Topic: Weird carb leak CB500

[Honda-Rog]

Found carb #2 dripping when the petcock was off, so assumed float was stuck - repeated banging on float chamber failed to solve it. Assumed float valve might be dirty or worn, so ripped carbs out, checked for dirt, blew out, couldn't see wear on the float needle, but swopped the needle with carb #1 to make it easier to access without removing the carbs. Checked carb 1 for dirt as well.

I have recently set the float heights correctly, bike has been running very well as a result.

Reassembled carbs, ran bike briefly, and turned fuel tap off. Very slight seep from carb 1, then after a few minutes a sudden little spurt from #2. As I say, weird!

So now I don't know what's what - should I replace both float valve assemblies, or is there another (cheaper) explanation?

Also having trouble with the petcock not closing off the outlets properly when off, despite fitting new gasket (the one with four holes in it). Any thoughts on that one? It doesn't help the carb issue when there's a constant trickle of fuel down the tubes.

Cheers everyone, must be worth the hassle eh, to own a HONDA? 

Rog
UK

[flybox1]

two ways to find out the offending part.

#1
take the offending bowl off, and its overflow tube.
fill it to the brim with water.
set it somewhere with a catch under the overflow tube
come back in an hour and see if the water has leaked out.
if it has, you have a cracked overflow pipe.
if not, the leak is from excess gas in the bowl probably from a bad/blocked float valve.

#2 - clear tube method (Lucky...this has nothing to do with float height setting  )
depending on your carbs, attach the clear tube to the overflow pipe, or insert it in the drain screw.
look at the fuel height  Height will show you where to look.
if the height is 3-4 mm below the carb bowl top, you have a crack in your overflow standpipe
if your fuel level is nearer the top of the bowl, you have a bad/blocked float valve

[Duanob]

Just a suggestion since you have two petcock drains, one flows to the #1-2 carbs and the other fows to the #3-4 carbs. You probably have crap and crud in the lines getting to the float valves. It only takes very tiny particals to hold the float valve open enough to overfill the bowl. Said particals can also make their way into the pilot jets and plug them up as well. It's enough to drive you nuts! I try and drain all gas from my tank and clean it as a winter maintenance routine.

[Honda-Rog]

Wow! Swift replies, thanks guys, I will follow up on these asap. I'd read about cracked overflow pipes, but why should one suddenly crack, when it's been left well alone?

Will update when further tests are done.

[flybox1]

repeated hot/cold cycles expanding hairline fractures induced during install maybe.. just a guess 
easily remedied with a brass sleeve over the whole pipe or a steady soldering hand

[Honda-Rog]

 . . . and I think my petcock issue may be to do with the fibre washers on the screws, as I don't recall seeing them.

[mickwinf]

Yes those washers will allow a leak, they are special size and very expensive, generic fibre washers don't seem to work.

[2wheels]

. . . and I think my petcock issue may be to do with the fibre washers on the screws, as I don't recall seeing them.

You can take your old petcock seal (the one with 4 holes) and using a small wire cutter cut out 2 new replacement for the fiber washers.  It is easy and quick way to find out it they are the problem.

[Honda-Rog]

I was about to buy float valves and seats for the carbs, then when I came across the fibre washer issue on the petcock, went to my local car spares shop, who shoved a tray full of 'em at me and said 'take what you want'. Found four looking about the right size, and they did the trick - no leaks anywhere, including the carbs. Cost - £0.

So once again thanks to those many folk on this forum who've posted their solutions and ideas - I just wish you weren't so far away.

Now, I have a small vacuum leak somewhere . . . here we go again . . . 

[dave500]

ive used copper washers in those fuel taps no worries,new seal kits have trouble if the valve lever sealing plate is pitted/corroded.