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Cb900C Hesitates Off Idle And Will Not Past 7k Under Load

curtie94

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Hey guys new to the forum. I'm not sure if my bike fits but it's a 1980 CB900c has 85k miles on it.

It has been re ringed by the previous owner but seems to run strong. What I'm dealing with is that just off idle so cruising though town just barely cracking the throttle I get a hesitation. If I continue to hold it it will crackle.

I believe it is running lean since the plugs are bone white.

What I have done is rebuilt some low mileage carbs new accelerator pump properly adjusted and is working. I also replaced the Air cut off valves since they where all cracked. All new orings as well. Stock jetting.

But it does have a aftermarket 4 into 2 pipe.

The previous owner said he paid someone to adjust the valves not even 100 miles ago so I haven't checked the clearances yet.

I also found that I cant rev the bike under load past about 7-8k. I figured it might be due to the 3/16" fuel line I'm using so I haven't put much thought into that problem yet.

I adjusted my idle mixtures using a color tune and the outer cylinders run best with the fuel screws just about ready to fall out. The middle cylinders run best around 3 turns out.

Any ideas.

Thanks

Curtis
 
Not revving out fully is usually lost compression from valves leaking and at that mileage not surprised. Not using OEM airbox will add to that if so.

Idle screws are way too far out, they then use up fuel reservoir that was intended to be for light acceleration added to the accelerator pump, the small holes behind the butterflies. 2 turns out, forget the colortune. May help to bump up both mains about 5 numbers each for the pipe if it flows well.

At that mileage the timing chains will likely be pretty stretched to retard cams, that will lessen performance too.
 
Thanks I will set the idle screws to 2 turn out reset the idle then try and see if its any better or worse. So your saying having the idle screw too far out will take away from part throttle fueling?

Also does the larger main jet thread into the emulsion tube. Right now my jetting is stock 68 primary and 105 secondary.

Im not too concerned with the wot performance at this time. if I can get it to run well off idle I would be happy.
 
'...having the idle screw too far out will take away from part throttle fueling?'

Yes. They both pull off the same reservoir of fuel, the chamber formed by the sealing plug you see under carb body right behind the mixture screw, covered partially by fuel bowl.

Not sure how second question is posed, ALL main jets add air in some way to make an emulsion, it is how the circuit activates faster and the mix is more thoroughly atomized. Smaller jet goes in front and bigger in back if asking that.

Yes to the third, the accel pump should squirt at even the slightest movement of the throttle. Later or less equals hesitation. Same with the holes behind butterfly, they act like miniature accel pumps on their own. If mixture screw is too far open then the holes feed less fuel, the screw used some of it up. Leading to overly rich idle and then goes lean at any throttle movement, a hesitation situation.
 
Thanks again that clears it up alot.

Is there a special way to adjust the accel pump I have 0 gap between the pump and the linkage but I dont get activation of the pump until past my hesitation. SO maybe I have play in the linkage?
 
Probably not that, rather the one way check valve in fuel bowl. It does not check as quick as it should when older and then the pump shot suffers until it positively seals. The pump rod has to seal that check by pressure before any builds up in the circuit to squirt out. People have fits with them.

You can see the top of it at the gasket flat with bowl off. It opens to fill pump up but must seal off with .002"-.005" movement the other way of the pump rod when set at zero clearance, what I set mine at, screw that slight airgap they call for there.
 
If you can carefully advance your ignition timing up to say .030" more than the OEM mark that will help hesitation too. Listen for pinging around 3K or so. If no let 'er rip.

That pump must really work with no lag for best result.
 
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