RPMs increasing when warm

jgoldin1 · Full Member Joined: Jun 25, 2007 Posts: 37

hello,
first, let me say that before i hop on the bike, i let it warm up and idle at around 1200-1500 rpm. after i've been riding the xs for a while (30 mins or so), i'll be sitting at a light and notice that my idle is up to around 2000. then a few minutes later, i'll be at another light and it'll be slightly higher. if the light is long enough, i can watch the rpms slowly, but steadily increase. i don't notice a change in RPMs when its actually driving, but maybe that's because i live in the city and my speed is rarely constant.

what's going on?

its on my to-do list to rebuild the carbs (know of a good link to help out a beginner?). i hope this will fix this phenomenon. until then, what should i do? lower the idle?

Cooltouch · Posted: March 14, 2008, 2:18 pm Post subject: Re: RPMs increasing when warm

What year model is your bike? If your bike has a mechanical advance, it could be getting sticky and causing this. Try lubing the advance weight pivots and see if this helps. You probably ought to just pull the whole advance rod and service it completely -- make sure there's nothing gummed up in there.

I have an '81, which has the electronic ignition, thus no mechanical advance, and my bike sometimes experiences this. So, in my case, it can't be the mechanical advance. Haven't figured out what causes it, and it isn't consistent either. I can blip the throttle and it will settle back down.

Best,

ichael

jgoldin1 · Full Member Joined: Jun 25, 2007 Posts: 37

i have an 80 (G) special II, and from what i've read, i've also got electronic ignition. i haven't checked to make sure. when i blip the throttle up, it settles down to the higher RPM and will continue to climb.

anyone else have an idea? if yours does it as well, maybe i'm worrying over nothing.

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

It could be a case of setting your idle speed too high. If you set it on a cold or not fully warmed motor, it will climb and idle too high when up to operating temperature (hot). You must set it on a hot motor. The idle speed will be low when the motor is cold, it may not even want to idle at all, but that's just the way these old bikes are, as are most other old ones as well.

The next time it happens, I would dial the idle speed down to 1200 and see if that helps. If it keeps climbing again after that, then you may have an air leak somewhere. The manifolds are the usual suspect but it might also be one of your throttle butterfly shaft seals. They're not indicated on any of the BS34 parts diagrams but they are in there. It's the same seal as used in the 38s.

Tomterrific · Posted: March 14, 2008, 6:15 pm Post subject: Re: RPMs increasing when warm

Listen to 5T. What you've discribed sounds exactly like you have set your idle when cold. These bikes don't have the fancy idle speed controls like modern cars so the brains is the operator. I have a plastic tire valve cap forced over the idle speed adjust screw so I can twiddle it with my fingers. I adjust the speed up when cold and down when the bike warms up.

Tom Graham

jgoldin1 · Full Member Joined: Jun 25, 2007 Posts: 37

great. that makes me feel better. thanks

xsjohn · Posted: March 15, 2008, 5:58 am Post subject: Re: RPMs increasing when warm

jgoldin1.....my spare set raiises when I hit the throttle...goes up to 3 grand.....if setting the idol doesn't stop the problem they might need a good cleaning......the pilot jet could be a 45 while your there....42.5 stock......allens in the float bowl "lowes hardware" and the floats and jets can be accessed later without taking the carbs off again.......I think on my second set air is getting in from the choke plungers and the pilot circuit still has crap in it....not sure yet cause I don't use them......just put them on once to try them and never tried them again.......I have a post in my account on setting the floats.....tried everything and those settings seem to work the best.........if you clean them careful with the choke rod....small ball and spring that is easily lost........wouldn't even consider taking them off the main bar.........

xsjohn

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