How to identify correct float needle? 1981 CB750c

[chickenstrips]

I have a bike that was running lean on all four. Opened the carbs and found right away that the float heights are all a consistent 17.5-17.9mm!

I doubt that all four floats are somehow off by the same degree and there is evidence that there has been carb work done before. The evidence doesn't give me the most confidence: the screws for the choke plates weren't flared to secure them and one plate was installed backwards!

My guess is that they bought four like needle valves which were incorrect size.

How do I identify if the valve is correct? It is seating properly, no question about it.

[Lucky 1]

Not even any photos to help us???

You can hire me to go find out all of this information.
Anything is possible when working on 40 year old carbs that amateurs worked on.

[chickenstrips]

No photos, no camera.

I'm very familiar with all pre-90's Honda's. This is the standard 80's-era plastic float and the needle without the wire clip at the top. It is correct for the era and fits the floats.

Question is, why are all four floats off to an equal extreme?

Must have this fixed. The low fuel is making the bike fall flat on its face and won't idle below 1,700rpm. Not good.

[Lucky 1]

You sound like you know what your are doing.
Carbs can get extreme problems when extreme
people work on them without a workshop manual.

[TOOLS1]

That sounds more like a vacuum leak then low fuel in the bowels.
TOOLS

[chickenstrips]

There's no shortage of small bodged jobs on this one. Another "first time" project. I don't imagine an even vacuum leak along all four, but one step at a time.

The first step is: the float height is incorrect across all four and needs to be rectified.

[TOOLS1]

A vacuum leak on one carb will cause the problems you describe. The little bit that the floats are off will not hurt anything as long as they close the needle valves, and keep the fuel level in the carbs from overflowing. I often have to set the floats higher (actually lower when turned over) on these older carbs to compensate for the 30+ year old floats drying out, and not floating as they should to shut off the needle valve.
TOOLS

[chickenstrips]

How do you adjust a plastic float to compensate? How does plastic dry out? These floats work fine. I've just compared them to floats in a known working CM400.

With low fuel levels, you can't sustain a wide-open or near wide-open throttle. The bike in fifth would reach about say 80 and then slump and only be able to sustain 75 at WOT. Pull in the clutch and wait about three seconds and you can repeat, reaching 80 then slumping as the fuel can't catch up.

The point is very simple, I just want the fuel level to be correct. Relative to the gasket surface, where should the fuel level be?

[TOOLS1]

You said "it would rev, but not idle". Now you are saying that it will only rev for a second then you have to let off, and give it throttle again. Now you bring in a CM 400.
As for the plastic floats, the plastic does dry out and become porous. That's why the floats sometimes need to be adjusted differently to keep the needle seated. Now if you just want to know what the stock origional float height is supposed to be set at, just download the factory service manual from the homepage here, and look it up.
TOOLS

[chickenstrips]

The float height is 15.5mm. I know this by heart. Hell by now I've memorized most vintage Honda specs.

But I'm not concerned about the FLOAT height. As you said, we may have to compensate. So what I'm asking now is if anybody would be kind enough to measure the FUEL level with a piece of clear tubing and tell me what's normal.

[bcombs]

Hello,
I have a 1981 Honda cb750c - and my mechanic is saying I need to rebuild the carbs.
The bike has been converted to a bobber and the filters have been replaced with Air Pods.
The bike is running really rich at times, then lean - then misfires then cuts off...  He said that I need to order Larger Main Jets (120-150) and Thinner Float Needles...

I've read when these bikes are converted to bobbers and Air Pods are put on the bikes and the exhaust is chopped - it doesn't create a well enough vacuum in the bike and requires larger jets.

Does anyone know where I can purchase thinner Float Needles?

Also, can anyone confirm that the Main Jets are Keihin Carbs Round Jets?

Thanks for ANY advice...

Best Regards,
Brett
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