Tom,
The distinction you make between returning to reasonably normal power by using choke, versus being able to restore only partial power as indicating a vacuum leak (versus a partially clogged main jet) is most interesting. When considering likely places for a vacuum leak, the manifold gasket, a broken scavenge tube fitting on the intake manifold, and the carburetor flange gasket would be good places to start.
Since restrictions within the main jet itself are so frequently discovered to be the cause of requiring choke after an engine warms adequately (I've been saying somewhat arbitrarily 99 times out of 100), we haven't spent a lot of effort looking for the exceptions. I now wonder if there might be more exceptions out there than we previously thought.
In addition to a vacuum leak external to the carburetor, I'm wondering if a review of the tech tip on "internal pressure distribution within a carburetor" might reveal a condition that could mimic a vacuum leak external to the carburetor which would respond positively to the use of choke. This would account for the (few) times I can recall when one of these choking issues resolved by replacing the carburetor, even though the old carburetor had a perfectly clean main jet.
Beyond the immediate consideration(s) of this particular thread, your suggestion also highlights the critical importance of paying close attention to "symptom observation and analysis" in the process of troubleshooting, and to not take even our most trusted assumptions too much for granted.
In any case, thank you very much for this suggestion. It deserves to be somehow highlighted, and I'm looking forward to the ultimate outcome of this particular episode.
Don
Last edited by Administrator; 05-02-2010 at 08:55 AM.
|