RPMs hang

[jlobaido]

Have 1980 XS1100 G carbs triple cleaned, rebuilt, floats set. Problem is when reving the bike in nuetral, the revs will drop quickly to about 2k RPMs then slowly return to idle. When shifting thru the gears while riding the RPMS pick up when clutch is pulled in making ever shift a speed shift. Experimented w/ removing the plugs over the pilot jets, different main jets and pilot jets and different settings on the idle mix screws. Tried to find a vac leak but there doesnt appear to be any. Additionally the bike has 4 into 1 pipe and a low restriction air filter. Watchya think?

[skids]

Synch the carbs (balance vacuum at idle) and then check pilot screw settings?

[jlobaido]

We sync'd the carbs and they are even but the columns do jump around alot.

[Snow]

Did you vacuum sync or bench sync? I had problems with this awhile ago, gave up, went to another set of carbs, and finally came back and fix the prob. What I ended up needing to do was bench sync the carbs first and then vacuum sync them at a steady 4k rpm. I dont know if it was necessary to have them up at 4,000 for sure, but that's what I did.

[skids]

If is works, it is hard to argue with, BUT I recommend adjusting vacuum at or near idle. The reason is this: The idle is sensitive to the relative positions of the throttle plates when they are nearly closed. If you adjust at higher rpms, the vacuum differences are less affected by the relative positions of the throttle plates and more dependant on things that are not in your control, like aerodynamics of the air box. I once tried to adjust at higher rpms (4000?) and when I was finished, it would not idle properly. YMMV

Originally posted by Snow
Did you vacuum sync or bench sync? I had problems with this awhile ago, gave up, went to another set of carbs, and finally came back and fix the prob. What I ended up needing to do was bench sync the carbs first and then vacuum sync them at a steady 4k rpm. I dont know if it was necessary to have them up at 4,000 for sure, but that's what I did.

[Snow]

And by no means am I a carb guru - in fact, I'm switching to fuel injection as soon as I can

[pgg]

Sounds like lean pilot jets. Or too rich. Or sticky throttle.

[jlobaido]

Both...

The carbs. have been bench sync and then vac. sync. ......along with the RPMs hanging, I cant get a clean idle,mid and top all at the same time. ......When these carbs were rebuilt, the kits came with what appears to be a restricter type plug with a hole in it. We have tried these carbs with the plugs in and with them out. plugs out, runs rich and gags and farts on the throttle but has a great idle. Plugs In runs lean has a great mid range and top end but idles like a chokin dog. I am looking at these restricter type plugs pretty hard.....opinions welcome !.......beyond this I am thinkin they might need an exorcisum.

[TheDjost]

With the plugs in, you sound like you have a lean idle. Try backing out the pilot screws 1/2 turn each until it smooths out. Even if the specs say 2 1/2 to 3 turns out, mine had to be set to 5 1/2 turns out to have a correct idle. Midrange and top end are fine. I probably need to rejet, but it works OK for now, and I'm contempling a couple of ways to get an injection fitted for DIRT cheap.

-Justin

[TheDjost]

I forgot: a quick way to check if idle is too rich is to add a TINY BIT of choke. If the idle smooths out, just back out the pilot jet screws.

-Justin

[pgg]

I'd throw the 'restricted plugs' out the nearest window.