Rising revs

lewisjohn

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I’m using my R80 project bike a lot now but have a strange issue.once the bike is warm, when I shut off the throttle the bike revs to around 2500rpm, if it’s in gear and I let out the clutch it’ll drop down to 900rpm tickover. It will also start building up revs on its own without touching the throttle. I’ve checked the carbs and pipework for leaks with carb cleaner , made no difference. I’ve given the advance and retard mechanism in the bean can a clean, but now I’ve run out of ideas.
Any thoughts?
 
One thing I've been told can cause this is air leaking in somewhere.

Don't know where but that's what I know.
 
Is this the bike with mikuni carbs?
 
Is this the bike with mikuni carbs?

Think the bike had bings refitted.
In theory the revs can only rise if the engine is getting air.
If the carbs have been apart check the butterfly plates have been fitted correctly and are seating nicely in the Venturi bore,
Also check for wear in the spindles and bushes,are the spindle o rings in good condition.etc
 
I'd still suspect the bean can advance mechanism. I had a similar problem. Stripping and cleaning the bean can didn't fix the problem but testing with another bean can cured the problem.
It might help to check the timing marks. Even though the revs drop on releasing the clutch the timing mark is always advanced indicating a stuck adv mech.
 
Is it balanced when the revs increase, if it is then hardly likely to be two carbs to simultaneously develop a fault unless both set up wrong such as something missed on assembly like an o ring letting air bypass somewhere. In my inexperienced but logical mind balanced while those revs increase pointing to a component that is shared by both carbs such as the bean can as per others comments.
 
I’ve never had a problem with the bean can although I’ll probably wish I hadn’t said that tomorrow:). I’ve always been surprised that mechanical advance retards work as well as they do, for as long as they do
 
rising revs

Hi it could be the float level are set slightly high, or a leaky float needle jet weeping allowing fuel to pass when the floats are at the top. check the chokes are closing, and also change the throttle cable return springs or at least check that they are working correctly, or it might be worth checking the valve clearances...good luck...Ian
 
Think the bike had bings refitted.
In theory the revs can only rise if the engine is getting air.
If the carbs have been apart check the butterfly plates have been fitted correctly and are seating nicely in the Venturi bore,
Also check for wear in the spindles and bushes,are the spindle o rings in good condition.etc
Yes Mike, I am now using the Bings, completely standard set up, I stripped the carbs to clean ultrasonically but didn’t remove the butterflies. I think I’ll check the float heights, have a look at the butterfly shaft o rings, and then see how it is. How does the Enduralst ignition system perform, would it be a worthwhile upgrade assuming the problem isn’t fuel related
 
Yes Mike, I am now using the Bings, completely standard set up, I stripped the carbs to clean ultrasonically but didn’t remove the butterflies. I think I’ll check the float heights, have a look at the butterfly shaft o rings, and then see how it is. How does the Enduralst ignition system perform, would it be a worthwhile upgrade assuming the problem isn’t fuel related
The enduralst ign??
I’ve fitted the yank system ( forgetits name but think it’s endurolast )in conjunction with the alternator ,,what a mess about,can’t adjust the timing dynamically,the rotor wants to chop its way though the front case.it works well but not as good as.
Emerald Isle alternator upgrade,available through motorworks 400 or 600 watt
Seimans ign kit also available from motorworks.
Ign kit is plug in ,set timing ,and forget !
we fit to all our special builds.( I think this kit is also Emerald Isle)
 
Yes Mike, I am now using the Bings, completely standard set up, I stripped the carbs to clean ultrasonically but didn’t remove the butterflies. I think I’ll check the float heights, have a look at the butterfly shaft o rings, and then see how it is. How does the Enduralst ignition system perform, would it be a worthwhile upgrade assuming the problem isn’t fuel related
I have a spare bean can if you want to try swapping it out to eliminate it.
 
Yes Mike, I am now using the Bings, completely standard set up, I stripped the carbs to clean ultrasonically but didn’t remove the butterflies. I think I’ll check the float heights, have a look at the butterfly shaft o rings, and then see how it is. How does the Enduralst ignition system perform, would it be a worthwhile upgrade assuming the problem isn’t fuel related
I think moto bin carb kits are 20 squid or so.for both carbs
Rebuild the things,the throttles want to snap shut,someone may have been there before you and fitted the butterfly upside down ,back to front who knows..
Hold the carb up to to light and look though the Venturi.
There should be virtually no gaps
 
I have a spare bean can if you want to try swapping it out to eliminate it.

Thanks Paul,I may take you up on that, I’ll give the carbs another shot over the weekend and see how that goes first.
 
Did you check the valve Clearances?? If nothing else has changed and you have all the holes and wot nots with their right bits, slack in cables, plugs, bungs and clips where they should be and tight? AND the Ignition timing is correct?

About the only place it can get air is slightly open valves when hot and the clearances have reduced ??

and another possibility is a broken spring on the bob weights
 
Well it’s not the bean can, I put Paul’s spare bean can on yesterday, did around 50 miles today,still the same, a thought came to me as I was riding, I remembered one metal inlet stub turning in the head as I tightened the jubilee clip on the rubber pipe.
I’m going to take the carbs off again and remove the stubs to try and seal them properly.
 


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