Topic: 1981 Honda CB650C dumping fuel, won't start

[frenchy_CB650C]

I got a 1981 CB650C that had sat for over 10 years. First thing I did was pull the carbs out and clean them very well. Split, boiled, brushed out all varnish from the jets, seat, etc replaced all gaskets, floats and needles. The issue i'm having is that when I try to start it, it sputters a bit and won't start. After trying to start a few times, it ends up starting to spew fuel out of the air boot drain. I've taken the carbs out several times to inspect and make sure I haven't missed anything, but they remain clean as a whistle. Any thoughts on what I may have missed?

[flybox1]

verify your carb floats are moving freely, that they are set at the proper level, and that your float valve actually shuts off fuel when depressed.

[frenchy_CB650C]

I replaced the floats and needles and made sure they were moving smooth. They aren't adjustable floats on these carbs, so I'm not sure how I could set the level. I'll take the carbs off the engine, remove bowls and see if the valves are all closing off fuel flow. Thanks for the reply

[flybox1]

your floats should be adjustable...these are.

[frenchy_CB650C]

those aren't the type for these carbs. these carbs have the plastic non-adjustable floats and valves without the hanger wire

[kkelley515]

Any resolution on your issue? My '81 CB650C is having the same problem. Cleaned the carbs thoroughly, replacing the float valves, main jets, needles, and gaskets. It's still leaking fuel back into the air box. (Additional complications from a petcock that doesn't actually turn off...new petcock has been ordered.)

[frenchy_CB650C]

I have yet to confirm, but as far as i can tell, the sync was way off. I've since bench sync'd them. I have a new kit coming today. Once i have that done and the carbs back on, i'll update on the result.

[pmanning]

Check the O-rings on the "T" fuel inlet tubes between the carburetors.  When dry and cracked they with empty your tank onto your crankcase.

[frenchy_CB650C]

Any resolution on your issue? My '81 CB650C is having the same problem. Cleaned the carbs thoroughly, replacing the float valves, main jets, needles, and gaskets. It's still leaking fuel back into the air box. (Additional complications from a petcock that doesn't actually turn off...new petcock has been ordered.)

Did you end up getting yours fixed? I'm having mine sonic cleaned and rebuilt. Hopefully that will work. Maybe they will see something I didn't.

[frenchy_CB650C]

So they've been ultrasonic cleaned and they still leak. I think that I've sourced the problem to a float needle. I had bought a couple of carb kits and it seems that the needles aren't the same as the original ones. The one that is leaking is not an original (don't have all four original) so I ordered one from eBay in the hopes this will solve the problem. For background, I've even swapped all four carb bodies and had the same issue. Will post back with results when I have them.

[ekpent]

  In a perfect world the carbs overflow out some overflow tubes and not back out into the airbox or engine. Check that your vent tubes on the carbs are clear as well as the overflow tubes if your carbs have them as they act as vent also.  Hopefully your new needles will do the trick also.

[frenchy_CB650C]

  In a perfect world the carbs overflow out some overflow tubes and not back out into the airbox or engine. Check that your vent tubes on the carbs are clear as well as the overflow tubes if your carbs have them as they act as vent also.  Hopefully your new needles will do the trick also.

For sure. We separated the whole rack to clean and ensured all the passages were clear. It would be nice if these had overflow stacks like other carb models!

[frenchy_CB650C]

Changed float valve again and it's still leaking. How is it that these carbs leak back up through the jets? I don't understand what's going on here....

[frenchy_CB650C]

So are there any vacuum related possible issues here? The floats appear to be stopping fuel from going in the bowls, so is it possible that the fuel is not stopping and just backing up out the jets?

[JeffK]

Check the O-rings on the "T" fuel inlet tubes between the carburetors.  When dry and cracked they with empty your tank onto your crankcase.

Sorry to hijack this post

Hey PManning, I had a situation a couple of weeks ago where I left the gas on reserve and the bike sat for just under two weeks. The crankcase filled up with gas like you described above. I just rebuilt the carbs this winter and did not replace those o-rings. I did inspect them and they looked good but they are about fifty years old now. I have never seen gas in my oil before. The only instance is when I left the gas on a couple of weeks ago. Is that how the gas got in my crankcase or are there different ways? I'm just looking for an opinion.

Thanks,

Jeff

[Eddiev]

Hay there, do you notice the fuel leak all the time or just after you crank the engine a few times? Can you get the bike started at all?

If it's leaking fuel all the time, I would suggest first taking a look at your fuel level in the bowl. Take some clear tubing (I just went to home depot and got a few feet), hook them up to the drain, open it up and do a visual comparison to the carb. Make sure your bike is fully level before starting.

[frenchy_CB650C]

I figured it out today. The rubber diaphragm in the accelerator pump that has the "ears" that go over those little holes was just slightly off. I reassembled it properly and boom, leak is gone. What an ordeal. Here's a pic of the part: https://drive.google.com/file/d/144bPhpZIV-LW2gp9aQ-iSrwCEUPOu0VY/view?usp=sharing