Idle mixture screws? - Kawasaki Motorcycle Forums
 
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post #1 of 9 (permalink) Old 03-27-2019, 10:51 AM Thread Starter
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Idle mixture screws?

I have a 84 zn700. What should the mixture screws be set to? I have a Kawasaki manual but all I can find is that I'm supposed to count the numbers of turns in so I set it back to that setting. The caps have been removed by someone before so I'm sure they have been adjusted in the past. I'm just looking for the correct factory setting. Thanks!
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post #2 of 9 (permalink) Old 03-27-2019, 11:35 AM
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When I bought my 85 ZN700 back in 2009 with only 1k miles on it, the idle mixture screws were set at 1 1/2 turns out. The caps had been removed so I don't know if this was the factory setting but since it runs fine and gets good gas mileage at that setting that is where I leave them. My bike gets 40mpg running around and 50mpg on the highway.

The way I used to set idle mix screws on auto carbs was to turn them in to lean the mix until the idle started slowing and then back them out just a little. It seems to be harder to get them just right using that technique on a bike especially on my V4s.

1985 Kawasaki ZN700 Shaft, 4cyl
1989 Yamaha Venture Royale
2007 Yamaha Royal Star Venture

Last edited by RoadRunner322; 03-27-2019 at 11:38 AM.
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post #3 of 9 (permalink) Old 03-27-2019, 12:17 PM Thread Starter
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Thanks! That what I have tem set to since that is the starting point for most bikes. Yes that method is real simple to use on the twins I own but noticing a difference on a 4cyl is a little difficult.
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post #4 of 9 (permalink) Old 03-29-2019, 10:03 AM
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Those screws control mixture between 0 and 1/8 throttle. Viewing the carb from the side if the screw is behind the slide it is an air screw and must be turned in to richen mixture, if in front of slide it is a fuel screw and must be turned out to richen. Same circuit either way. For an air screw start at about one and a half out, warm up engine and quickly blip throttle. if there is any hesitation progressively turn in the screws 1/8 at a time until you can blip throttle with little or no hesitation. Fuel screw is similar but you may want to start at about one turn out and continue to turn out in 1/8 increments until hesitation is gone. If hesitation persists most likely cause is plugged pilot jet. Carb adjustments are--- 0 to 1/8 throttle= air/fuel screw---- 1/8 to 1/4= pilot jet----1/4 to 3/4= needle jet, jet needle combination---- 3/4 to full throttle= main jet. Of course there is overlap and an example of that is that the air/fuel screw will do nothing if the pilot jet is plugged. Also on enricher equipped carbs, and most are, be sure the plungers are free and cable or linkage is adjusted properly before any tuning. To much free play and the plunger will not open starting circuit. to little free play or a stuck plunger and the starting circuit will remain open and affect mixture.
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post #5 of 9 (permalink) Old 03-30-2019, 11:13 AM Thread Starter
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You sure about that whole front/behind in and out thing when adjusting mixture screws?
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post #6 of 9 (permalink) Old 03-30-2019, 11:47 AM
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Yes, the air screw commonly found on two strokes is controlling air only as it enters the low speed circuit. The fuel screw controls fuel and if you think about it, it makes perfect sense why the must be turned in opposite directions to richen the mixture. Try to find a cutaway diagram so you can see what is happening and it will make much more sense. But no amount of adjustment will help if even one pilot jet is even partially restricted.
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post #7 of 9 (permalink) Old 03-30-2019, 12:10 PM
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I searched and found this, it should clear up any confusion. Google and watch this video ---- How does a CV carburetor work - Quick and simple explanation. Look closely at the fuel screw and you can clearly see how turning it out would increase the amount of fuel delivered.
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post #8 of 9 (permalink) Old 03-30-2019, 01:05 PM Thread Starter
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So yes if the screw is behind the slide (or between the carb and engine) screwing in the mixture screw cuts off the fuel supply leaning the bike screwing out allows more fuel through makes it richer. If its in front of the carb (or between the carb and air cleaner) its opposite in is rich out is lean.
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post #9 of 9 (permalink) Old 03-30-2019, 01:44 PM
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Generally front means the front of the motorcycle, but other than direction you are correct. Upstream, air box side= air screw, downstream, cylinder head side = fuel screw.
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