Topic: CB500 carbs dumping gas after reinstalling tank

[spinefine]

Hello all,

I'm coming out of lurker mode to ask a question. Hopefully someday I'll have answers for others, but for now I'm a definite newbie. I have a 1972 CB500 with recently rebuilt carbs. It was running fine and the overflow tubes were completely dry.

A few weeks ago I siphoned the gas out of the tank and removed the tank so I could bring it to a shop to have it painted. Since the bike was going to be sitting for a while, I drained the carb bowls using the drain screws.

Tonight I reinstalled the tank, filled it with gas, ran the lines back into the petcock, and turned the petcock from STOP to ON. What happened next was a flood of gas from the bottoms of all 4 carbs as well as the overflow tubes. I didn't start the bike. The leakage stops if I put the petcock back on STOP.

I know there are various issues that cause carbs to overflow, but I'm curious what you guys think in this case, given that the carbs were not leaking before I pulled the tank. I wonder if it's related to the tank being empty for nearly a month and possible particulate matter in there...though there was no apparent issue with that previously.

[harisuluv]

Did you tap the bowls to see if they would unstick?  Sounds low tech I know but this is common, especially if they've been dry awhile.

[bollingball]

Hello all,

I'm coming out of lurker mode to ask a question. Hopefully someday I'll have answers for others, but for now I'm a definite newbie. I have a 1972 CB500 with recently rebuilt carbs. It was running fine and the overflow tubes were completely dry.

A few weeks ago I siphoned the gas out of the tank and removed the tank so I could bring it to a shop to have it painted. Since the bike was going to be sitting for a while, I drained the carb bowls using the drain screws.

Tonight I reinstalled the tank, filled it with gas, ran the lines back into the petcock, and turned the petcock from STOP to ON. What happened next was a flood of gas from the bottoms of all 4 carbs as well as the overflow tubes. I didn't start the bike. The leakage stops if I put the petcock back on STOP.

I know there are various issues that cause carbs to overflow, but I'm curious what you guys think in this case, given that the carbs were not leaking before I pulled the tank. I wonder if it's related to the tank being empty for nearly a month and possible particulate matter in there...though there was no apparent issue with that previously.

Did you tighten them back up?

[harisuluv]

Says they are 500 carbs so if they were loose they would leak out the plugs and not the overflow like pd style.

Real question is are they all hooked up to the drain tubes and hence is possibly only one carb with a stuck float bring interpreted as all four leaking.

[spinefine]

Did you tap the bowls to see if they would unstick?  Sounds low tech I know but this is common, especially if they've been dry awhile.

Thanks--I tapped the bowls as you suggested, using a wrench with a rag wrapped around it. That might have helped a little. Still dumping gas, but it looks like only carb #3 is now leaking from the carb body itself, while carbs 1 and 2 are leaking through their overflow tubes. Carb 4 seems dry. The tube coming out of the airbox is also dripping. Should I bang harder on the carbs, or should I remove the float bowls and loosen the floats by hand?

And yes, I had re-tightened the screws, but that's a perfectly valid question given my level of ability.

[harisuluv]

You seem to be flooding pretty bad. Perhaps when you siphoned the tank some sediment at the bottom got to the carbs or something.

If you have a clean tank now, sounds like those carbs are going to have to come off. You will need to either clean the seats of debris or set the float heights or both. While you're in there clean out those jets and fuel circuits and lines. 

[spinefine]

You seem to be flooding pretty bad. Perhaps when you siphoned the tank some sediment at the bottom got to the carbs or something.

If you have a clean tank now, sounds like those carbs are going to have to come off. You will need to either clean the seats of debris or set the float heights or both. While you're in there clean out those jets and fuel circuits and lines.

I was afraid you were going to say something like that. But thank you for the advice and the reality check.

[spinefine]

Update: After letting the bike sit overnight, I opened the petcock again this morning and the carbs overflowed again, but this time tapping on them (with a hammer and screwdriver handle) stopped the overflow. So perhaps the floats loosened up after sitting in gas overnight? I'm assuming the problem was that they were stuck in the down position, but maybe it was something else.

I also noticed that the petcock itself was leaking around the bowl gasket. Maybe it was dry for long enough when the tank was being repainted that the gasket (which is probably old) degraded somewhat. I tightened the bolt and that seemed to fix it.

In any case, I was able to go for a ride with no overflow. I stopped for an hour, and then when I opened the petcock, carb #1 started overflowing again. So obviously there is some crud in there. I was able to free it up by tapping on the carb body again, but that's not exactly a permanent solution. But hey, neat trick: just hammer away at your carbs when they're stuck. I guess that's why I bought a vintage bike! I'm working up the nerve to pull my carbs for the first time and see if I can clean them out. In the meantime, would it at least help to remove the float bowl and manually push the float up and down to try to free it up a bit? Honestly I haven't yet seen the inside of a carb, so I'm not sure what I'll find.

[73cb500]

also ive found that if the float valves are not springy and kinda plush when the floats fall down that can give problems (assuming there new it would more than likely be trash in the seats. these carbs are also very very float height sensitive however I agree with everyone else pull those carbs I know ive been there and its not fun:(

[73cb500]

Update: After letting the bike sit overnight, I opened the petcock again this morning and the carbs overflowed again, but this time tapping on them (with a hammer and screwdriver handle) stopped the overflow. So perhaps the floats loosened up after sitting in gas overnight? I'm assuming the problem was that they were stuck in the down position, but maybe it was something else.

I also noticed that the petcock itself was leaking around the bowl gasket. Maybe it was dry for long enough when the tank was being repainted that the gasket (which is probably old) degraded somewhat. I tightened the bolt and that seemed to fix it.

In any case, I was able to go for a ride with no overflow. I stopped for an hour, and then when I opened the petcock, carb #1 started overflowing again. So obviously there is some crud in there. I was able to free it up by tapping on the carb body again, but that's not exactly a permanent solution. But hey, neat trick: just hammer away at your carbs when they're stuck. I guess that's why I bought a vintage bike! I'm working up the nerve to pull my carbs for the first time and see if I can clean them out. In the meantime, would it at least help to remove the float bowl and manually push the float up and down to try to free it up a bit? Honestly I haven't yet seen the inside of a carb, so I'm not sure what I'll find.

a hair dryer or boiling the boots will make putting back on easier just some stuff I learned when I rebuilt mine was DONT forget about those emulsion tubes I used a wooden chop stick to push em out and also take note on the condition of the main jet towers for pits or chips and always a good idea to also check your floats in a bowl to see if they float. my number 3 carb was leaking on me and I checked it and checked it taking carbs off 4 times tearing them down finally got some fourm tips the brass tube in the bottom of the bowl was split I used clear finger nail polish got the idea off a lawn mower repair site also this stuff called SEAL ALL in a yellow tube is good stuff just let it dry good

[73cb500]

Update: After letting the bike sit overnight, I opened the petcock again this morning and the carbs overflowed again, but this time tapping on them (with a hammer and screwdriver handle) stopped the overflow. So perhaps the floats loosened up after sitting in gas overnight? I'm assuming the problem was that they were stuck in the down position, but maybe it was something else.

I also noticed that the petcock itself was leaking around the bowl gasket. Maybe it was dry for long enough when the tank was being repainted that the gasket (which is probably old) degraded somewhat. I tightened the bolt and that seemed to fix it.

In any case, I was able to go for a ride with no overflow. I stopped for an hour, and then when I opened the petcock, carb #1 started overflowing again. So obviously there is some crud in there. I was able to free it up by tapping on the carb body again, but that's not exactly a permanent solution. But hey, neat trick: just hammer away at your carbs when they're stuck. I guess that's why I bought a vintage bike! I'm working up the nerve to pull my carbs for the first time and see if I can clean them out. In the meantime, would it at least help to remove the float bowl and manually push the float up and down to try to free it up a bit? Honestly I haven't yet seen the inside of a carb, so I'm not sure what I'll find.

a hair dryer or boiling the boots will make putting back on easier just some stuff I learned when I rebuilt mine was DONT forget about those emulsion tubes I used a wooden chop stick to push em out and also take note on the condition of the main jet towers for pits or chips and always a good idea to also check your floats in a bowl to see if they float. my number 3 carb was leaking on me and I checked it and checked it taking carbs off 4 times tearing them down finally got some fourm tips the brass tube in the bottom of the bowl was split I used clear finger nail polish got the idea off a lawn mower repair site also this stuff called SEAL ALL in a yellow tube is good stuff just let it dry good

one more tip a manual will be a live saver get it read it cover to cover any thing you do will be in that book some things are a bit unclear but just keep reading I still read in mine every day as I too am new to the cb500 four those carbs will seem very intimidating but once you get them off they are not so bad if you tear em down to rebuild them and clean open the slides with the idle screw and bench sync all four slides with a small drill bit using the smooth end that will get you very close the gauges to fine tune if you don't have the gauges there is some nice manometer instruction online

[mono]

I'd say that if it's running fine and the overflow is your only issue, then just:

-clean the float needles to make sure they're traveling properly
-clean the float pins and float hinges to make sure they can move freely
-set the float heights to spec
-replace the float bowl gaskets too perhaps.

Because if it's running fine and you're not seeing any sediment in the float bowls, there's no need to go tearing your carbs all apart.  maybe pull the jets and make sure they're extra clean, but again if it ain't broke don't fix it.