high speed engine problem

[HobbyMan]

I am new at this and have just rebuilt my dad's two 79 XS1100F's and my 80 XS1100G. They all run great, but on my bike(1980), when I get over 80MPH and let off the throttle slightly it seems as though someon hit the kill switch momentarily. I am thinking it is the vacuum advance line but don't know for sure. It only has around 21,000 miles. Any suggestions?
Thanks,





I am 17, if I can do it, anyone can.

[TopCatGr58]

Hey HobbyMan,

Welcome, and Congrats on such fine work on such fine machines!

Okay, your 80 seems like it falls on it's face after you reach 80mph, and then back off the throttle slightly like you are trying to maintain that speed, but it won't accept the throttle, right?

The "kill switch" analogy is probably your best diagnostic here. You're close, not so much the vacuum advance hose or module, but it's involved. Under strong throttle the timing plate is being rotated actually LESS due to reduced vacuum to the vacuum advance(really a retarding unit!) under strong throttle so that it keeps the timing advance in the lower range for the best power under strong throttle. When you back off of the throttle, then the vacuum actually increases to the advance unit, and it then increases the timing which is desired for cruising speeds when you have less fuel input.! Sounds Oxymoronic doesn't it!

Anyway, this flexing of the timing plate and the attached pickup coil wires which could be aged, cracked, etc., and are breaking contact which stops sending signals to the TCI effectively "turning off the switch" until you change throttle again enough to change vacuum, rotating the plate and flexing the wires again until the make contact sending pulses again to the TCI!

Recommend you to to the tech tips, and review the repair pickup coil wires tip for diagnostics and repair info!

But also, check the vacuum advance module, make sure it and the timing plate are rotating smoothly, not binding etc. and that the vacuum hose doesn't have any cracks, holes, leaks!!
T.C.

[HobbyMan]

Thanks, I read some forums and it all helped out a lot. The previous owner of the bike took the left case cover off and replaced the seal incorrectly. There were signs of water intrusion and I cleaned and lubed everything. I also replaced the vacuum line to be sure. It seems to have solved the problem. Now let the good times roll
Thaks for the help