occasional rough running

[JohnK]

I have a really odd situation. Periodically my 81 runs rough, almost as though it is not getting enough fuel. It begins behaving badly when i'm down to about 3/4 of a tank, not low enough to go to reserve though. The engine idles badly and take off is very sluggish. It sounds like it is not getting fuel or maybe even missing. If I switch to reserve this does not help. If I fill up it will continue to run poorly for 20 or 30 miles. Medium and high end are good at this time, though rolling on the throttle it sometimes does not respond smoothly, but from a standing start it is down right embarrassing!

I have checked the choke linkage, the fuel diaphram, the petcock screen, and the air cut-off valve; all of the linkages and diaphrams are OK. I installed a K&N air filter OEM replacement. Could it be the throttle pump? I had the carburetors cleaned about 1,000 miles ago and I use 93 octane amoco Ultra gas. One more clue: after riding for many miles and before this whole low end thing starts in earnest, it does not want to idle smoothly. This seems to start before my main complaint here.



Any advice would be greatly appreciated.



John

Indianapolis

kowalkejohn@msn.com

[texascbx]

Could be vacuum lock,check vent in gas cap,had that in my last 82.I wasn't completely blocked,just enough to make it run like crap.

[bob]

I have a Russel gas filter on my 81 and the line goes in a loop from the tank to the carbs. As the tank runs low I believe the lack of gravity makes gas flow poorly which make the bike run bad sometimes. Even starting with low tank or switching to reserve takes a bit to go. Bob

[CRAZY DAVE]

I think TEXAS CBX is on to something. Put your spare key in gas cap lock and when it starts to run bad unlock it and turn the cap loose and see if the bike picks up. Just don't try any stoppies with the lock in the upright position !!