Runaway rpm's

[marcmax]

My LS started idling a lot lower about 2-3 weeks ago (400 rpm). At speed it runs just fine but coming to a corner I have to keep the throttle tweaked or it will stall. I adjusted the valves (not far off to start) and installed new plugs. Still ran good at sped but very low idle. I adjusted the idle up as best I could and yesterday I rode to a friends (about 100 miles) and we used his TwinMax and got them dialed right in. Bike was running great and peppier than it has been in a while.

About 15 minutes into the ride home I came to a stop sign and when I pulled in the clutch the rpm's shot up to about 4000. Throttle was backed all the way off. I ended up holding the front brake and easing out the clutch and once the rpm's fell to about 1800 it dropped back to a normal idle of around 8-900 rpm's. Same thing happened a couple more times in the next 15 minutes and it progressively got more frequent. It finally got to the point that even pulling away from a corner once the rpm's got above approx 1800 they would continue to climb until around 4000. If I braked and let the engine lug until the rpm's dropped below 1800 it will drop to a normal idle. The entire time, once I get up to speed it runs smooth as glass.

I am stumped as to where I should even start. It is obviously a carburetor issue but short of a tear down and rebuild I don't even know where to start.

Any suggestions? Please?

[Milo_357]

I just found out the throttle arm on my spare carb was bent.  I never noticed it.  Could it be something like that?

Something is sticking somewhere. I'd check both throttle & choke cables, and my carbs.

Good luck!!

[Bob_Roller]

A sticking centrifugal advance in the 'bean can' can cause a similar problem, the flyweights on the advance mechanism stay in the high rpm full advanced ignition timing position .

It doesn't sound like it will be your problem, but performing a carb synchronization with an engine that's not up to operating temperature will cause similar problems .

[montmil]

A sticking centrifugal advance in the 'bean can' can cause a similar problem, the flyweights on the advance mechanism stay in the high rpm full advanced ignition timing position .

It doesn't sound like it will be your problem, but performing a carb synchronization with an engine that's not up to operating temperature will cause similar problems .

If you and your bud enjoyed a cool beverage prior to the the Twinmax work, the LS cooled off enough to mess up your efforts. Engine needs to be at operating temps. Best deal would have been to hook up the synch tools immediately.

Go for another ride and get the engine toasty. If you do not have your own syncho tool, I'd suggest backing off the throttle stop screws until they no longer touch the throttle adjustment lever. See if the revs go down.

If revs remain high, tighten up the mixture adjust screws on the bottom of the carbs. Do no more than 1/16 of a turn on each and see what happens. May need to blip the throttle. Lower revs but still a bit too high? Continue on.

Try to bring the idle revs down using the mixture screws only. You can adjust the idle to be higher, if need be, using the throttle stop screws... those big ones up topside.

When I find an operational concern on my bike, I always return to this last item I was messing with.

Let us know what transpires.

[tvrla]

When I'm confronted with something unusual like this, it's time to run down the list of possibles and tick them off one at a time. It doesn't sound like an air leak, though you could check. It doesn't sound like the piston is hanging up in the carb. How's the ignition timing? It doesn't sound like you messed up on the carb synch since the idle will return to normal. Something's sticking is what it sounds like. Perhaps the cable splitter. I think that's where I'd look first. Take it all apart, clean and lube with silicone grease, and then seal it up with some heat shrink tubing after getting it all adjusted. The shrink tubing will keep debris out and prevent it getting sticky.

[Barry]

First things first. The initial symptoms were very low idle speed and the cause of that was never determined. Could have been some partial blockage of the idle mixture ports in the carb. The idle speed was increased with the idle stop screws and then later whatever it was that caused the initial low idle speed has disappeared so you now have idle speed too high.

I would go back to base settings and start over. You are aiming to adjust the mixture screws for maximum idle speed and then turn out the throttle stop screws to lower the idle if necessary to the desired level. You are always aiming for the minimum possible throttle stop setting to achieve the correct idle speed and that means the mixture screws have been set in a position that gives the fastest idle.

There are no circumstances when you should attempt to lower idle speed using the mixture screws that is a recipe for a runaway idle because you can get things so messed up that the transition ports come in to play. The transition ports are fed with air/fuel mixture from the idle jet but they are not controlled by the mixture screw. They should not flow at idle and what normally keeps them out of circuit is having the throttle plate in the correct position which is almost closed. In any situation where you  attempt to reduce the idle speed with the mixture screws it means the throttle plate is aleady too far open and in danger of bringing the transition ports in to play.  A proper understanding of the transition ports is the secret of CV carbs. There are other possibilities but this is one of the classic scenarios for the idle speed to hang up.

[marcmax]

The thing that is so perplexing about it is that the increase in rpm always starts at about the same place 1800 rpm. If I keep the engine below that it drops back to a normal idle every time. Once it has started to run away if I force the idle down by lugging the engine once it reaches the 1800 rpm mark the idle drops to normal by itself.

Thanks to all who have posted suggestions. I think it is time to back up to square one and start the process over. Thinking back on things I thought we used the TwinMax shortly after I got there but it was actually probably at least 30 minutes while we caught up on things before we got to work. Plenty of time for things to cool down to a point that adjusting the idle wasn't accurate.

All I know is that the runaway rpm's made for an interesting ride home last night. I am glad the traffic was light and that I know which backroads to use.

[Bob_Roller]

Have you done a valve clearance check recently ?

One tip I got from an independent BMW mechanic, was to do a valve clearance check before adjusting or performing a carb synchronization .

These bikes do act odd, if the clearances are off !!!

By chance, did you do a check with the Twinmax, after you adjusted the throttle cables, I was sceptical the first time I heard it, but after doing it, it does make a difference .

Just something else to throw into the fray !!!!!

[bruce_launceston]

I concur with all the learned gents that you will almost definately get a high idle (when warm) if the bike has cooled any amount before doing the carb sinc.

[marcmax]

Spent the morning tweaking things and have it very close. I checked the valves again and they were spot on, same with the ignition timing. I then adjusted the idle using the mixture screws to a high idle and then backed it down using the idle screws. It now idles fine, accelerates fine and doesn't run away when letting off the throttle. The rpm's don't rapidly drop off when the throttle is closed like they used to but they do drop in a few seconds.

That is acceptable for now (the day is beautiful and I feel the need to ride). I will continue to tweak and see if I can get it closer.

My yardstick manometer seems to do a better job than my friends TwinMax.

[Bob_Roller]

I don't think this will make a difference, but in the R65 owners manual (US version)l, it mentions that once you have max rpm with the idle mixture screws, to turn the mixture screws in one fourth of a turn .

I don't know if this was for exhaust emissions or not !!!

[Barry]

Now that you are close I would mark the position of each mixture screw slot so you can return to this bench mark if needed. Then turn the mixture screws in by small increments - just the width of the screw driver slot at a time and see what difference that makes.

There will be a range of mixture screw adjustment where the revs don't change. Many carb tuning methods will describe that you should be in the middle of this range. I tend towards turning in the mixture screws until the revs just start to reduce and then back out a touch. That gives the weakest mixture that still maintains maximum idle revs and should give a faster return to idle.

[marcmax]

Went out riding and did about 150 miles. The more I rode the more steady things became. The rpm dropping when the throttle is closed is so close to where it was previously I don't think I am going to mess with it until the next over all service. It was a pleasure to ride and the ride couldn't have been better. Temps in the high 70s, blue skies and riding into orange grove country all the orange blossoms are in bloom. It is the most heavenly smell on earth. I didn't want to turn for home!

[Mike V]

marc,

A little late getting to the post, and hopefully the rpm issue is gone.  But did you check the vacuum take-off port for leaks, or if you have a small screw plugging the port, is the fibre washer intact and in good shape?  Check your intake horns (tubes) and both rubber collars for leaks, take some brake or carb cleaner and spray it on all the fitting ends with the bike idling, any change in rpm?