Problem with front brakes and maybe carbs

[sorny]

Ok, I cleaned the carbs on my 81 CB 750 and got it running (wouldn't even fire a bit before) and now it fires up pretty quick (second or two usually) and runs pretty smooth. After idling a few minutes with the choke it usually idles by itself (a little high tho, I need to adjust it). Right now I don't have the airbox on because I can't ride it anyways and one of the rubber boots connecting the airbox to a carb just kind of broke apart in my hands. But anyways, runs good until I crank it WOT and it just hits about 3500RPM and stops. Like its hitting a rev limiter, not popping or jumping around, just kind of stopps reving up. I sprayed some carb cleaner in the carbs while I was revving it up (so it wouldn't die) and when I done that I could get it to rev way pay 3500RPM, about to redline! So that makes me think its starving for fuel, but would carb cleaner burn that good?! I didn't think it would burn good enough to make up for lost fuel from 3500RPM +.

The other problem I'm having (and the BIG thing w/o being able to ride) is that the front brake is out. It's hydraulic and has fluid in it but when you pull the lever it goes back to the grip w/o any resistance. I'm thinking master cylinder, but it hasn't leaked a drop. I opened the bleed screw all the way and when I pulled the lever it spurted a little bit out, a few drops. So Now I'm confused. If it was the master cylinder, wouldn't it leak or at least not let there be ANY pressure (not even pushing a few dribbles out the bleed screw?

I appreciate everyones help. I got a manual in the mail today (Clymers)and its pretty good, but only when you KNOW whats wrong, not troubleshooting.

[sanglant]

Ok, I cleaned the carbs on my 81 CB 750 and got it running (wouldn't even fire a bit before) and now it fires up pretty quick (second or two usually) and runs pretty smooth. After idling a few minutes with the choke it usually idles by itself (a little high tho, I need to adjust it). Right now I don't have the airbox on because I can't ride it anyways and one of the rubber boots connecting the airbox to a carb just kind of broke apart in my hands. But anyways, runs good until I crank it WOT and it just hits about 3500RPM and stops. Like its hitting a rev limiter, not popping or jumping around, just kind of stopps reving up. I sprayed some carb cleaner in the carbs while I was revving it up (so it wouldn't die) and when I done that I could get it to rev way pay 3500RPM, about to redline! So that makes me think its starving for fuel, but would carb cleaner burn that good?! I didn't think it would burn good enough to make up for lost fuel from 3500RPM +.

The other problem I'm having (and the BIG thing w/o being able to ride) is that the front brake is out. It's hydraulic and has fluid in it but when you pull the lever it goes back to the grip w/o any resistance. I'm thinking master cylinder, but it hasn't leaked a drop. I opened the bleed screw all the way and when I pulled the lever it spurted a little bit out, a few drops. So Now I'm confused. If it was the master cylinder, wouldn't it leak or at least not let there be ANY pressure (not even pushing a few dribbles out the bleed screw?

I appreciate everyones help. I got a manual in the mail today (Clymers)and its pretty good, but only when you KNOW whats wrong, not troubleshooting.


On the brakes, assuming it's in otherwise good shape, it's simply air in the system. Bleed the whole thing, from the master down to the caliper, working over each banjo fitting. A trick to this is to take the whole system off the bike and then invert it so that the caliper bleed screw is the highest thing in the system. That will encourage the air to go to the top when you bleed. Keep bleeding and keep bleeding. Tap all the joints and wiggle the hoses. Bleed it some more. You will get pressure pretty quick this way, but it still takes some work to completley purge the system. A vacuum bleeder is nicer, but I'm assuming you don't have one of those or you wouldn't be squirting fluid out the bleed screw.

For the carbs, I'd bet the slides are sticking or you have a vacuum leak somewhere (like a torn diaphragm). Fire the engine and open the throttle all the way. Look down the carb throats (a dental mirror arrangement is priceless) and see if the slides are pulling all the way up. I doubt that they are.

[sorny]

Here's a quick question about the carbs; when the engine is idling should the slides be jumping up and down with the vacuum of induction? Because they're bouncing around like crazy, right along with the revolutions of the motor. I've never seen one work before so I wouldn't sure if thats how it should be or not. Makes sense since it'd plug the jet and keep fuel from coming in when that valve on that cylinder is closed.

And on the brakes, I didn't put a hose on the bleed screw and in a jar of brake fluid because I didn't think it could be as simple as air in the lines (with my luck ;) ). When I just opened the bleed screw and pumped and pumped the lever and nothing much ever happened, should I be surprised? Wouldn't it still pull the brake fluid through the resevoir and right out the bleed screw?

[LoDownSinner]

If you just opened the bleeder screw and pumped the lever repeatedly, you probably introduced more air into the system. Sanglant hit the nail right on the head with the procedure. plan on taking some time (an hour or so, at least) to do it right.

One other tip, if the bike has been sitting for a while, and you're going to be doing carb work, have plenty of extra (new) spark plugs around, and check/change them pretty often. Plugs tell you a lot about what's going on with the motor.

[sorny]

If you just opened the bleeder screw and pumped the lever repeatedly, you probably introduced more air into the system.

HA! Damn I'm smart! ;)
Thanks for the tips guys. I'll try that with the brakes.