Slow to lose revs

[plc]

Hi All,

My R65LS has a curious problem.  When it has warmed up, usually after about 20 minutes of riding, when I try to slow down or when I come to a stop it takes a while for it to settle to idle. Now once at idle it idles fine, no stalling at all. This is a real nuisance when I try to throttle off for a corner and find I am still a few revs above idle. This means engine braking is effected.

If I am sitting at lights with it idling and I give it a quick little throttle burst the motor revs then slowly loses its revs until it settles back to idle.  I thought the throttle cables may be sticking so I lubed the throttle mechanism and replaced the old upper cable but this didn't fix the problem. In fact when I rev it at the lights as described I can see the throttle linkages on the carbs return to idle position.

My only experience with carburettors was the old SU on my Austin 1800 I used to own.  Is there some problem with the pistons in my Bings? Could it be the diaphragms? Has anyone else had this happen to them?

Any advice for this newbie would be appreciated.

Best wishes to all,
Paul

[mikethebike]

Make sure that the carbs are fully returning.  It doesn't need a lot of throttle opening to give the symptoms you describe.  If the carbs are returning fully to rest it could be the auto advance on the timing sticking.  You can check it with a strobe light on the timing marks (bung next to oil filler).  Sometimes the lube dries out or they rust.  (It's located behind the front cover on the engine)  The factory manual says it's not serviceable but you can actually strip, clean and lube it.

[airhead]

Normally something like this is the carbs needing to be set up and synced when the engine is hot after a run of about 10 miles or so. Set the carbs cold, or not up to running temp and the revs will run away when it warms.
It could also as be as Mike says, sticky advance weights. There is a small access cover on the side of the beancan, oval in shape, that once popped open will give direct access to the weights without having to strip the whole unit, which if you are new to Airheads, may be a little daunting. It's much better to strip, but a few drops of light oil through the access ports may help.
To tell if it is the advance weights sticking, the next time it revs up, slowly let out the clutch in gear to load the engine till almost stalled, then pull the clutch back in. If the revs stay down it is a fair indication it's the weights, if the revs rise again, it's carb adjustment.


Bill........................;-)

[MrRiden]

This is a common complaint with our bikes. Mine even does it a bit and I've been through the advance weights. I'm just a bit too lazy to recheck the carbs lately. 110deg forecast for here today. trolle posted a to-the-point procedure and you can find it here http://suraklyn.com/yabb/YaBB.pl?num=1212944300
let us know how you make out!!
rich

[Ed Miller]

You can test for cable sticking with the bike not running.  Just shine a flashlight on the throttle mechanisms on the carbs, turn the throttle grip, and snap it closed.  The throttle levers should snap back closed too.  If not, something is hanging up, maybe in the splitter.

[nhmaf]

Yeah, once I noticed that it was an LS, I began to think "splitter", too.

[plc]

Hi All,

Thanks for the replies so far I really appreciate them. Tomorrow I will try the simple way of just lubing the advance weights as Bill suggests and see if that works.  

Regarding the splitter it seemed OK to me when I fitted the new cable, but I have to say I may have made the inner piston "less smooth" than it was trying to fit the new cable, what a nightmare job.  I removed the tank so I had good access, disconnected the cable from the grip mechanism, but the inner cable length to outer cable length seems wrong to me.  With the splitter adjuster wound right up to create the most cable slack the inner piston did not clear the splitter tube, no matter what I did I could not get it out. So to remove the old cable I cut the cable. But then when I went to fit the new cable I had the same problem, the upper cable's inner just seems too short.  With difficulty I could get the upper cable in, but then the two lower cables could not be fitted, because the inner piston did not come out far enough, try as I might to get clearance, with angles, brute force and swearing the lower cables would not fit.

A mate at work had a solution, he shortened the upper cables outer and that gave me enough length to get the splitters inner piston clear to fit the lower cable.

I read on another spot on the forum this is a tricky job, but you have detent in the splitter that allows the lower cables to slide in. I simply could not get it to fit, particularly with half of the splitter piston still in the splitter tube.  My upper cable was not a BMW original but a UK made Venhill, I assumed it would be fine.

The problem still exists regardless of old cable vs new cable, mind you I did not change the splitter because it seemed OK to me. Maybe that would have been a simple solution with the tank off at the time.

Next time if I have to change a cable I will disconnect the lower cable at the carbs, fit them and then get the upper fitted. It still seems wrong to me that the inner piston does not clear the splitter's case.

Best wishes to all,
Paul

[plc]

Hi All,

My LS is a 1984 model with electronic ignition. Does that mean there are no advance weights? The Clymer manual talks about a trigger unit which I assume has no advance weights, is that correct?

If so the search for the slow rev loss continues.

Best to all,
Paul

[mikethebike]

The electronic ignition does have advance weights in the "bean can" under the front cover.

[samc]

I couldn't get the lower cables installed until I disconnected the upper end entirely from the throttle. then there was barely enough room, but it did work.

[nhmaf]

Yup, those splitters can be a real pain to deal with.  I think that I figured out a trick to getting a bit more slack, but will have to go back to my notebook to find it.

Yup, you've got a bean can in that bike, the 'trigger' unit is a hall effect sensor  inside the can that picks up the passing of a metal 'vane' or tab that passes by it as the crank rotates, and sends a low voltage signal to trigger the ignition control module.

[mikethebike]

Check out the post on lightening the throttle. I just had a problem with sticking throttle after greasing the throttle twistgrip gear and refitting it in the wrong position. I lined up the wrong marks on the twistgrip which gave similar problems to yours.
Mike

[plc]

Hi Guys,

Just a little update and a request for more advice.

In my continuing search for the cause of my slow rev loss I took off the little oval access port and lightly sprayed the advance weights (or what I could feel of them) with WD40 style spray.  I turned the rear wheel to rotate the weights a little then applied some more WD. I know the WD will evaporate but I thought it was more likely to penetrate any dried grease on pivots.

It didn't make any difference, so I am now in a quandary as to whether I bite the bullet and disassemble the bean can, whether I chicken out and buy a reconditioned one, or if I embark on a carb overhaul first.

I should mention I have checked the cam gear in the grip mechanism and there is a lot of free play there and it does not seem to stick.

I am curious if people think the following more detailed symptoms suggest it is most likely to be the trigger unit:

1. Slow to lose revs when warm, but will idle once revs are lost.
2. If I open the throttle only one carb at a time the motor still is slow to lose revs, regardless of which side it is.
3. In the first 15-20 minutes of riding the problem is completely absent.

Sorry to be so needy, I am just a little puzzled about it all.

Best wishes,
Paul

[airhead]

Still sounds like the carbs need to be dialled in when fully hot by someone who knows how to do it if you aren't able.
Can also be a problem with hardened/aged O rings on the throttle shaft and/or mixture screw. These can both introduce extra air at idle allowing revs to rise. So many variables and I'm not sure how many you've investigated since my first suggestions or if you have only had a quick look at the advance weights. You can move the weights through the little access port to see if the springs are ok and pulling the weights back, though you will have to be brave and remove the beancan from the bike to check it better.


Bill...........................;-)

[drewboid]

I had the slow to return to idle problem with my /6 ( I know - points not electronic - but the weights are similar) Since increasing engine speed with either carb causes the problem I would look again at the ignition. My weights were so worn I opted to use the Omega electronic ignition system. It mounted to the end of the alternator and has a built in advance system. At $265 it is about 1/2 the cost of a new beancan and about the same as a rebuilt one.  YMMV