Hello everyone it's been quite a while since I've posted on here. But now is the time to post a question. My 85 k100rt has had a idle issue since I bought it, but because of not riding it much because of having another bike I have neglected trying to fix it. When idleing at 900-1000 rpms it has a miss, sounds like a short in a switch or something. I had suspected the fuel pressure regulator because when I remove the vaccumn hose that goes to the #4 throttle body and plug the nipple off, it smooths out and hardly misses at all. But when the vaccumn hose is reconnected it starts to miss again at idle. The bike runs great out on the road, but just at idle do I notice the miss. I had trimmed the vaccumn hose when I first got the bike because the hose was cracked and leaking where it fits over the nipple on the throttle body, but I never thought to trim the end that goes on the pressure regulator. Probably do that tomorrow and check for other vaccumn leaks.

Does anyone know how the vaccumn from the throttle body affects the pressure regulator? Is it a must to run the vaccumn hose on the pressure regulator? Thanks for your help, bob

Hi Bob,
Any chance the vacuum hose or pressure regulator has an air leak in it?
Regards,
K-BIKE

Bob, some are running their K's without that vacuum hose. K1100's even didn't have it fitted. It is intended to equalise the fuel pressure at the injectors depending on the 'vacuum' at the throttle plates, so that the pressure difference at the injector tip (between the fuel and the area where it sprays into) is constant.

Pressure gauge is the only way to check for pressure leaks and pressure regulation.

That vacuum connection is not required, in fact, IMHO it is nothing than a neusence. I have been running without the vacuum connection for many months and find no difference in performance and fuel consumption, except that the bike behaves much better at idle and low revs.

This was covered earlier see : http://k100rt.aforumfree.com/technical-repair-and-troubleshooting-f7/fuel-pressure-fluctuations-t866.htm?highlight=fuel+gauge
Have a look

I took the air box out Saturday and replaced the vacuum hose and it seemed to help some but it still idles erratic at times, think I'll plug it off and see how it idles. This bike runs really good otherwise, think I'm going to sell it, just because I don't ride it enough to let it set in the garage neglected. Will put some pictures of it in the for sale section, thanks Bob.