Keihin Carb problems
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3rd Gear
Joined: Dec 2016
Posts: 7
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From: Bay Area, CA
Keihin Carb problems
Hello everyone, i was hoping someone could possibly give any insight to the problems I'm experiencing with my carburetor. I have a 1987 sportster 1100 with the stock 34mm keihin butterfly carburetor. The only real modification to the bike is a thunderheader exhaust. When I'm riding, if I punch the throttle under 3000 rpm the bike will sort of bog down like its choking for a few moments and then pick up. The carb responds just fine over 3000 rpm. I've been playing around with different pilot jet sizes (50, 52, 55, 56) and even tried a bigger main jet (155) and re-adjusted the fuel/air each time but almost all the different jet sizes just made the problem worse. I also tried checking for vacuum leaks around the carb and intake with the bike running by spraying carb cleaner around the sealing areas but the idle wasnt affected at all, so no leaks as far as i know. The accelerator pump diaphram and spring has also been replaced. At this point im stuck. Could it be a timing issue? Am i missing something? Any insight would be appreciated.
Club Member
Joined: Jun 2007
Posts: 45,429
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From: Bedford UK
I suggest a clue is that your jet changes have made the problem worst. I suspect the change of exhaust has made the mixture richer and that you should consider slightly smaller ones than stock. Change your jetting back to stock and start from there.
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3rd Gear
Joined: Dec 2016
Posts: 7
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From: Bay Area, CA
Went out and bought a 48 pilot jet (52 is stock supposedly). Gonna try the stock jet setup first and then the smaller ones and see if it helps. Thanks!
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3rd Gear
Joined: Dec 2016
Posts: 7
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From: Bay Area, CA
Yes I had the air cleaner off to check and I saw the fuel from the pump spraying into the carb. I'm going to check again though just to make sure.
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3rd Gear
Joined: Dec 2016
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From: Bay Area, CA
UPDATE: checked the accelerator pump fuel spray and noticed it was spraying too far right so I re-adjusted the spray as close to the middle of the intake as possible. I also re-adjusted the throttle and idle cables to eliminate that as a possibility of messing with my carb adjustments. As of now it has a size 50 pilot jet and 150 main jet (stock sizes for that year are 52 pilot and 150 main). I warmed it up and rode it down the street a few times, I am still encountering the same problem, its worse now. In second gear if I hit the throttle too quickly under 3000 rpm it'll sputter down and sort of choke for a few moments, and then catch back up, but it acts just fine over 3k. As far as the A/F ratio, I've been running it with the screw out anywhere from 1 to 1-1/2 turns out. I have a Clymer book for the bike but there's nothing in there about adjusting the A/F so I've been just going by what I've seen on forums for older keihin carbs as a starting point. Maybe the A/F ratio is where I'm going wrong I'm just going to start back from square 1 and do more research, having an oddball year bike is definitely not helping.
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Joined: May 2010
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I'm no expert, but I'd go with a 45 pilot and a 170 main.
Last edited by frankjc; Mar 1, 2018 at 02:39 PM.
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Road Warrior
Joined: Jul 2016
Posts: 1,370
Likes: 439
From: va
I highly suggest if you have not already to soak the bowl in a good strong carb cleaner as there is a tinny hole in the bowl that feeds the acc pump gas , you can't see the hole until it is opened up , it is the biggest reason that Keihin got a bad name and many good carbs were trashed
What I mean as a good carb cleaner is take the bowl to a auto machine shop ( or so ) have them soak for a while , carb cleaner in a can is not strong enough
What I mean as a good carb cleaner is take the bowl to a auto machine shop ( or so ) have them soak for a while , carb cleaner in a can is not strong enough
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Cruiser
Joined: Aug 2017
Posts: 118
Likes: 9
From: Los Angeles
As have been stated,
-take apart the carb
-check seals
-find out the exact stock jetting and replace with that
-clean out carb as best as possible
I would suggest reading up on tuning the carb. Best advice I can give is bring it back to what is stock and only change one aspect a time. The less variables you worry about the faster it is to isolate and figure out what's causing problems.
-take apart the carb
-check seals
-find out the exact stock jetting and replace with that
-clean out carb as best as possible
I would suggest reading up on tuning the carb. Best advice I can give is bring it back to what is stock and only change one aspect a time. The less variables you worry about the faster it is to isolate and figure out what's causing problems.