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a/f screw oddities after carb sync 16 Sep 2020 06:39 #835153

  • Bama704
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Had a strange thing happen to me yesterday while I was working on the 1977 KZ900. with 28mm late model carbs. I could not get it to idle well, so it was time for some work.

The carbs are clean with new jets. I synced the carbs and got the needle super close to each other, great. Then I took out my Gunson colotrune to work on my a/f screws. I went and did carb #4, 3, 2, then 1. All was going ok until I got to the #1 cylinder. I fully seated the a/f screw and used the Gunson colortune to dial it in like the others. I am literally 1/8 - 1/4 turn out from fully seated when running the best and blue color. My other ones are around 1 - 1 1/2 turns out. Anybody experienced this?

The bike runs great with this set up and it idles better than it ever has since I have owned it. Pulls hard through all gears with no bogs finally, pretty happy I finally have my jetting dialed in. (I know a/f screw only controls idle).

Should I not worry about this and just enjoy the ride, or let my mind wander and start to diagnose?
1977 KZ900
Carbs are late model 1975 28mm

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a/f screw oddities after carb sync 16 Sep 2020 07:28 #835154

  • zed1015
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Just run it.
If the screws have been over tightened in the past then that will distort or enlarge the seat making it flow more than the others at the same amount of turns hence having less turns to achieve the same idle mix.
The important thing is getting the correct mix and not matching turns on each screw..
The following user(s) said Thank You: Mikaw, linuxrob

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a/f screw oddities after carb sync 16 Sep 2020 07:43 #835155

  • krazee1
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The idle mixture screws on the 900s are air screws, opening them further leans the idle mixture. 11/2 to 13/4 turns out from fully seated would be considered to be a normal setting, since your #1 carbs A/F ratio is correct with less than a normal amount of air I think you possibly have a restriction in the fuel circuit, the passages in the carb itself and the pilot jets are really small. I guess another possibility is the air screw O ring is torn letting more air flow which would also affect the screw setting. I don't know if I'd worry about it too much if the ratio is right and the bike idles good.
Sorry if I'm explaining what you already know!
Mike
Former M.E. at Kawasaki Motors Manufacturing, Lincoln, NE
1966 W1 (the Z1 of 1966-50H.P. and 100mph!)
1974 Z1A (disassembled)
1976 KZ900B LTD (SOLD!) it's in GOOD Hands!
1978 KZ1000 LTD
1976 KZ900B LTD pile O parts!

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a/f screw oddities after carb sync 16 Sep 2020 07:45 #835156

  • Bama704
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Thanks, that is the assurance I needed to just run it like I stole it.

Zed, thanks again for your air corrector jets! Made a huge improvement.
1977 KZ900
Carbs are late model 1975 28mm

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a/f screw oddities after carb sync 16 Sep 2020 08:21 #835159

  • zed1015
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Bama704 wrote:
Zed, thanks again for your air corrector jets! Made a huge improvement.


Thanks.
It would be great if you could post a similar comment on the air corrector thread just to help keep it up at the top and to show people that they work.

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Last edit: by zed1015.

a/f screw oddities after carb sync 17 Sep 2020 19:32 #835248

  • Zeerx
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I have a 78 LTD with 77 carbs. When I was in the final stages of tuning I was having problems with getting the carbs fine tuned. Come to find out one of the fuel screw tips had broken off and stuck in its hole in the carb. Those tips are VERY thin and can break off if set too tight. Might be something to check.
‘99 ZRX1100, ‘05 DRZ400S, ‘78 KZ1000LTD, ‘12 F150 (Ford)

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