build
Perry Stephenson
MGB GT + Rover V8
9.62 @ 137.37mph
Now looking for 8 seconds with a SBC engine
