adjust high idle?

[26667]

Ok, I'll just admit ignorance here. Even my guru w lots of airhead experience was left scratching the olde noggin.

Stopped by Chi BMW and the airhead expert there said it sounded (the footpeg vibration at all speeds) like the high idle wasn't adjusted; in addition to balancing the carbs as per usual at idle they should also be balanced at about 3000 rpm.

If you adjust the cables to the proper fee-play, then fine tune w the stop screw, then readjust at 3k, don't you just screw up your previous balance???

My guru's gonna ask his guru, but any how-to advice appreciated.

 

[manicmechanic]

What you're attempting to do is balance the carbs at idle, then when you increase engine speed with the throttle, you are trying to make sure both carbs are responding the same and both cables are pulling equally.

 

[26667]

That makes perfect sense, but don't you screw up your idle balance if you go changing.... wait. So ok. You go ahead and change the adjuster nut/free-play, then RE-adjust at idle w the stop screw?

So you sort of do a rough idle balance with the free-play adjuster, then a fine idle bal with the screw, then another adjustment of the free play at 2500-3000, then go back and re-fine tune the idle?

 

[Bill Burke]

I use the spoke method, shorting each side at idle and using the idle adjust screw to balance the sides AT IDLE. Then rev it to 2500 or so and HOLD IT at that RPM using the cruise control screw or a friend, and from there use the throttle CABLE adjuster to increase the rpm on the "weak" side to achieve balance at the higher RPM. This does not result in messing up the idle balance at idle RPM. Does this make sense? (Be sure all idle adjustments are done with a completely warmed up engine - 10 mile ride), and work quickly so as not to overheat the engine during the adjustments (or use a floor fan to cool the cylinders). Hope this helps, I have to hit the road.

 

[sumran]

The idle adjustment and the cable adjustment should be unrelated. I have no idea what the term "high idle" means.

The adjustment of the idle speed screws is done with free play in the cables. Whether you use the shorting method or a vacuum tester or a combination, it has to syncronized at idle using the screws because that is what is holding the throttle plates open.

Once the throttle is engaged the idle speed adjustment screws aren't doing anything. The throttle plates are now being held open by the cables. So the cable freeplay has to be adjusted until the carbs are syncronized. This is usually done at about 1,500-2,000 rpms, depending on who you ask.

If the engine is in balance at idle and it is in balance at 1,750 and it is not in balance at 3,000 rpms, the problem is not a simple cable adjustment. The CV carbs should deliver smooth performance throughout the rpm range if things are right.

I would hunt for airleaks if there are significant differences in balance at 1,750 and 3,000. You could be leaking air at the rubber hose between the carb and the head or between the head and the intake spigot. If there is a leak, it will behave differently as the vacuum level changes.

I am curious about the loss of performance you mentioned. It is probably connected, but I'm not sure how.

 

[26667]

Thanx, Bill and Sumr'. I too use the spoke method, but misunderstood how each adjuster related to the other. I don't believe it's an air leak. Reason being that we put on new throttle cables just prior this footpeg buzz phenom'. (Tho' it semed dandy for 500 mi) Maybe they 'stretch'. Whatever the reason it sems to me ya gotta think it likely that the cables are pulling slightly differently, and if that's so, it would also account for the slight drop in giddy-up.

The term "high idle" was one used by the BMW tech, and believe me, it's not used anywhere else that we could find. My guru looked at two different BMW shop manuals at Chi BM while we were there, and one that he owns that's translated from the German. Nothing about this procedure. I need to go look at my Bing book. Doesn't it show how the different needles etc overlap to give a smoothe transition from idle to full throttle?

But the tech says that's the likely prob. And you guys make sense, so we'll see if that don't do the trick.

This @#$%&*# bike is so close to perfect I can taste it. And it won't be long before the snow flies here in the middle west...not that a little snow should keep me from riding