Topic: Leaking Carb Overflow

[emerson]

After fighting with other issues, the bike ('73 CB500) was finally in good shape, then it started leaking gas out of the #2 carb overflow. After double checking the float height and if it still floats, I determined it might be the float valve. A buddy of mine at work mentioned that might be the issue, as all o-rings in the carbs were replaced last year. So pulled and cleaned it well, but still no luck.

Then I thought I would give it a shot with the 800 grit lapping compound that I used on my head rebuild. Put a tiny amount in there, and rotated either manually or with a low speed drill (lightly chucked to the valve body).  After cleaning well and re-assembly, no leaks!!! Beats purchasing a whole new valve (those little things are pricey!) 

Just a little FYI I thought I would share...

[Johnie]

I have used MAAS metal polish for the same thing. Good results...

[P_Camps]

Emerson I'm having the exact same issue on the exact same carb!!  I tightened the screw but no luck.... 

I wonder if blue magic liquid metal polish will be ok... as I currently dont have lapping compound (which I've never even heard of before)  lol... what is it?

[Johnie]

I think any light polishing compound would be fine.

[emerson]

It is very inexpensive. You can find different types on mcmaster.com (an excellent hardware store online).

[Shooter]

Have you applied the lapping compound to the entire valve or just the pointy tip where it seats in to the housing?

[Johnie]

The pointy tip & seat. You should not have to do the shaft of the tip unless you think there is some rough edges hanging the float needle up.

[emerson]

Yes, what Johnnie said, the pointy end is the sealing surface that you are attempting to repair. The sides are critical to keep the whole thing aligned when it slides up and down (open and close), so avoid any abrasive on the sides of the needle.

[roadkng48]

Tooth Paste works well for this.

[72hondacb750]

I've purchased lapping compound in pep-boys before .. Call around if you need some you might find it closer to home than you thought. and its cheap.

[Squally]

Mothers Polish works as well as long as you use it on Brass. 

[lucky]

Yes, what Johnnie said, the pointy end is the sealing surface that you are attempting to repair. The sides are critical to keep the whole thing aligned when it slides up and down (open and close), so avoid any abrasive on the sides of the needle.

The tip on the float needle is RUBBER not metal.
Just buy a new float needle .You do NOT need the brass seat.
The rubber float needle tip gets out of round or or hard and does not seat properly.

[emerson]

On my model there they are all metal. I have rebuilt all 4 bowls and there is no rubber tip on the float valves.

[lucky]

On my model there they are all metal. I have rebuilt all 4 bowls and there is no rubber tip on the float valves.

Those all metal old style float needles should be replaced with the rubber tip style needles along with the seats.
The reason why is this...
If you take out the all metal float needle get a magnifying glass and look carefully at the shiny ring around the beveled tip. It is actually a step or groove worn into the beveled face.
It gets that way from engine vibrations.
It will always leak.
That is why Honda went to the rubber tipped float needles and it rubber tip can dampen out the engine vibrations and still do its job of making a complete seal.
I was a mechanic for Honda during the years of metal float needles and slide needles that were not free floating and it caused lots of poorly running motorcycles.

A leaking float needle that is overflowing can burn your house and or garage down if the fuel gets ignited by the hot water heater.

[cookindaddy]

I'm thinking after reading this that there is still a possibility of crud from the gas tank causing the needle valve to stick open and leak gas, no matter how well it is polished or it's condition (if rubber tipped). Probably it would be the case if the leak occurred on different carbs, not just one. You will see after you run for a while.

Possibly you addressed this but it is not clear from the postings. I mean by that the condition of the in-tank filter.

It's what I found when I pulled my filter and checked its condition. The PO had creamed the tank yet left the damaged filter in place. It had a hole in it. With Cream removed (that was difficult) and new filter installed, no more overflows going on three years now.

[lucky]

I'm thinking after reading this that there is still a possibility of crud from the gas tank causing the needle valve to stick open and leak gas, no matter how well it is polished or it's condition (if rubber tipped). Probably it would be the case if the leak occurred on different carbs, not just one. You will see after you run for a while.

Possibly you addressed this but it is not clear from the postings. I mean by that the condition of the in-tank filter.

It's what I found when I pulled my filter and checked its condition. The PO had creamed the tank yet left the damaged filter in place. It had a hole in it. With Cream removed (that was difficult) and new filter installed, no more overflows going on three years now.

Think about this.....

The fuel cannot get into the fuel line until it goes through that very fine screen inside the gas tank.

Then the fuel has to through a paper filter before it can go into the carbs.

Do you really think that something big enough to actually hold open the float valve is getting in there? Even after going through a paper filter? And it would have to stay there.


Do this...
(well vented place). Take a float bowl off . Have a rag handy.
Attach a fuel line or if it is still on the bike gently hold up the floats and turn the fuel on. If the fuel is coming out and you are holding the float up then it is leaking right?
Now let the float drop down for a second and let gas wash out the jet then hold the float up (closed again) . If it is still leaking it is not "crud", it is a bad fit between the float needle and its seat.

That is what those metal float were famous for. leaking when worn.
Look for the shiny ring on the needle.

See what I mean?

[cookindaddy]

Lucky,
I was asking if the in tank filter was in good shape.  It might have a hole in it, or not be present at all. Some don't even know it is there.
Your logic assumes that the in-tank filter is ok. (I don't think that there is a paper filter on a stock 73 CB500).
After valve polish or replace, if that does not solve the problem, I was offering another thing to look at - from my own experience.

[lucky]

OK...Now I understand what you are saying.

I feel that all motorcycles should have a filter between the gas tank and
the carbs.

The screen in the gas tank petcock just keeps out the big stuff.
I understand that many bikes did not come with a filter between
 the gas tank and the carbs when first sold.