TOPIC:

by pilot screws you mean air screws, right? I think the stock position is 1 1/4 or 1 1/2 which is what makes me think I'm running this circuit super rich. (after pulling the plugs I know I am, they were very sooty).

I have only tried to rev it up a few times, and the only times I can get it to rev up is when i'm running the pilot super rich. This makes me think the high speed circuit is running lean and sucking lots of air (not fuel). If there's extra fuel coming in from the idle circuit it's using that to rev up, but ideally the main circuit would just work.

Is this circuit adjustable? Does my logic make sense, or is this something completely different?

Yes, pilot screws. Some bikes have them between the airbox and carb, and turning them in throttles air, hence more fuel. Some have them between the engine and carb, and throttling them means less fuel. Not sure where yours are.

I'm not 100% sure on your carbs, but usually the main circuit is not adjustable, other than moving the position of the needles (if yours have that capability) and going to a different size jet. If everything is set up 100% (including exhaust and airbox) and your electrical end is up to snuff, I'd say you have a vacuum leak or clogged carb or fuel level isn't correct. You got fresh gas, correct? There are lots of internal passages in these carbs and if some of them seem to blow air and carb cleaner, they still might be partially clogged. My 550 pilot ciruit on one carb was partially clogged. When you blew air through it, you noticed it. But when compared to another one, it was about 75% of the total air flow.

Even if your pilot circuits aren't right, once you get into the more open throttle positions I would expect the bike to pull out of the dead spot.

Also, if your spark is weak, it might have a harder time at higher RPMs. I had a bad coil once on my 750. It would idle ok and rev with no load, but when I'd try and ride it, anything more than just creeping down the road and it would miss and bog. At the higher rpm the coil would crap out and I'd be essentially running on two cylinders.

Take the carbs off and use the little red hose on a can of brake clean to clean out ALL of the passages in the carbs and float bowls. Remove the air screw and spray cleaner through that passage too.

Make sure the cleaner is exiting from somewhere else in the carb body, that will ensure a clean passage. Don't just spray it like you would some air freshener either.

Set the carbs to factory settings, install and tighten the intake boot clamps.

Go from there.

abrambila wrote:
I have only tried to rev it up a few times, and the only times I can get it to rev up is when i'm running the pilot super rich. This makes me think the high speed circuit is running lean and sucking lots of air (not fuel). If there's extra fuel coming in from the idle circuit it's using that to rev up, but ideally the main circuit would just work.

Is this circuit adjustable? Does my logic make sense, or is this something completely different?

In mine, the gas feeds in through the main jet and feeds all the carburetor. The "idle" and "idle bypass" are fed by the pilot jet and work up to about 3K RPM and up to about 1/4 throttle open. beyond that, the vacuum piston rising feeds the carb from the main jet and needle assembly.

I would make SURE you have no holes or slits on the diaphragms as that will cause this kind of starving/hesitation since the pistons don't rise correctly.

Good catch bounty... never thought his daiphramgs might be the problem. On my 750 4 cylinder the "true pilot hole" and the "transition holes" were fed from the pilot jet. The fuel screw was in between the two, so when you adjusted it, it only throttled the pilot hole and left the transition ones unchanged.

Hey, my carbs dont have a diaphragm. But after adjusting the floats and syncing, it runs great up to like 2500rpm, then it feels like its struggling, but if I pull through that about another quarter-turn it catches and takes off like crazy and then runs great above the struggle zone. Each time you shift up it has to pass through this zone again.


Any ideas why it would be struggling through this transition/what can be done to make it smoother?

abrambila wrote: ...runs great up to like 2500rpm, then it feels like its struggling....

Does adding a little choke worsen the struggling, or perhaps reduce the struggling?

Good Fortune!

Doesn't it almost sound like the transition holes on a set of keihin cv34's are plugged. (for the 750-4).?

Or maybe the needle is 1 clip too low (if it's adjustable?)

If you have a look at this diagram for your carb, look close at how the Needle goes into the slide valve. Look at how the spring holds the needle farther up, and that the adjustment clip doesn't sit down against the bottom of the slide. Check this, as I found a set of carbs miss assembled like this once.

abrambila wrote: ...runs great up to like 2500rpm, then it feels like its struggling, but if I pull through that about another quarter-turn it catches and takes off like crazy and then runs great above the struggle zone. Each time you shift up it has to pass through this zone again....

If not already done, would assure the timing advancer is properly functioning, as it may be "sticking" in the closed position.

Good Fortune!

Just as an update, there is a loss of power around 2500 to 3000 rpm. Turning the throttle another quarter turn or so will give it power again, but it revs up like crazy when it gets to this point... kind of dangerous. Then it settles out fine and above 4000 rpm runs like a champ.

If I run it with the choke on high the problem goes away. It's also much less noticeable at high speeds because i can just stay above the 3000 rpm zone. Its really the worst in traffic and going uphill. The problem is I can't always run with choke on.

I'm just confused because it runs so well except in this one band, and there are only two carb circuits I'm aware of. Pilotjet for low speed and main jet for high speed. I've adjusted the needle and it hasnt helped. does anyone know what's the story with the transition hole/diaphragm?

thanks

It sure sounds as if you still have blockage. What is your set up? Stock carb box with stock exhaust? Pods or 4 into 1 exhaust. This makes a big difference as to what jets should be used in the carbs. The metering rod height also may have to be changed. You also may have cracked carb holders, all this stuff has an effect on the proper running of the engine. Bob