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mroman59

Adjusting pilot air screw - help please

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mroman59
I believe my engine is was running rich due to the heavy black soot on the frame at the end of the exhaust and the heavy black carbon build up on the spark plugs. I want to adjust the air pilot screw(s) to see if I could correct the situation. I found a pdf manual for my Kawasaki engine FH641D online. The manual says turn out 1 5/8 turn. It also says: A moderate amount of air comes in around the throttle valve at an idle, so adjusting the pilot screw changes the fuel/air ratio. Turning the pilot screw (Idle mixture screw) out (Counterclockwise) enrichens the mixture; turning it in (clockwise) leans the mixture. Is this statement correct? I found another description online that says the following, which appears opposing views: The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed. One is saying that turning out or counterclockwise makes it rich and the other says "turning out, opens the passage ... resulting in a lean mixture. My question is, if 1 1/58 turn is where it currently is set or I set it there and there is no change, do I turn the screw out (counterclockwise) or in (clockwise) to try to prevent the build up black carbon? Thanks, MR

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HubbardRA
With the engine running, turn the screw clockwise till it runs rough, then turn the screw counterclockwise till it runs rough, then set the screw halfway between those two positions.

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mroman59
Thanks for the info, however if it is a twin cylinder engine and has two pilot air screws, then what is the procedure. I suppose it is the same, just one at a time. Why would you richen it just a tad? My engine, though running rich, is not running rough, so I would assume that I may be in between both extremes already. Any thoughts?

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HubbardRA
After setting the jet to the central position, I always check by opening the throttle quickly. If there is any hesitation when the throttle is quickly opened, then I richen the mixture just enough to eliminate the hesitation. That way there is no sputtering when the governor opens the throttle suddenly when hit by a load. Since the carb on the Kawasaki is a dual venturi (two barrel) carb there should be two idle mixture adjusting screws. Try to adjust them both by the same amount each time you adjust.

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mroman59
Hey guys, I looked at my carburetor in order to adjust the air pilot screws and there is a black plastic piece over the screw. This plastic housing has cross hatches where you can put a screw driver in and turn, but it also has a tap on it that hits another tap on the carburetor that keeps it from turning. Therefore, I cant adjust them. It looks like if I remove the plastic piece, then the metal screw underneath has no cross hatches so that one can adjust with a screw driver and count the turns for adjustment. If I pry on the plastic pieces a little, they appear as if they will break off and I think they are needed in order to keep the screws in place once the desired placement is found. Any thoughts?

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larry8200
I've seen such plastic caps on a lot of carb adjusting screws that were there to limit or prevent adjustment, and keep emission standards. I'm not familiar with yours, though.

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mroman59
Yes, mine are there to limit adjustment, however I clipped and filed off the tips so they can be adjusted. The adjustment made no difference. With the engine running and turning the air pilot screw in either direction, I could not hear and change in the engine sound. Also, even before I made the adjustments, when I would turn off the engine I would hear backfire. Usually when I hear this I think it is time to rebuild the carburetor. This Kawasaki engine is only 3 years old and I would not expect to need the carburetor rebuilt, so I am puzzled about the whole thing. I know it did not backfire when I first installed the engine. Mike

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metaldr
Mike, The pilot circuit only effects how the engine runs at idle and just off idle. If you are adjusting them with the engine running faster than that it won't have much if any effect. If your engine has an adjustable main circuit that's where I'd look to make your adjustment. If not, I would make sure your air cleaner isn't dirty or the choke partially on. As for the backfire, I have a Toro zero turn with a Briggs engine. Since day one it will backfire if I shut it off at low engine speed. The dealer told me (as does the manual) to keep the rpm's up when shutting it down. I thought it was because the newer engines run leaner now but that doesn't explain why your tractor backfires. The plug described in your original post sounds like a rich condition, not lean. Don't know if any of this will help but that's my 2 cents worth. Bruce

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