Colthebrummie
Well-Known Member
Sounds like fuel evaporation. Some carbs needed cooling to prevent this. I'm not familiar with your particular engine, does it have a heat shield fitted to the carb? Does it need one?
Col
Col
Marshall8hp
Well-Known Member
Basics first. Does you engine have an ignition amplifier on the distributor? (A 79 model shouldn’t have, but you never know what has been done in the past). If so, then hot start problems are a sign that the amplifier is failing and needs to be replaced.
Put that to one side, if the engine starts cold OK (i.e. on choke) and not hot, then it is a sign that the mixture is too weak. Have you looked at the diaphragms in the carbs? (Take out the 4 x Phillips head screws securing the top cover and have a look). If they are damaged, that will prevent the needle lifting of the seat and giving a rich enough mixture to start. For clarity, the choke system on these puts fuel into the carbs without the use of the main jet.
I assume your carbs are top adjust, so there shouldn’t be any issues with the jet being off Centre. If they have been replaced with bottom adjust carbs, then if one or both of the jets are off Centre, they can jam the air valve and needle at low air flows and cause fuel starvation.
Next time it won’t start hot, pull out the choke and see if it starts. If it does, that will give you a hint that out is a fuel starvation issue. If it doesn’t then it is likely either an ignition issue or a carb flooding issue.
These carbs do not suffer from vaporization, provided the fuel return line is open and fuel is flowing around the loop. If not, then you will have issues.
Put that to one side, if the engine starts cold OK (i.e. on choke) and not hot, then it is a sign that the mixture is too weak. Have you looked at the diaphragms in the carbs? (Take out the 4 x Phillips head screws securing the top cover and have a look). If they are damaged, that will prevent the needle lifting of the seat and giving a rich enough mixture to start. For clarity, the choke system on these puts fuel into the carbs without the use of the main jet.
I assume your carbs are top adjust, so there shouldn’t be any issues with the jet being off Centre. If they have been replaced with bottom adjust carbs, then if one or both of the jets are off Centre, they can jam the air valve and needle at low air flows and cause fuel starvation.
Next time it won’t start hot, pull out the choke and see if it starts. If it does, that will give you a hint that out is a fuel starvation issue. If it doesn’t then it is likely either an ignition issue or a carb flooding issue.
These carbs do not suffer from vaporization, provided the fuel return line is open and fuel is flowing around the loop. If not, then you will have issues.
.Hicap phill
Well-Known Member
Marshall8hp
Well-Known Member
If it has electronic ignition, factory fitted, then almost certainly the issue is the ignition amplifier module, fixed to either the distributor on some models and mounted near the coil on others. When they go bad they do exactly as you describe.
