Trouble setting up VHB carbs

[750S3]

TwincefFollwed Thisoldtractor procedure for setting up the carbs on the 750S3: loosen cables, attached Carbtune, set idle speed at around 1000, adjusted fuel mixture, all seems ok, both columns in sync., idling nicely just above 1000. Took the bike for a ride and after about 20 miles or so it wouldn't keep the idle speed, the engine raced like crazy to 4-4500RPM. That's twice it did this. Any thoughts?
Thanks

[guzzisteve]

You are supposed to do this on a warm bike, 10mi it will be fully warm.

[750S3]

You are supposed to do this on a warm bike, 10mi it will be fully warm.

Yes did that, fully warm after a 15M ride

[Antietam Classic Cycle]

May not be a carb issue, could be a sticking advance mechanism in the "distributor".

[pehayes]

Sticking slides?  Did you change to lighter carb springs?

Patrick Hayes
Fremont CA

[750S3]

Will investigate the advance in the distributor, lighter springs replaced by standard weight ones.

[canuck750]

If the idle will not come down after releasing the twist grip throttle there could be an air leak, check that the carburetor top caps are tight, both small bolts torqued down and the paper gasket under the carb top cab is properly fitted.

Unfortunately these VHB car caps warp with time as the alloy will expand and distort with age.

Sometimes I wipe a thin smear of silicone around the cap lip once the cap is torqued down, a very thin smear of silicone will seal the cap to the body.

The other source of an air leak is the rubber boot that wraps the throttle cable to the carburetor cap and the flange bolted to the head, check all three 8mm hex head flange bolts are torqued down and the double set of intake flange gaskets are fitted together without any foreign material, sealant preventing a tight seal.

The carb to flange clamp must be tight as well. For the RPM to rise that much once warm there has to be a lot air getting into the fuel mixture path.

[Chuck in Indiana]

^^^^ wat our canukistanian sez..

[750S3]

Will definitely check all items listed, it is still odd that it would behave like this after a ride. As I said everything looked good after the Carbtune setting and the first 15-20 miles were good, idle went back to normal (if a a trifle high at 12/1500RPM) at traffic lights, then suddenly while stopped it raced up to 4-5000. At on traffic light I had to turn the engine off. Back in the garage engine racing I unscrewed the knurled idle screw almost all out to finally reduce the engine racing.  Weird !