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View Full Version : carb tuning, rich/lean


canadianskier
06-19-2008, 01:37 AM
does anyone know what is the proper setting for the air/fuel mixture on a `92 prostar, 351, 4160 carb. engines runs great, 160temp, idles at 750rpm, no hot start problems, but when idleing exhaust smells rich. plugs look good. Is there a way to test the CO% and HC levels, as in autos? or is there a spec some MC tech on here knows about for an idle drop test with adjusting mixture while reading rpm?

drbesvold1
06-19-2008, 08:57 AM
Holley Performance Products 600 CFM Four Barrel Carburetor 0-80319-1 (http://www.holley.com/0-80319-1.asp)

Hunterb
06-19-2008, 12:53 PM
I've wondered the same thing. I don't think you can use a standard analyser in a wet exhaust.

You can set your mixture screws to the suggested setting by simply screwing them all the way in (gently) and then backing them out 1 1/2 turns. You can then adjust them slightly to lean or enrichen the mixture by turning them out to make it richer and in to make it leaner. What I try to do is find the spot that creates the best, smoothest idle. You may have to switch back and forth from adjusting your mixture screws to adjusting your idle speed screw as each one affects the other. If you're smelling fuel at idle you may be a bit too rich. You could try turning the mixture screws in a 1/4 turn and see what happens. If you don't like the results you can easily turn them back out. The mixture screws are only effective at idle. Once off idle they do nothing.

Good luck. I have found the 4160 carb to be pretty reliable once set up properly.

Make sure to post back if you find a way of using an analyser.

Bruce

TMCNo1
06-19-2008, 02:05 PM
A small clear bowl or glass 1/2 full of water on top of the flame arrestor cover will show you how smoothly the engine is idling by the ripples in the water. Adjust the screws till the water is not moving or moving the least.

JimN
06-19-2008, 02:20 PM
If someone has access to emissions testing equipment, they should be able to dial it in a lot more accurately than using a bowl of water. Whether it has a carb or injection, the air:fuel ratio should be around 14.7:1 and if someone was doing this adjustment without any test equipment, go rich and lean it out as you notice how it's running. Running it lean will kill it but running it rich will just cause it to be a bit funky at times, waste some gas and blacken the plugs.

Hunterb
06-19-2008, 02:38 PM
Jim,

Can you use a standard emissions tester on a boat exhaust? I thought all the water shooting out would make it pretty tough. It would certainly be the best way to do it if it's possible.

Bruce

JimN
06-19-2008, 02:49 PM
I don't know how it would be done in the field. In a dyno lab, I would think they would take the readings from the gases before they meet the cooling water. That, or connect it to closed cooling and a regular exhaust.

Gonzo
06-19-2008, 03:53 PM
I've wondered the same thing. I don't think you can use a standard analyser in a wet exhaust.

You can set your mixture screws to the suggested setting by simply screwing them all the way in (gently) and then backing them out 1 1/2 turns. You can then adjust them slightly to lean or enrichen the mixture by turning them out to make it richer and in to make it leaner. What I try to do is find the spot that creates the best, smoothest idle. You may have to switch back and forth from adjusting your mixture screws to adjusting your idle speed screw as each one affects the other. If you're smelling fuel at idle you may be a bit too rich. You could try turning the mixture screws in a 1/4 turn and see what happens. If you don't like the results you can easily turn them back out. The mixture screws are only effective at idle. Once off idle they do nothing.

Good luck. I have found the 4160 carb to be pretty reliable once set up properly.

Make sure to post back if you find a way of using an analyser.

Bruce

A small clear bowl or glass 1/2 full of water on top of the flame arrestor cover will show you how smoothly the engine is idling by the ripples in the water. Adjust the screws till the water is not moving or moving the least.

These both sound like great ideas, I was idleing rough this past weekend but got better as it progressed and was the first weekend out.... I was woried I effected some things when I did the fuel line adjustment.... if I need to I will try these remedies.

TCrate
06-22-2008, 02:24 PM
Turn in idle mixture screw until the boat starts to stumble, then back out 1/4 turn. Do this on each side.

Or put a vacuum gage on the carb. Do the same thing by turning in the idle screws get to where the engine starts to stumble. then start backing out each screw about 1/8 or 1/4 turn until you hit maximum manifold vacuum.

Make sure you do this while the engine is hot. Make sure the fuel is not dripping down through the venturis and thus your float level being too high. You can see this at idle by pulling the flame arrestor and looking down the carb while it is idling. You should not see any fuel dripping at idle. Good Luck.