fuel in throat

[Marty Levenson]

:mad:
Dear Don et al,

I returned to our boat two days after a brief sail and motor, and smelled a hint of gas. On inspection, there was a tiny bit of fuel in the carb throat, and some (fortunately not much) had leaked. My other problem, which I'm wondering if its related, is difficulty getting a good, steady idle. The best setting (not great) is with the mixture adjustment screw turned all the way in. My carb is the aluminum body (68), with a fixed jet. So, I'm thinking, is this one problem or two or more?

1 - Is there an air leak...maybe between the upper and lower faces of the carb body. I disassembled it to clean it last year, but didn't know about that potential problem, so didn't check that then.

2 - Could the float valve have a problem? I did clean with laquer thinner last year, before I read that that can destroy the float gasket. Could that be letting gas through after shut-down?

3 - Could the idle jet or associated passage be clogged? I thought I cleaned them well, but they could have accumulated crud since then. (I do have a good filter sytem, and no recent evidence of dirt in the fuel).

4 - I have the Indigo crankcase sytem, and the scavanger tube in place. Do I have that set up correctly? It goes from the bottom of the carb to the fitting in the spacer between the intake maifold and the carb. Shouldn't that keep gas from collecting there while it is running?

5 - The fuel seems to have leaked (though I'm not certain!) through the outside (port) edge of the choke shaft where it exits the carb. I wondereed if gas dripped onto the horizontal choke plate and then found its way down the shaft. Should that shaft be air/fuel tight? Or is that not really a problem if gas isn't dripping down the carb in the first place?

Not sure where to start...any advice/suggestions greatly appreciated.
-Marty

[Don Moyer]

Marty,

Here are my responses to your questions. Questions (1) and (3) relate to your idle problem, and the rest to the fuel appearing in the base of the intake throat:

1 - Is there an air leak...maybe between the upper and lower faces of the carb body?
Yes. If the gasket does not seal well around the idle passage in the center of the housings, the idle system will suck air at that point, and require that you completely block any additional air through your mixture adjustment. This possibility is increased somewhat if the mounting tabs for the four screws holding the two halves of the carburetor together are bent together. You can take a flat file and work it across the tops of these screw tabs and "flatten" them a bit, which will pull the two halves together better (at least for a while).

2 - Could the float valve have a problem?
Yes. I'm not sure about lacquer thinner being the problem, however. It's probably something more common, like dirt around the seat. I'd take a bluntly pointed 3/16" wooden dowel and dress up the seat before doing anything more serious.

3 - Could the idle jet or associated passage be clogged?
Possibly, but I'd place my bets on an air leak.

4 - I have the Indigo crankcase system, and the scavenger tube in place. It goes from the bottom of the carb to the fitting in the spacer between the intake manifold and the carb. Shouldn't that keep gas from collecting there while it is running?
You have the scavenge tube connected correctly, and it probably is working while the engine is running. However, fuel will continue to trickle in until the residual pressure from the pump is dissipated.

5 - The fuel seems to have leaked (though I'm not certain!) through the outside (port) edge of the choke shaft where it exits the carb. I wondered if gas dripped onto the horizontal choke plate and then found its way down the shaft. Should that shaft be air/fuel tight? Or is that not really a problem if gas isn't dripping down the carb in the first place?
All your assumptions on this point are correct. It's common for fuel to drip out through the choke shaft. There is really no safeguard to prevent it, and it isn't a problem if the float valve is working.

Best regards,

Don

[Marty Levenson]

Thanks once again for your complete reply. In the next few weeks I will pull the carb and check the fit of the upper and lower housing, as well as the float valve.

-Marty