Carb is leaking from overflow.... Why?

[FastCletus]

my # 1 carb (drivers left) is leaking but not from seal, but from the over flow valve?  I just rebuilt them and it was my first rebuild.  took lots of time and felt I put all things together right.  Apparently not.  Before I take the entire bank of carbs back off bike is there anyway of letting me know what I should be looking for? None of the other carbs leaked.  

Also is there a guide that shows me what each part on a carb is called?  

[alehman]

You probably have either a sticky float, cracked overflow tube (on the portion inside of the bowl), needle valve not seating correctly (which is unlikely because you have rubber needle valves), or a drain screw not seating correctly. Its also possible the float is leaking and takes on gasoline, so it sinks.

[alehman]

Sorry, I should go more into specifics. I had this same issue. If it's a sticky float, debris is keeping the float from moving freely. Whacking the bowl of the problem carb with a non-metal object (to prevent sparks) can help dislodge it, and cleaning the carbs thoroughly is a more permanent solution. Debris can also cause the needle valve to not seat correctly, and whacking the bowl can sometimes help that too. I've heard that varnished gas and filth can build up on the needle valve seats, so what I did was give the seats a quick polish with a q-tip and toothepaste (you can use fine abrasive too).

If the drain screw isn't seating correctly, take the bowl off and clear out the debris with compressed air. I'd be gentle with physical cleaning tools because that threading is aluminum and probably pretty delicate.

What ended up being my problem were tiny splits in the brass sections of the bowl's drain tubes, which really do require magnifying glasses and good light to see. You can also test for them by filling the removed bowl with gas, connecting a rubber drain tube to the drain plug on the bottom, covering the opening of the brass tube with your finger, and blowing into the tube (holding it above the gas surface, of course). If there are bubbles, you have a leak. Testing by seeing if water leaks out of the bowl, which is what I did because it was less messy and available right out of the tap, won't work because it won't seep through the cracks like gas can, so don't try it. If you find cracks, you can braze or JB weld them shut. I'd recommend brazing because I don't think JB weld is gas-proof in the long run.

You can also test to see if your floats are leaking by a simple test of submerging them in gasoline overnight and seeing if they take on gas. Pretty straightforward.

Good luck!

[Lucky 1]

You cannot polish your way to a good carb rebuild!!!

You must replace the float needless.

The rubber tipped ones get out of round and distorted. The rubber tip is not a perfect cone.

You can look at them with a magnifying glass.

You MUST replace the float needles.

[icerigger]

Yeah, 30 year old rubber doesn't flex much. Would you trust  a 30 year old condom?

[FastCletus]

My rebuild kit didnt come with rubber needles.  metal ones are what I put in.
 I just took the bank off the bike and opened up carb 1.  float seemed ok and cant see any cracks or movement on the overflow tube.  with as fast as it was flowing out I would of seem a crack I believe.  the screw at the bottom of the bowl.  what it is this screw for.  it has a rubber grommet on it.  is this screw just used for draining carbs

[Lucky 1]

Yes the screw on the bottom of the bowl is to drain the bowl.

If you take the float bowl OFF then you can fill it with gas and see if the drain screw is causing
the leak OR if it is the overflow tube.


While you have the float bowl off you can turn on the gas while
holding the float up with your finger and see if it leaks.

BTW if you replaced the float needles and not the
worn out seats all bets are off.

If the previous needles WERE rubber tipped
and you put in metal needles into the old seats that
will also not work because the angles are different.

[Lucky 1]

Were the previous old needles rubber tipped?

[FastCletus]

previous needle were not rubber tipped.  How do I replace the seats?  I couldnt get them out

[Lucky 1]

I see the problem.
All of the parts diagrams show rubber tip needles for the 1980 model.

Go to Parts-N-More web page and you will see what I mean.

When you have rubber tipped needles you do not need to replace the seats.
The metal tipped needles in a seat designed for rubber tipped needles is a parts mismatch.

Do you have a 1980 CB750?
Are they 1980 CB750 carbs?

[FastCletus]

I see now.  I think i found my nemesis, and I learned 2 things.

1.  The tips are rubber
2. always look close at needles before insertring them.  I apparently missed this

[FastCletus]

One on the right is the new one

[FastCletus]

well it seems that this one isnt going to work.  i pulled the rubber excess off the tip but i dont hink its going to seat.  and now for some reason i cant get the gasket seal to work.  think I am just going to let the manufacturer know and have them send me a new one.  all of the others carbs seem to be fine.  its just this one.  will keep ya'll posted

[Piute]

   Talking to yourself again
 
              90% of my post go unanswered cause the RIGHT One
       will come from a person that rides and own a CB 750 F2 Super Sport
       There is not 1 on this sight that post,So my build (MEMBERS BUILD)
            is for my records of dates & remember better what was done,
              Saves space on my putter with pic's here also,

[Lucky 1]

He was just thanking me for finding the problem.