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Old 09-05-2017, 09:01 PM   #1
Dhawk12
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Default 89 Kat 750 with a Vance & Hines Supersport Exhaust

Hey guys, I have a question for you experts. I have a 89 Kat 750 and I recently acquired a Vance & Hines Supersport 4-1 exhaust system for it on the cheap. Anyways I put it on and it runs great past 4000 rpm, but going through 3000 rpm it stumbles and surges something fierce. It had a bit of a stumble here previously with the stock system but I minimized that by cleaning the carbs, visually syncing them (waiting for my tool), and making sure all the air/fuel screws were adjusted equally at 2.5 turns out. It seems that the new exhaust has amplified this stumble. So here is my question: Would adjusting the air/fuel screws help to fix this problem, as it only seems to happen when the bike is on its pilot circuit (and my understanding is that these screws adjust the ratio for the pilot circuit)? There are no pops on decel, and it seems to idle fairly stable at around 1200 rpm. It does not have a jet kit installed (to my knowledge, however the a/f screws did not have plugs on top when I got the bike), and if I can get away without spending that money by adjusting the a/f screws to fix this problem all the better. The only other thing I can think to mention is that it seemed to run better when cold, although I could have just been so excited by the new exhaust that I didn't notice it. (I have only run it from cold once on the new setup, so I will update when I have more info.) So the second question is, if adjusting the pilot a/f mixture would help, which way should I adjust and by how much? I suppose I could trial and error it, but I was wondering if I could get some advice before I remove the tank and the carbs just to give these screws a 1/4 in the wrong direction. Thanks in advance!
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Old 09-05-2017, 09:11 PM   #2
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we cant say anything viable till a proper sync is done. out of sync cabs can and will do strange things.
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Old 09-06-2017, 06:24 AM   #3
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Quote:
Originally Posted by 92xjunker View Post
we cant say anything viable till a proper sync is done. out of sync cabs can and will do strange things.
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Old 09-06-2017, 07:32 PM   #4
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Synce them first.

Then check out these directions for specific rpm range adjustments to tune out the issue.

Factory Pro High RPM Carb Tune Directions

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