Board index Flathead Power-Technical Questions, Answers, and Suggestions Sporties Tuning a Keihin Carb question

Tuning a Keihin Carb question

Post Sun Jun 06, 2010 7:45 pm

Posts: 1676
Location: Interlaken, NY USA

O.k, I'm into something new to me here and need what I hope is an easy question answered.
Is the Idle mixture adjustment on a butterfly Keihin carb,(not CV) as found on say a 79 sporty, get richer as you back the screw out? That is turning it counterclockwise? And is turning it clockwise make the idle mixture leaner?
Got called to look at a bike a friend was working on, and it would barely run at idle. It had the rubber "bands" to seal the manifold, not "O" rings. They were loose, and the custom clamps weren't doing a good job of sealing. No, I didn't have any way to do a manifold leak test. Anyway, tried to install "O" rings, but the custom clamps wouldn't clamp down tightly on them, they were meant to fit a larger diameter, such as you have with the rubber bands. So, I installed a new set of rubberbands, and did the best I could lining up the clamps to seal.
With new rubberbands it runs but the idle mixture screw needs to be out about 3 turns to get a halfway decent idle.
So, I need to know if we're making it richer to get it to run, leading me to believe there's still a manifold leak.
Also, I'm thinking this bike came with "O" rings originally, anyone know for sure?
Thanks,
DD

Post Sun Jun 06, 2010 10:06 pm

Posts: 251
Location: Hudson, Florida
Doc, we had a similar problem on my sons Sporty, I had to open the low speed mixture screw 4 1/2 turns to get it to run decent. Son removed carb an installed a Super B I had on the shelf an never looked back. I looked at the carb again when I needed one for an 80 Sporty I was building, figuring with the mixture that lean I would install a larger jet. On closer inspection I found the intermediate jet blocked with what appeared to be a rubber plug, near as we could figure the fuel lines had melted from the alcohol in our gas an what remained had formed a plug that would not come out with out the use of a drill. Once drilled out the carb tuned perfect at 1 1/2 turns out.
In answer to your question my Clymer manual states Turning the idle mixture screw to the right (clockwise) leans the mixture. Turning it to the left ( counterclockwise) enriches the mixture.
They also state to turn the limiter cap out (counterclockwise) to the leanest setting that gives a smooth idle. I've found this conflicting line to be false in practise and go with the first note I presented:)
The 77, my sons bike had an o-ring intake seal. I had to change the intake to the flat bad style on the 80, not sure which 79 uses, I've heard stories of 79 being both ways:). Seems to be that way sometimes good luck Neil74

Post Mon Jun 07, 2010 12:58 am

Posts: 2688
Location: Los Angeles, CA
A friend owns a 1991 Sturgis. During the last few years it has been ridden little. When coming home from work and opening the garage door they found the garage floor soaked in gasoline. The OEM fuel line on the bike had disintegrated and dumped the contents of the tank on the floor. The petcock was also destroyed. All this thanks to modern fuel.

Post Mon Jun 07, 2010 6:11 am

Posts: 1676
Location: Interlaken, NY USA

Thanks guys,
I think we're on the right track. This bike has sat for quite awhile, the gas had a bad stale smell to it. I'm betting the carb needs a thorough cleaning, and maybe a reaming at the least.
DD

Post Mon Jun 07, 2010 6:30 pm

Posts: 251
Location: Hudson, Florida
Couldn't hurt, anymore I pull the carbs off an go thru the fuel lines and carb if it's sat idle for a month. Wonder whats going to happen when they stop selling us the good stuff:)

Post Tue Jun 08, 2010 10:55 am

Posts: 112
THE RUBBERBAND INTAKES AND HEADS DON'T HAVE THE GROOVE FOR THE O-RING.

Post Tue Jun 08, 2010 2:28 pm

Posts: 251
Location: Hudson, Florida
WHAT??? OOOO RIGHT YOU ARE:)) Good point, I took an intake off one time had the o-ring groove on the heads an they used the flatband intake. Whoever installed it had way more patience than me, they had the o-rings jammed in under the flatbands. I can't even figure out how they got it assembled, but with determination like that on their side I'd bet anything is possible.

Post Tue Jun 08, 2010 3:30 pm

Posts: 1676
Location: Interlaken, NY USA

Interesting point, this motor has a groove for the O ring, but as I said, had rubber bands on it. very interesting.
DD

Post Tue Jun 08, 2010 8:54 pm

Posts: 251
Location: Hudson, Florida
I personally like the 0-rings, only problem seems to be on assembly of the heads and cylinders you really have to take your time and alien them for best fit. I've cheated for years now by using 7 inches or so of electrical tape around the intake nipples an haven't had a confirmed leak yet:)Sounds like you need a carb rebuild and new o-rings and clamps.

Post Wed Jul 07, 2010 8:54 am

Posts: 93
Location: Norway
Hi Frankenstein. The carb is a good carb. No need to replace it if not broken. As said above, the idle richens if backed out (counterclockwise). The rubber plug that blocks the channel from the idle jet to the float bowl is supposed to be there. do not remove it. Often the oem idle jet is too smal if aftermarket exhaust is used.
There are available plastic rings that fits in the o-ring groves when converted to rubber band seals.
Also pay attention to float hight. This is important on these carbs. If rubber parts is damaged do to old gasolin, remeber that the float valve has a rubber tip. Do not use any other than oem floatvalve if replaced.
Torstein

Post Wed Jul 07, 2010 11:38 am

Posts: 1676
Location: Interlaken, NY USA

Thanks for the info, we traced the problem down to bad rubber tip on the float valve. robbed the needle from another kehin we had for parts, and immediately solved the problem.
DD

Post Wed Jul 07, 2010 1:11 pm

Posts: 251
Location: Hudson, Florida
Nice you found it:) Good call Sidewinder, funny how we always over look the two first rules on checking carb problems, is there gas in the tank and is it getting to the carb:))Scary part is, what's in store for us with the next round of fuels headed our way.

Post Wed Jul 07, 2010 1:24 pm

Posts: 1676
Location: Interlaken, NY USA

Right you are Neil, the tip was swelled and misshappened, presumably from the gas, (or whatever that sh*&t is we're paying $3 a gallon for)
DD

Post Wed Jul 07, 2010 8:41 pm

Posts: 251
Location: Hudson, Florida
You know Doc,at times I'm kind of cheap and I've been known to do a small job around here using gas to clean the parts. Wasn't an uncommon thing in my younger years to be elbow deep in gas for several hours.
Try that now for 15 minutes with this new stuff an its damn near as bad as using Carb cleaner, burns like hell an dries your skin till your fingers crack around your nails.


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