ENGINE WILL NOT DIE off the THROTTLE - CLUTCH? CARBS?

ANLAF

ANLAF

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I have been running in my rebuilt top end. Intermittent refusing to die off when I come off the throttle - bike keeps revving until I either turn down the idle screw or (if I am in gear) until I pull in the clutch and release to bite the clutch again. And there is popping as I come off, too.

Is this a clutch adjustment issue or a carbs issue.

What I have done
New clutch recently, followed by going in again a couple of weeks ago to check shifter-shaft and star wheel selector action (clutch had to come out, of course).

Today I lubricated the throttle twist mechanism on the handlebars and made sure the throttle cable was working properly (it is).

Took off pods and felt for the operation of the float chambers. The right hand side was operating quicker than the left - so I dismantled the carbs, thorough clean and reset floats (they were out and unevenly adjusted) to 25.5 mm.

Result on tonight's test ride - popping gone, but still some intermittent racing when coming off the throttle.

Hmmm! could it be a failed diaphragm? or must I go back to check the clutch adjustment?

Anlaf
 
Are you running the stock advancer? If so it could be sticking. If the advance advances and then sticks it will not idle down when you back off the throttle.
There is a video around somewhere showing how if you manually open the weights on the advancer and release them how they should snap shut.
If they don't snap shut you need to pull the advance rod, clean out the old grease out of the ends of the cam and off the rod.
Regrease with a moly grease. A thin coat on the rod and retest the snap back.
One thing with out anything hooked to the rod it should spin in the cam with no to very little resistance.
One the springs you can tighten the springs by clipping off the loop on one end and bending up the next turn of the spring to make a new loop. Do this to just one spring and retest the snap back.
I suggest marking one spring with a sharpie. This way if you need to further tighten the springs you can do the other one. The sharpie mark will let you know which spring you cut first.
Leo
 
Thanks, XSLeo, but the Boyer ignition does away with the mechanical advance mechanism - but wait! does that mean if I have not tightened the nut properly that binds the Hall effect rod at the advance end it will cause the same sticking effect? That's one thing to check.

Anlaf
 
I should have looked at your signature better. With the Boyer you have a solid connection the cam No stickiness issue there.
Is your throttle cable too tight? It needs some slack to let it not pull the throttle open when turning the bars.
Air leaks can cause a lean mix and let it idle high.
Your clutch adjustment has no effect on the idle. Pulling the lever and releasing it just pulls the engine speed down, just like slowing down by using the clutch.
Leo
 
The throttle has the requisite slack to allow for pick-up.

The leak theory sounds interesting. The only thing I have changed is to turn the securing clip that holds the carbs in place, so that the nut and bolt is on the bottom. I can't see how that would affect things, though (I'll put it back to the original place).

Carb diaphragm then?

Anlaf
 
Hey ANLAF, have you built the homage to the carb gods yet?

Yeah, a bad/leaking diaphram can admit air into the system, enuff to make lean mix at part to mid throttle, but doesn't really affect idle. Could try retarding the idle timing to the most retarded position, 13° BTDC, or even 10° BTDC. This will reduce the idle rpms, and require a slight upping on the idle stop position, which could move you out of a possible/questionable low idle mix zone, like when throttle shafts/body are worn or butterflys not seating properly. Ideally, the throttle stop screws should be no more than 1 full turn from fully closed...
 
The idle is fine - in fact purrfect. It is revving as if it doesn't want to come down from mid range. I might have mislead you and XSLeo by talking about adjusting the idle screw.

I dismantled the carbs this evening and noticed a little pool of petrol on top of the left-hand carb diaphragm - I have never seen that before.

Is this a sign of a failed diaphragm?

Anlaf
 
Moving the clamp around won't cause a leak, not getting it tight enough will. Tighten till the end of the clamp are touching.
There are many places to get an air leak. The carb to holder joint is just one. The vacuum barbs have hoses and caps that can get loose. Throttle shaft seals.
Just had a thought carb sync can cause throttle hanging too.
One way to check for air leaks is to use a spray bottle with even just water in it. Spray a bit of water on any place that you suspect. The water will seal the leak for a few seconds. You will know this by changes in the idle. Backing the idle down some may help you notice the differences easier.
Leo
 
Yes, XSLeo, the clamps are always tightened to joining.

Synching crossed my mind, too, but I would need one of those vacuum things to test the balance, would I?

Where are the hoses, caps and seals you speak of?

Anlaf
 
Sounds sort of like what I was experiencing somewhat recently when my low end was running lean. I ended up adjusting my idle mix screws out and upping my pilots and now it's ok. Might not be your solution but maybe something to think about. I'd put my faith in what the experts have to say first!
 
Thanks, RetroLS1, I will eliminate the obvious diaphragm issue first. I have a feeling that will be the source of my hanging throttle (new terminology enters the Anlaf vocabulary). Before the rebuild the jets/needle settings worked well, but of course, the bike was running on a wing and a prayer.

Anlaf
 
Just a thought - if diaphragm has failed, with this affect the combustion? - I am trying to work out if more air or fuel will go into that failed side cylinder, and why?

Anlaf
 
You can swap slide/diaphragm assembles if the carbs are the same. But aren't your two sets different years? '77 and earlier ones won't swap with '78-'79 ones. It's not the diaphragm that differs but rather the slide. They will physically fit but not run right because of vastly different needles.
 
Check your throttle shaft seals, while it's idling place your finger over the end of the throttle return spring. See if it changes the idle any. Mine were leaking and I couldn't get the left cylinder to die while trying to do a dead sync. When I placed my finger over the end of the throttle shaft it would idle down and almost die like it should.
 
Thanks, 5twins - I will take off th e76/77 catbs and replace with my 78/79 carbs.

The 78/79 carbs have 37.5 pilots and 140 mains - is htat within range.

And should I boost the jet needle up to #2 as I did on the 76/77 carbs?

Anlaf
 
I have just done the diaphragm function test at Post 4: Slides and Floats (The Carb Guide).

Both diaphragms are working and have not been breached. I will do a visual check and hold them up to the light.

If the diaphragms are fine, it looks like a seal has failed - throttle shaft seal, perhaps.

Anlaf
 

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