Topic: 1st CB need some help please!

[MX267]

Hi everyone I'm new to this forum and the CB world. I just bought a 1972 CB500 and am having some problems with it and have come to a dead end. It starts and runs great for about 15 to 20 minutes. After that it starts to cut out and sounds like it's only running on 1 cylnder and has hardly any power at all. And if you let it die, like it wants to, it takes about 15 minutes to get it started again, and then still runs like crap until it sits for a couple hours. I replaced plug wires, cleaned the tank of rust, installed a brand new petcock, and fixed the main fuse under the seat that was rigged together. Any ideas would be greatly appreciated, thanks for taking the time to read this.

[Retro Rocket]

The breather on the cap could be blocked, it creates a vacuum when blocked and fuel delivery dries up,i would try fixing that first...

Mick

[curtis]

it sounds like that could be it i had a yamaha that did that same thing replaced allmost every thing else before checking the simplist thing open the gas cap and run it see if it still dies if not thats your problem

[Lavis500]

+1 Mick, Curtis

Easy way to test that is just let it try to run with the gas cap open.

Then I'd check the fuel lines.  Make sure that they are the right 5.5mm size, because if they aren't then your gas tank can't deliver fuel to your carbs as fast as they consume it.  Therefore, your bowls are underfilled, making your bike stutter, cough, and die.  Even if they are the right size, make sure they are clean and not clogged.

Also, how did you clean the tank?  If you didn't coat it then I'd poke around with a flashlight and a tiny inspection mirror to make sure you haven't had it rust back up on you.  Too easy, and it can't hurt, right?

I'd also suggest going through the carbs and syncing them, but only if either the bike has been sitting for a long time or if your problems persist. This is of course just my $.02, of course.  What do I know?  My bike is still in a million pieces in baggies on shelves!

[MX267]

Thanks a lot guys for the input, I filled it up with good 92 octane gas tonight and hammered on it down the road a few times trying to get it hot (I know it sounds corny but when it's hot is usually when it acts up). And it ran fine, I'm deffenitley going to look into everything you guys suggested, thanks a bunch for the ideas and if anybody else has any keep them coming! I want to fully restore this CB and am stuck on this problem.

[TwoTired]

Look at the spark plugs.  If they are sooty, then check the air filter. Replace.
Use new plugs and restart the troubleshooting.

It need a complete tune up every 3000 miles, btw.  If the last one is unknown, it needs it now.

Cheers,

[zanorak]

I have the same bike and had the same problem. I got 10 minutes from home and it started to act lean - so I put the choke on knowing it had undersized primaries - and it gradually ran crappier until it died. Found out that the pilot jets were 7mm too short - must have been the wrong kit or something. Maybe your carbs have the wrong pilot? The ones that were in there were 21mm long, while the 424-24 jets that it has been running (really well) on are 28mm! The floats would empty a bit and then the pilots wouldn't touch the fuel supply - overheat and die. Then wait for it to cool off. Same as you. Just sayin' you might check it just in case it's a common mistake at the parts counter.

You came to the right forum though! These guys know EVERYTHING about old CB's (and probably a lot of other stuff and if they don't they'll figure it out. Im just a greenhorn.