Idle mixture uncontrollable?

[cgerstle]

I have come to the conclusion that my (relatively new - has 4 seasons on it) carb has a problem. The engine appears to run too rich:

1) Plugs are instantly sooty
2) Running under load at idle fouls the plugs in a matter of a few minutes to the point of stalling or having no power
3) Using a ColorTune spark plug (a wonderful thing), I can see that over all RPM ranges both in and out of gear that the mixture is very rich

In order to safely operate the boat, I have taken to making sure that whenever it is in gear, that I keep the RPMs at 1300 or above to make sure that I am running on the main jet which seems to keep the plugs operating OK. This does make for some interesting maneuvering though!

I have noticed that adjusting the idle mixture screw (from all the way in up to 6 full turns out) has NO effect on the ColorTune-visible flamefront color and no effect on engine RPM. While I am interested in the adjustable main jet as a good-to-have thing, I don't believe that it alone will solve the general mixture problem, especially at idle.

Any suggestions on tracking down the cause of the idle mixture screw having no effect?

Suggestions on tracking down a generally too-rich carb mixture?

And, before pulling the carb apart for general inspection and cleaning, should I need a full carb service kit on-hand?

Thanks in advance.

[Don Moyer]

Carl,

While the idle system is far more problematic than the high speed system, it is almost always the case that the idle system is too lean. Your symptoms are actually more consistent with a dirty needle and seat in the float valve. This condition can lead to such a rich condition that an engine won't idle at all.

I recommend removing the flame arrestor housing so that you can inspect the inside of the intake throat while the engine is running. If the float valve is not seating properly, you should be able to see a small trickle of fuel draining down from the vertical throat of the carburetor.

I wouldn't send for any parts at this time, other than perhaps a new bowl
gasket.

Don

[cgerstle]

Perhaps not the float valve?

Don - I took your suggestion and inspected the carb throat with the flame arrestor removed while the engine was running. At high and low speeds, in gear and in neutral, the carb throat remained dry. If dripping gas in the carb throat is the best indication of a poorly seating needle valve, perhaps there is another cause for the out-of-control (too rich) mixture?

[Don Moyer]

Carl,

You have a very interesting problem indeed!

What we know for sure (from experience) is that turning the idle mixture out even 2 or 3 turns off the seat will lean out the engine enough to cause it to quit running. My best guess therefore is that you'll find some kind of gross crud in the upper housing within the air channel that is normally regulated by the needle of the idle mixture adjustment. A can of aerosol carburetor cleaner with the long thin red plastic tube should work well to blast that area clean.

I would not suggest buying a service kit at this time.

Don

[cgerstle]

Cleaned passages, still no idle mixture control

While I'm sure that the folks at Gumout love the amount of their product that I used (and I did get some amount of crud out of the various passages), the behavior of the carb remains unchanged. With the carb broken in half, I sprayed lots of Gumout through each passage in both directions until nothing appeared on the paper toweling on which I was working.

What I am wondering about at this point is if it is worth attempting to mechanically ream out the passages (with the jets still in place). Perhaps some fine single-strand wire? Any suggestions as to what would be appropriate to attempt this?

Carl

[Don Moyer]

Carl,

I've always able to blast jets clean using a regular aerosol carburetor cleaner (Gumout being one of the best), with occasional help from one of the bristles on my wire brush.

Don