carburetor problems

[duluth dog]

I had bought a '77 cb750k last fall as a project and have since gotten it cleaned up and running but it hasn't been quite right.  Im sure there's something off with the carbs but I don't have enough experience with bikes to know exactly whats wrong.  I was hoping some of you on here might be able to diagnose it.  so the problems are as follows: I almost always have to run it at full choke, if its warm I can take it off choke but it will make a loud sharp popping noise from the exhaust.  it has also been leaking some fuel, it looks to be coming out of the overflow, and Ive only seen it after I get done running it, although its hard to look down there while driving.  and lastly if Im at a cruising speed, say about 40, and try to accelerate quickly the engine will lose all power and die unless I let off the throttle.  The guy I bought if from had recently moved from the mountains in colorado to minnesota where I am, so I was thinking the fuel to air ratio may be off now that its at a lower altitude. any hints, tips, or suggestions?

[TOOLS1]

It sounds like you have a vacuum leak, and your carbs are dirty. You need to remove them, and do a through cleaning.
TOOLS

[Re-run]

I agree on the running lean. Maybe vacuum leaks but a really dirty pilot will act much the same.

Either way, it is pulling off the carbs and removing the idle jets and cleaning them and the rest of the idle circuit.

[gary c.]

Most of the time that a bike has to use the choke to keep running is due to the low speed jets plugged or partially plugged. Also bikes as old as yours, the rubber insolators (intake boots) get too hard and will not seal. When you clean the carbs you will want to clear the low speed jets. It is best to soak them in a carb cleaner then try to dislodge any thing in the jet with a brush bristle or something stiff enough to remove what is clogging the jet. You never want to use a wire or anything hard enough that can wollow out the jet . When you clean the jet you will notice the opening gets smaller inside the jet. That is the metering portion of the jet. Prior to that, this bigger opening can have a build up of residue.  If this residue is not removed it can dislodge with fresh gas and reclog the jet. Then you have to do it all over again. You will want to spray carb cleaner through all the passages. WEAR SAFETY GLASSES you will definatly get a direct shot in  eyeball if you dont. Let me tell you, that sucks. If you do get carb cleaner in your eye dont close it. It will burn worse. Take your dry clean fingers a hold the eye open. The cleaner in your eye will evaporate fast and it will burn less. In a minute or so you will be ok. If it still burns after 3 or 4 minutes rinse it with water. Most of the time it will evaporate  if you hold the eye open.  After you have the carbs clean and ready to assemble you will want to have a manual with specs on setting the float level. These old bikes can be tough to dial in. I am having problems with our CBX right now. The bike runs perfect in every aspect except the idle is real choppy. The carbs are  in perfect sync.  I either have a float sticking just enough to go rich but not leak. Or a wollowed out low speed jet or a worn main jet needle allowing fuel through the main jet at an idle. Take your time be patient with yourself. Try to put the jets in where they came from. You may need to switch them around to troubleshoot a problem later. Example lets say the # 2 carb is excessivly rich. If it is a wollowed out low speed jet. When you move the lowspeed jets to another carb and the problem moves with it then you know the jet has been wollowed out. If the problem doesnt move with the jet them you know it is something else. Useing what you have to troubleshoot is alot cheaper than just buying parts. Sorry if this is so long but there are so many veribles when dealing with carburation. Thanks, Gary C.

[duluth dog]

thanks for all the ideas, ill be working on it the next few days, and ill be hoping for the best.  also ive been thinking about cutting down the muffler or buying another aftermarket.  my question is will i have to adjust anything significantly if i do this? i have an old honda ct70 and i know back pressure in those little two strokes is important, is it as important in a larger 4 stroke engine?

[Re-run]

Technically, anything can clean the jets as long as it fits. I myself bought a carb jet cleaner with the honda name on it. It has a variety of sizes for different jets. The primary key is not going all roto router on the jets and damaging the venturi inside the jet.

It is easiest to pull the jets out when cleaning them. Just wrap the jet in a bit of cloth or rubber(helps with grip), grab with pliers and twist while pulling. I do not recommend wiggling the jet as that could crack the housing for the jet.

As part of this, turn in the idle mix screws while counting how many turns it took for each screw. Lightly seat them and then pull them out with their associated washers, orings, and springs. These passages need cleaning too. Remove the accel pump too and make sure both ears in the diaphragm are clear and the passages too.

When I talk of cleaning carbs, I mean clean enough that you would drink from them.