Topic: Quick carburetor questions for 76 CB750 - PROBLEM SOLVED

[cshanek]

I have what is probably a dumb carburetor question. I recently rebuilt my OEM carbs by scrapping pieces from 2 other sets and making a pristine, clean set. I set them up in the garage last night to test for leaks and after all carbs/bowl filled up #3 and eventually #4 began leaking out of the drain tube. Now on to the dumb questions ...

• This should not be an expected behavior correct? My assumption is that this occurs when the float sticks.
• If my assumption above is true why do the drain tubes exist? Just for when things go wrong?
• Other than cleaning out jets and adjusting floats, what can I do to prevent this drainage?

Lastly I have looked at a few options for treating the rock hard carb boots, and one of the appealing ones was to soak them in heated break fluid. It certainly seems to soften them a little but I am afraid they will age or crack quickly because of this treatment. Any thoughts?

[ekpent]

 Swap the two inside carb bowls with each other so the drain plug will be on the correct side. As for the overflow leak in a perfect world with perfect parts it would not do that. There is a reason Honda put a petcock that needs to be turned off when not in use though. If the leak did not happen quickly you may be fine when running. Could be the float needle and seat not mating well,floats set wrong,Float hanging up and jammed or even stuck on gasket,plugged vent etc.. As for your hard rubber manifolds people have used wintergreen oil and other concoctions. I use a heat gun and heat them up well so they are pliable and easier to work with. Tighten the clamps while they are still warm
   Good Luck on the project.
  PS- There are rubber vent lines that attach to the two nipples on the upper part of the inside carbs incase none are in your parts stash.

[BenelliSEI]

Another good option is to put the boots in boiling water for a few minutes, then be quick on the install. If the6 are really hard, do yourself a favour: replace them.

[cshanek]

Easiest to put on head first then mount carbs?

[ekpent]

 That's how I do it and they have to be at just the correct angle for the carbs to line up. I little lube on the rubber manifolds helps mounting boots and carbs also.

[BenelliSEI]

+1 to on the head first. Also, note that rotating the manifolds one way or the other (a very small amount) can slightly lift or lower the angle of the carbs. If your airbox does not line up with the mounting tabs on the frame, have a close look and make the small rotational adjustment....

[754]

If your overflow tubes are cracked, it can leak gas.

[pjlogue]

What 754 said.  Also, you said you mixed and matched stuff from two sets of carbs.  The fuel valve needles and seats tend to wear together as a set over time.  Very often mixing these with different seats will cause leaks.  Another thing that can happen is the brass sheet metal edge of the float can wear a grove into the float pivot pin post causing the float to hang up and the fuel valve to not seat.

Carefully check your brass overflow tubes in the float bowl for cracks as 754 said and if you did mix the fuel valves and seats get new ones.  OEM is better than aftermarket but they are not cheap.  Check your float posts for groves/burrs.

-P.

[cshanek]

Lots of great info here ... I will take a look later in the week. If I do have a crack I should be able to swap a carb out since I have a total of 2.75 sets.  I tried mounting the carbs before the post, with rock hard boots, and did notice that rotating them one way or another was absolutely necessary. Once I did get everything lined up I figured I might have my wife help me coordinate rubber mallet taps on each side to get them on.

[BenelliSEI]

Easy on the mallet..... you shouldn’t need that much and things can break with the shock. I usually use a paint stripper heat gun on the boots, once they are on the head. Keep moving across, under, over and around. When they are just barely too hot to touch, the carbs will pop in easily.

[ekpent]

Easy on the mallet..... you shouldn’t need that much and things can break with the shock. I usually use a paint stripper heat gun on the boots, once they are on the head. Keep moving across, under, over and around. When they are just barely too hot to touch, the carbs will pop in easily.

+1 - Great minds think alike as I suggested that to him also in an earlier post. Use the mallet for something else and use a good pair of leather gloves to protect your knuckles when wiggling them on.

[HondaMan]

Those overflow tubes are just the right height to prevent raw gas from overflowing up the mainjet and into the engine if the bowls' float valves stick. If they are leaking from there, they are doing their job!

[cshanek]

A late update, I had another go at the carbs this weekend since I was in an all out blitz to get everything done and get the bike together. #2 would overflow out of the drain tube a good minute or even two after I turned on the petcock. I decided to get a little creative to make a better working environment (See attached pic), which allowed me to flip everything over safely to inspect bowls and floats and clean. I tested all 4 bowls by filling them with fuel to the lip and they were all fine. I then went back to spraying carb cleaner in all of the ports and used my cleaning files. I re-assembled everything, test again, and not a drop of fuel leaked (pictures shows a carb cleaner puddle that was left from flipping them back over).