Quick DOHC Carb Question

80 model cb750f super sport, keihin carbs.

Quick question: (there seems to be debate within my group of friends)

The top puck, with the slide needle,..... is the slide needle supposed to be loose, where you can wiggle it? Or tight, where it doesn't move?

They are of the non-adjustable variety. Meaning there's no clip.

I say they are supposed to tight. What's the proper way?


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hillsy said:

They have to have some movement or else they will gouge the seat / themselves out with operation - so, they shouldn't be "tight". There should be a spring either holding the needle down in the slide or up against the retainer. Either way, there should be allowance for some lateral movement.

Click to expand...

Interesting. A spring between the top of the needle and the threaded bushing that holds it down? The retainer in the puck, right? I've got 3 carb banks and none of them have a spring in there. There is a spring under the cap, and on top of the puck. Is there usually another spring?

The needles on my running bike are loose, but I'm having idle issues. That's the reason I've been asking around. I know it's likely an air leak issue. I'm just trying to eliminate all possibilities before I break the bank apart. I'm just being lazy lol

I've had people tell me both ways however


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the needle should be loose as you describe

no there is not supposed to be a spring pushing on the needle

its on the top of the slide under the cap

Ok. Good. That's the way they are.

I've probably got an air leak somewhere. It's the only logical explanation at this point. I can't find it on the boots, and I don't think it's there. I'm thinking it's probably in the rails. That or air shutoffs.

One more question... there's a nipple on the bottom of the stock airbox, with no hose on it. What does that go to? Just a moisture drain? The hose on top is attached and runs to a collector box, but nothing is attached to the bottom nipple


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Yes the bottom nipple is a drain. If it is open then it should have a hose on it with a one way end so unfiltered air cant enter.

Gotcha. Thanks a lot for the info.


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Thought I would give you guys an update. I finally bit the bullet and broke the carb bank apart. The idle problem was %100 the air cutoffs. They were completely corroded, to the point I had to pry them out with a screwdriver.

I got everything back together, and was finally able to get a good sync. She runs like a scalded dog. I feel pretty good about it


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