Topic: CB350/F Carb Overflow

[grapppa]

Now that I have the 550/4 working nicely, I've turned my attention to restoring the 350/4.  Over the winter I rebuilt the carbs completely as the bike had been sitting for 6-7 years. Did the usual routine renewing all the internal seals after thorough cleaning.  I hooked up the carbs to my external tank and after they filled up for 10 sec or so, they started gushing gas all over the place from at least 2 carbs.  The gas came up from the main/aux jets and even had enough force to come out of the vent tubes.  So I took them back apart to examine the float needle and seat.  The float heights were uniformly 21mm to spec for the 350.  I sunk the floats in gas to see if they were leaking - nope. The needles are in very good shape. The seats are clean and the internal brass surfaces are clean and sharp.  I replaced 1 that looked tired with a 550 seat.  The parts I believe are interchangable. Reassembled and got same result.

I suspect the needle/seat interface for leaking.   I cant understand how they could be leaking so profusely.  How can I test them for leaking when removed?   Should the needles be "lapped" in to give more surface area for the needle to seal?

Many thanks members for your time.

[pwright]

Don't forget to check the brass standpipes for cracks. I had two on my 350F that had very tiny cracks in them, and they'd overflow quite a bit. However, the overflowing only happened when hooked up to the tank with the pressure of 3 gallons of gas on them. You can take the bowls off, fill them up and see if they leak... I fixed mine with JB weld and haven't seen a drop since...

Later,
-Pat-

[grapppa]

They are not only overflowing from below but back up through the idle circuit and up the 2 vent tubes.  One thing I did not mention, but may be relevant, is that the float pins are loose in their mounts.  I am going to remove the bowls again and see if any of the float pins came out - but I dont think this is possible.  What I might do, which I did with the 550, is to peen one end of the pin to create a snug fit.

[Jay B]

The float pins shouldn't be able to come out, there are tabs cast into the bowls to hold them in place when assembled. The floats must be free to pivot with no resistance. One thing that really helped mine was to polish the tang on the float assembly where it contacts the springy pin on the needle. I used Mothers Mag Wheel polish per somebody's instruction on this board, it didn't cure things 100% but they hardly ever dribble now. Also I've found when filling empty carbs for the first time it's easy for the floats to stick. A couple taps on the float bowl usually stops them.

[TwoTired]

Just a suggestion.
You might want to see it is on operation as to why it is failing.
Take off one of the outer bowls, hold the float up with your hand, turn on the fuel petcock and and see if the the raised float will ever turn off the gas flow.  You should be able to manually manipulate the float to observe its operation.
This can be messy, but it can help diagnose the problem source.

Cheers,

[grapppa]

Two Tired, I took your advice and suspended the carbs up in the air using string, hooked up the gas and let it flow.  Each float needle sealed properly.  While doing this I found what the problem is... when the floats are fully extended down the float needle can get stuck against the float because it can come out of its housing partially.  Just enough to be a problem.  So what I did was set the floats so they dont bind up and refilled the carbs with enough gas to keep the floats from bottoming out. Guess work mind you as you cant see whats going on inside.   Put em back on and not a drip   Thanks for the idea!