Over revving

[UKDave]

Hi all, new to the marque and this website so please be gentle with me
Loving the bike it’s a 74 B in great condition, slowly sorting the items that need attention.
Going to strip the carbs, ultrasonically clean them then balance them which I believe needs doing.
So the question, bike over revs regularly sometimes up to 4,000 rpm for up to 30 secs. that doesn’t happen that often but regularly remains at about 2 k when trying to slow down.
it’s not the cables, the throttle linkage at the carbs are closed onto the stops.
Any help what I should be looking for?

 

[jetmechmarty]

Are you sure the cables are synced properly? That sounds like an out of sync condition.

 

[UKDave]

Are you sure the cables are synced properly? That sounds like an out of sync condition.

No idea, it looks like they open at the same point, but surely if it ticks over great and when it happens I check that both throttles are back against the throttle cable stops then it wouldn’t be that? Forgive me if I’m missing something! This only happens spasmodically, though fairly regularly

 

[jetmechmarty]

No idea, but surely if it ticks over great and when it happens I check that both throttles are back against the thro cable stops then it wouldn’t be that? Forgive me if I’m missing something!

The butterflies have to move precisely together. They have to allow precisely the same amount of air to pass when closed. Otherwise, you get what you describe. I imagine it’s possible that you have an air leak on one side.

You have two throttle cables, correct? One to each carburetor. What method did you use to sync them?

 

[nj1639]

Changes in air inlet or exhaust from stock can give a hanging idle too.
Post some pics of that ride of yours and welcome aboard!

 

[Jim]

A hanging idle, if it's not the cables, is usually an indication of an air intake leak... though that's usually fairly consistent and usually gets worse the warmer the engine gets.
To find a leak I use a can of CRC electrical contact cleaner. It's flammable, has a straw for good control of where it goes and evaporates without leaving a residue on the carbs.
Wait 'till the engine acts up (hangs) then methodically/systematically spray the contact cleaner on the carbs and boots. if there's a leak, the rev's will immediately increase when the cleaner gets sucked in. Once you've isolated it, do it a few more times to verify exactly where/what's leaking. It's usually the butterfly shaft seals and/or the intake boots.

 

[Kevin Werner]

Welcome to the madness. You get great advice above. One other thingmcan cause that hanging idle is a lean condition but I don't remember it being as high as 4000 rpm . By all mean post some pics, particularly showing carbs and exhaust.

 

[UKDave]

Thanks for the advice all, i haven’t done anything to the carbs yet, was getting some pointers as to what to look for when I take them apart. Friend has both an ultrasonic cleaner and balancing tool so will take advantage of that And also be looking out for air leaks as suggested.
the exhausts are Dunstal.

 

[UKDave]

Could it be one of the diaphragms split, I assume that’s what’s up in the round housings at the top of the carbs??

 

[Kevin Werner]

There is a lot of work you can do without removing the carbs if indeed you do not have an intake leak at the boot/manifold. Removing the tank gives you access to the diaphragm covers, diaphragms and slide/needle assembly. Removing the float bowls gives you access to the main and pilot jets. Buy or make JIS bits before you attempt either.

 

[Kevin Werner]

Well, I guess we should mention that most of the Phillips screws on this bike are not really Phillips screws but rather J.I.S. type cross points. They use and need a different type of screwdriver. Normal American Phillips screwdrivers don't fit them correctly. That's why you often find many of the screw heads buggered up on old Japanese bikes. What I use most are the Vessel 2" long power bits .....

I suppose one day I should break down and buy some real JIS bits too.
I've always just used a bench grinder and modified phillips bits. Never stripped a screw with these, so....
Yeah... I know... somebody flog me.


View attachment 199494

See the messages above for JIS bits. Phillips bits will strip your screw heads.

 

[Jim]

See the messages above for JIS bits. Phillips bits will strip your screw heads.

My "modified" Phillips have served me well.... I don't strip screws

 

[AUTOMAN]

I'll bite.
how'd ya modify?

cliff

 

[Jim]

I'll bite.
how'd ya modify?

cliff

 

[jetmechmarty]

https://www.google.com/search?q=jis...P4gMEGAAgQYgGAZAGCg&sclient=gws-wiz-serp#ip=1

JIS bits search

 

[AUTOMAN]

Other than slight angle. It looks good.

 

[Jim]

Other than slight angle. It looks good.

It doesn't mar and or strip JIS screws. That's pretty much all that counts.