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MkIII 850 commando uncommanded revving

Submitted by michael_wallace on Fri, 24/09/2021
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Commando

I have just read an entry from 2017 on this similar subject.

My 850 owned 20 years but only about 2k MLS in that time. Single mk2 Amal conc. Starts as should, warm up, off down the road, within a few mins will rev up to about 4k rpm uncommanded.  This has been happening intermittently for a long time but is now unmanageable. It seems that at a particular warm up point of expansion it occurs.  As in the past I could ride through it and goes away.

Air leaking past throttle slide? Has some play but I don't know if excessive, anyway what do you guys think and can anyone recommend resleeving company?

Submitted by john_holmes on Fri, 24/09/2021

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If its still using points ignition then a stuck AAU will do same, to find an air leak spray WD40 progressively over the joints, when you get to the leak the engine revs will change.

Kevin Traill at Alverstoke Restorations in Gosport, Hampshire

His details are: KJTRAILL@qinetiq.com
Telephone: 02392 580708 (after 16:30).

Details are old so may not work but his work is excellent.

Submitted by boo_cock on Fri, 24/09/2021

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Remove the slide from the carb. Take a CLOSE look at it. I bet there is a chunk missing from the bottom corner. Let me know how it goes . 

 

Submitted by michael_wallace on Sat, 25/09/2021

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It's on Boyer electronic, no points.  I had a close look at the slide there is no chunk missing but a small abraided area the size of a pea at the bottom that can't be fealt with the finger nail.  There is also some vertical scoring in the wall of the housing and a fair bit of play with the throttle slide in the closed position.

I think I'm going to get carb off for recon.  When I can find one.

Thanks guys

 

Submitted by robert_tuck on Sun, 26/09/2021

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I used to rub the slides and body with some powdered lock graphite . Now I have an air filter I use the thinnest smear of moly grease. Any more will affect the allowable air leaking and richen things up.

Submitted by john_holmes on Sun, 26/09/2021

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Before you take the carb off, just to be sure its not an air leak at a joint do the spray test, include the carb float to carb joint and all the manifold joints. A worn slide alone would have to be really worn to get to 4000 rpm, as standard they have 4 thou clearance.

Submitted by david_cooper on Sun, 26/09/2021

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The couple of times that happened to me it was because the slide did not close.  4000 seems a lot for anything else. Is the cable sticking...maybe too sharp an angle as it comes out of the top of the carb? Test run without air filter so you can see it or touch it. Does it go down smoothly even if you touch its side lightly?

Try taking a look at the manifole joints, Start the engine then spray the manifold with WD40. If the revs change, then there is an air leak. Alternativelt, smother the manifold joints with vasaline and see if that has the same effect. 

 

Submitted by michael_wallace on Mon, 27/09/2021

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Thanks Gents,

Well I'm going initially for a new cable, it could be sticking at any rate far from doubt free.  The throttle stop screw thread is interesting, can't be my problem as it's intermittent. Isn't this an example of coriolis effect induced vibration along with screw thread serrations.

New cable first. 

Submitted by michael_wallace on Sun, 03/10/2021

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Isn't it often the simplest most obvious cause of a problem, throttle cable in this case.  I've had the commando many years and the p o had fitted a splint to the broken cable conduit at the cranked end above the single carb.  This "fix" was now sticking. New cable, no runaway revving  but!

Misfiring and one cylinder dropping out.  I know these symptoms it's plugs.  Replaced the NGKs with a pair of champion N7YC from RGM.  I continually have plugs failing, copies maybe, not genuine I don't know.  With new champions went for short run and checked plugs they looked a bit "white".  An old plug colour chart I have from OBMart says could be weak mixture, alert!  And when I was in the carb I noticed the needle was on lowest setting,top notch,  weak mixture?  So I raised it one notch and now running like a different bike for the better.  I'll go for a decent run and check again.           

 



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