1982 CB750F...Better Devil

Jimbonaut

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I don't know what the specs are for this bike but usually the float level is lightly lower than the top of the bowl. also you can polish the needle seat by putting a cue tip in a drill with some polish. seems to help when using old float needles
However, tell you what I didn't do before and that's stick the q-tip in the drill. Along with switching out a known good needle (from non-leaking carb 2) with the needle from carb 1 (which was leaking), I re-polished the seat (thanks a lot for the tip) and then tried hooking up a gas supply to the carb rack with them on the bench. Big fail - I guess the carbs need draw from the engine pistons to create gas draw through them, so all I achieved was a minor Exxon Valdez reenactment on my workbench. Installed them back in the bike but minus the airbox (just for ease of installation) and fired the bike up to see what gives.

Well, it ran but like utter shit, only to be expected I guess without the airbox. However once I finally got the bike firing on all four it definitely looked like the leak from carb 1 had stopped and - with what I thought may have been a dodgy needle now in carb 2, no leak from carb 2 either. All this was done at 8.30pm last night with thoroughly irritated neighbours so had to abort before confirming the test. I'll reinstall the airbox and properly install the carbs again and see what happens.
 
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Maritime

Over 10,000 Posts
Sometimes all it takes is one little bit of dirt to cause a leaking carb. I fussed over a friends sons leaking carb for an hour. I had asked did you clean the needle and seat, he swore he did. I put a little twist of PT up and a little spec of black something came out and all of a sudden it stopped leaking gas. your re-polish of the seat may have done the same.
 

Jimbonaut

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Sometimes all it takes is one little bit of dirt to cause a leaking carb. I fussed over a friends sons leaking carb for an hour. I had asked did you clean the needle and seat, he swore he did. I put a little twist of PT up and a little spec of black something came out and all of a sudden it stopped leaking gas. your re-polish of the seat may have done the same.
At this point I'm not even being cautiously optimistic. These carbs have been in and out so many times now that I've learnt not to expect much until I know I'm in the clear. But one new thing I've learnt is the q-tip in the drill routine (thanks doc). I'd just been manually twizzling the thing in there with a bit of polish - but drill routine definitely gets them cleaner
 

pidjones

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Another thing that I learned with them is to tap on the leaking carb with a screwdriver handle. Guess the needles hang or don't self-center and the tapping is like starting it.
 

doc_rot

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Another thing that I learned with them is to tap on the leaking carb with a screwdriver handle. Guess the needles hang or don't self-center and the tapping is like starting it.
This^^^ I've installed brand new carb kits and had the needles stuck wide open. a couple quick raps with a screw driver and they seat and stop flowing.
 

Jimbonaut

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Well, stopped the leak. Got the carbs properly installed with the airbox (polished all the seats this time with the drill/q-tip), fired it up, no leak.

Next step is to take the bike out again for a ride and see if gets past 45 mins without crapping out.
 

Jimbonaut

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Hasn't happened yet - hoping to get it out tomorrow but forecast is for wall to wall rain. Weather's sucking balls here
 

Maritime

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Ha, I just tracked the CB125 float and they are out for delivery but it is also going to piss rain until Sat so same boat here.
 

Jimbonaut

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Still can't get the bike running right. Better maybe, but still not right. I replaced the floats with new ones - they sit higher in the bowl (when measured against the gasket surface - specs say they should be 15.5mm, these new ones are +1mm higher) but the fuel level in the bowl is lower -

HgQNRg6h.jpg


The clear-tube test is a good comparison too. The old floats -

nzHSi8Yh.jpg


and the new -

gLb1Y9kh.jpg


I've also bypassed the vacuum fuel pump on the carbs and am now running a fuel line direct from the tank to the carb fuel rail. Yesterday the bike seems to run better, definitely a solid idle for the first 35 mins or so around town but a slight drop by the end of the ride, and then pretty rough when I fired the bike up again after sitting for half an hour. Also rechecked the float level and it had risen -

VbSNRJqh.jpg


How in the hell does the level go up? There's no overflow from the tubes. Some kind of back pressure in the fuel line?
 

Jimbonaut

Over 1,000 Posts
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Maybe just gravity, now that the fuel is gravity fed rather than vacuum fed?

Christ it's a good job there are no parties at the moment. Because if there were I'd be no fun at them
 

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