Carb adjustment

[jd]

Just bought a CB750K and finally got it to start up. However the carbs are pooring fuel out the drain tubes. Does anybody have some recomendations on possible cures. I did take the air box off and have not yet checked the float valve adjustment.

[LukeM]

Fuel coming out of the overflow tubes (we're assuming here) means that the float needle valve isn't sealing on the seat, allowing gas to flow all the time.  My guess is there's fuel coming from one or two carbs, and not all 4.  If it IS all 4, then there's some extra pressure in the tank somehow.

Some of the DOHC models have a fuel manifold valve that only opens when the engine is running.  It's a safety device that cuts off fuel to the carbs when the engine stops.  It also keeps one from having to turn off the fuel tank petcock every time.

It's probably best to pull the whole carb bank, disassemble one carb at a time, and check the rubber portion of the float needle valve. It shouldn't have any markings or cuts on it, and should be rather supple and not hard or stiff.  Also, check the pin that holds the float.  It should allow the float to move freely. If the float is hanging up, it may hold the needle valve open.  

I'm not sure what the spec is for float bowl height.  A carb rebuild kit should have a little gauge to set that, but I wouldn't go that far unless something in the carb needs to be replaced.

Sorry for the drone on here: I'm overdosing on House episodes on USA, and I get overly diagnostical after watching that show.  One of my favorites.

Luke M

[Kube]

I've had this problem as well and I just can't track down the problem. I totally replaced the needles, used a qtip and toothpaste to try to clean the seats, pretty much every method suggested on here and I still have fuel leaking out of all 4 drains. I'm interested in what you said about extra pressure in the tank. I don't currently have the tank on the bike as it's very rusty inside and it's much easier to work on with the tank off. Instead I use one of these.

http://www.jcwhitney.com/auxiliary-2-liter-motorcycle-fuel-tank/p2004173.jcwx?filterid=j3

Could I possibly be hanging it too high or something and adding pressure? I'm about at my wits end. If it weren't my dad's old bike and a bit sentimental, I would sell the damn thing and put the money toward something that works.

[LukeM]

It's a gravity feed set up:  the petcock and connecting hose needs to be higher than the input to the carb bank. There shouldn't be any pressure in the tank, unless it's painted black and sits out in the sun, but that should be venting out the tank and not out through the carbs.

However, you mention you're using a different gas tank.  So pressure should not be an issue.

Another problem may be a crack in the brass drain tube.  If it's loose or cracked near where it attaches to the float bowl, that will keep the level artificially low.  They should only drain if the level in the float bowl gets to the top of the drain tube. One way to troubleshoot this is to block the outlet part at the bottom of the carb, and apply pressure to the top of the drain tube (removing the gas first, preferably).  If there's a crack or leak, you'll probably hear it.

I hope this helps.  Keep us in the loop.
Luke M

[toytuff]

Float level is 14 to 15 mm on the 77 cb 750K7. 26 mm on earlier models. (I believe)

tt