Hi Tom. I hope I can help resolve this issue. I'll give you a few things to check.
With any of the carburetors that you have tried, did you verify function of the accelerator pump or pumps, and does the carb have a choke hooked up to it that is functional? Were you able to get the float levels set?
When you are having this issue, verify that the accelerator pumps are squirting fuel when you work the throttle open and closed, and make sure the choke is closing about 90% of the way.
If there are idle mixture screws, make sure they are near the base setting of 1.5 turns out.
Hope that helps get you on the right path.
Mike
A rollover valve could only shut off the fuel supply for safety. Could there be a fuel leak somewhere? Is it a mechanical fuel pump, and could it be leaking fuel into the engine? You need a way to isolate the fuel system components to know where it is going. You need to know is the fuel bowls in the carburetor are empty when this happens, or is only the fuel supply line to the carburetor empty. Once the engine is running well, shut it off and excercise the throttle while looking at the accelerator pump squirters. Does fuel squirt out? Now let it sit overnight and check it again. If the float bowls are empty, the accelerator pump will not squirt. If that is the case, then it is likely either leaking into the engine or out onto the intake. May I ask, how are you priming the fuel system?
Tom, This is sounding to me like a choke misadjustment or an accelerator pump mismatch.
If the fuel bowls were truly empty, a little fuel down the carb bore would result in a start and stall for a few times until the fuel bowls filled up.
Do the carburetors you are using have chokes on them? If not, then your cold starting is going to rely only on the accelerator pumps. This is why you need to verify that the accelerator pumps are functioning when you open the throttle. If there is no choke on the carburetor, the accelerator will need to be pumped 10 times or more to get enough fuel into the intake for starting. Once the engine is initially started, the fuel demand will be less.
No apology necessary my friend. This is a valid question, and sometimes it's difficult to portray what is going on in text.
In order for the fuel to go back to the tank, the fuel pump would have to have an internal leak or the regulator would have to have a leak (if it isn't part of the pump). Does this fuel system have a return line on it with a regulator, or is it a single line system?
i believe the fuel pump is of the single line system but i am not positive at the moment. i am guessing that the pump has a built in regulator.
If you disconnect the fuel feed line at the carburetor, you may be able to use a fuel line or other method of testing the fuel supply to see of it leaks back. You should not be able to blow backwards through the fuel pump.
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20 years experience, all makes and models.