Carburetor KEIHIN Carburetor troubles

[Charlie Brown]

I've got a 1977 Sportster, factory standard, and I've developed a cough through the carb which occurs under moderate acceleration . The bike starts fine, ticks over nicely, revs ok stationary, if I pull away really gently all ok but under moderate acceleration it is unridable. I've stripped and cleaned the carb, all the jets and air ways are clean and clear, accelerator pump is in good working order, valve clearances are good, now I'm stumped. Anyone out there got any ideas?

[the_french_owl]

Can only be one of two things:

- Iddle jet too small, which is unlikely as the early Keihin are pretty good carbs and properly jetted, unlike later ones that veers on the lean side...

- More likely, your iddle is not perfectly set.

Either the throttle stop screw on yours is out too much, so throttle is closed too much and prevents good transition between iddle and main jet hence the caughing, or the transitions holes are partially sooted up. They are just in front of the iddle passage in the top of the body, 3 or 4 small holes that are uncovered as the butterfly opens.

Proper iddle setting is the good compromise between a partially open throtle, iddle screw setting and the revs at iddle not being too low.

Patrick

[Charlie Brown]

thanks for reply.
The tickover is at 900-1000rpm,the needle bounces!
The transition holes appear to be clear,i've blown smoke through via a tube.

[The Yukon Sizzler]

Has it only just started playing up Char...?? Sounds like classic carb "icing" , starts to happen this time of year as the temperature falls away.
Various fuel addatives available to cure it.

[Charlie Brown]

Hi vince.
It has just become a big problem however it has done this cough now and again in the past at a constant speed ie 60 mph ,not often enough to be a concern.I've got no air leaks,checked for that also.

[The Yukon Sizzler]

Try the fuel additive first Char.....it wont do any harm , and , if nothing else, rules it out of the equation.
Work through it one remedy at a time...by doing 2 or more things in one go means you'll never know what cured it.
Could even be coil breaking down under load.....
Keep us informed dude .... I can sort you a spare carb or coil to try if need be.

[Charlie Brown]

Coil! Good call Vince.That theory fits.Having been through the carb inside and out i am begining to think the problem lies elsewhere.The coil is probably the original.I will look into that.

Cheers.

[Phil]

If your bike was running well & this developed, I might be looking for a vacuum leak causing a lean mixture. Classic symptoms.

[kevscrivener]

Originally Posted by Phil

If your bike was running well & this developed, I might be looking for a vacuum leak causing a lean mixture. Classic symptoms.

Agree with Phil on that. Had similar on my 72 when I first had it, spent a fortune on new electrical bits then checked manifold and found leak.

It's not always obvious, only needs a tiny pinhole or crack which can be invisible to the eye.

[Charlie Brown]

Carried out a thorough crack search on the inlet manifold with a magnifying glass, none found but looking over the carb in the same way I found what looked like a crack and yes, petrol brushed on the outside appears on the inside! The crack is on top of the carb just inboard of the choke valve. Can this be repaired? If so with what? Is is a weld at best or a new carb at worst?

Advice appreciated.