odd carb discovery

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Aug 6, 2005
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ok, upon having a fantasic running commando now, I put a carbtune to the carbs and discovered almost no vacuum to the left carb and good healthy, normal vacuum to the right carb. Adjusting the slide screw or air screw has no affect on the left carb and the slightest tweak on the right side has a lot of effect on the right carb. The bike really runs great with no popping, missing, accelerates great, holds steady at any throttle opening without missing a beat. The carbs literally have evrey part new in them, sleeved, idle passages drilled out and set screw put in where blank was, new needles, jets, floats gaskets, everything replaced. New cables routed, carbs set up properly, double checked by a 30 year mechanic. I switched the carbtune hoses around and the readings followed the carbs. Any area other than carbs that could cause this and still allow the bike to run so well? Secondly running it this way surely cannot be good for the motor can it? I just thought it kind of odd that it runs so well and there seems to be such a difference in the carb balance. A mystery I'd like to solve for sure. Maybe I missed something simple and just am not seeing it.
 

L.A.B.

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A low vacuum reading on one carb seems to point to the throttle slide being further open on that carb?

Check that the slide is dropping all the way down and is actually resting on the throttle stop screw.

If the cable is over adjusted then the slide will hang on the cable and not rest on the throttle stop screw, so turning the screw will have no effect.
 
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I second L.A.B.'s diagnosis. I have purchased brand new carb cables that did not have enough movement in the inner cable and had to trim the outer cover to stop the slides staying slightly open.
 
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Ah, found the left slide is hanging up intermittently and the cable from the throttle to the junction does not allow enough slack. could be the answer, thanks guys. I'll find the problem and reset and hopefully be back in business soon.
 

L.A.B.

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If one cable seems tighter/shorter than the other it could be worth checking that the carb cables and nipples are correctly located in their slots inside the throttle cable junction box?
 
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