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Carburettor fuel measurement

The Sea Raven

CB750 New member
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Hi all,
My first post, unfortunately is a question rather than an answer. The short of it is - can anyone tell me the fuel measurement on a 1981 CB750K?

To be clear its not the float height I am after, I know this is 15.5mm, its the amount of fuel this will set i.e. would this be on the joint line of the carburettor body, 5mm below, 10mm below etc,

There is a long winded reason for why I am asking this but for the moment, I wont complicate the issue. I have research all over the place but cannot find this and despite my best efforts, and being confident the float height is set as good as I can, there appears to be a wide discrepancy in the amount of fuel in the bowls between two and the other two.

The overflow is about 6-7 mm above the joint line, so that would suggest the amount of fuel should be quite high but ???

I'm not familiar with that particular carb set, but typically the factory float height spec yields close to 2-3mm below the gasket surface (joint)
I'm not familiar with that particular carb set, but typically the factory float height spec yields close to 2-3mm below the gasket surface (joint)
Hi thanks, that is what I have read with other carbs, and bikes, either at the carb body, 1-2 mm below or 3mm below. Problem is I have two about 2 mm below and the other two are about 10mm below. I have a 81 model and these are supposed to have the non adjustable floats but they have the adjustable ones from the 79 model. To try & figure out what it should be, I bought the non adjustable set up to try on 1 carb, but that also came out as setting the fuel level low (about 10mm below the join line). I can only assume the carbs are the correct ones for the bike and the floats were wrong, though they seem interchangeable anyway and the new non adjustable float is also bang on the 15.5 mm float height.

Given the float height has a plus or minus of 1mm, and this sets the fuel measurement, something is wrong either way by quite a bit. Either two are low &/or two are high. Carbs have been ultrasonically cleaned and initially the bike started & sounded great, I carb balanced but then I had an overflow issue and have been chasing my tail ever since.

Bike is a total non starter at the moment but the overflow issue is no longer a problem.

I could get the non adjustable floats for all but this would be setting the levels lower than I thought they would be. I wanted to have an idea if this would solve the problem before throwing money at it to no avail. I don't mind the cost if it sorts it

You would expect the fuel level to be higher given the overflow is so high, maybe I am overthinking that though
Do all the float needles match? How about the needle seats?
Hi, yeah everything matches. The floats are the steel plated adjustable ones from the 79 model with the wire clip valves, they are in good order, viton tips are not worn and the seats look good. Its no longer overflowing, so they are sealing at some point but a vast difference in the fuel levels, the float heights are as close as you can realistically get. Only issue may be with the plungers on the valves, they all spring but don't seem to be that positive.

What I can't figure is what the correct level should be . You would expect that to be within a 1mm of each other, If I knew that at least I would know which are problem carbs. Just thought that number would be given somewhere as its the fuel level that is the key point

I put a set on from the later models (should be the ones that are on anyway, the plastic non adjustable float with the correct valve) and that came out with a low amount of fuel in the bowl, so thinking the two that have high fuel levels are wrong and the fuel level is lower than I expected. I have ordered a set of these to try across all to see if that at least gives an even amount of fuel levels and hopefully sorts it.
Yes you are overthinking it, I miss the brass ones,... like you said the plastic ones are non-adjustable so we have swapped them around to get basically the same measurement and what I call the same bounce when the carbs are turned upside down and lightly dropped like a drumstick... here's the measurement you're looking for if it doesn't flood you've got it... it would be nice if they made the needle seats a little more easier to remove since they're pressed in.