*note* I got carried away. See below for short version.
excessivedemon wrote
Hey Guys,
I've been trying to fix my 81 CB750F for quite some time now (after it suddenly died on me during a ride a couple of months ago and never wanted to start since)
There's no way you were out riding and then just suddenly lost all compression. Of course, I know that you understand that. So I'd guess that you're problem is elsewhere. Sudden failures are almost always electrical. When it died was it just like flipping a switch-instant- or did it sputter to a stop?
excessivedemon wrote
previously, I was able to check for good spark on all 4 plugs
99.9% of the time this is a good test. But it is possible for a spark to look strong and blue when tested,but then not fire when in the bike. It's a sign of weak ignition. Could be faulty cables,weak battery,poorly grounded engine.
excessivedemon wrote
...so I managed to have the carbs thoroughly cleaned....but don't seem to see any fuel coming out of carbs #1-3....I can see that there is some fuel on carb #4.
Where are you seeing fuel? You should see a thin stream of fuel squirting from your accelerator pump nozzles every time to twist the throttle. If you can actually see any other fuel it might be flooding.
If you know that you have good fuel flow coming out of the tank, then check the carburetor bowls. I open the flowt bowl drain screws to check for fuel. If the hose runs a steady stream then you're good. Check all four carbs. With a small drain pail it will take less than a minute to check and then you'll know for sure if the carbs are getting fuel or not.
excessivedemon wrote
I also noticed that as I try to start her up I can hear and feel that cylinder #4 sucks and spits out air as the starter turns - this does not happen on cylinders #1-3 - does that mean cylinders #1-3 do not have any compression at all?
No worries bud, that's not what that means. Since you were just recently riding I imagine you have at least adequate compression. It would be good to check it,though. I got a nice compression tester from Harbor Frieght a while back and am totally please with it. For $25 it's worth every penny and it reads right along side my Snap-On.
I've seen engine suck and spit like you describe. My chopper did the exact same thing on #1 cylinder but it ran just fine. It could be valves out of spec or even timing slightly off ( cam timing,not ignition timing).
If an intake valve is set too tight it could allow it to spit back slightly as the piston reaches TDC. But again, this is not completely unheard of and it won't cause you bike to suddenly stop running. But this can be a good sign too. Since it can suck and spit then the engine is obviously compressing air. Usually this is also a sign that the cam is turning since its able to compress,but your engine is twin cammed. It's possible that one cam is turning and not the other. Do you feel pressure coming out of the exhaust pipes? Since your bike stopped suddenly it's possible you lost a cam chain. Can you verify both cams are turning?
excessivedemon wrote
I also noticed when I put the bike in first gear and press on the clutch as I try to start it, it crawls forward (even with the clutch on)
Completely normal. It's just the nature of these wet clutches. The warmer the bike get the less it will creep like that,but when they are cold they creep pretty bad. You might possibly need to adjust your clutch if it's really bad, but they will always creep some. You should be able to sit on the bike in 1st gear (engine cold, clutch lever pulled in) and hold the bike still without any brakes. Just using your feet and legs. If you are having to hold your brakes your clutch may need adjusting.
Short version:
1-make sure all four carb bowls have fuel.
2-make sure all four carbs squirt fuel when you pump the throttle
3-check electrical connections for ignition ( plug caps are tight and healthy,no rotted wires or loose connections,etc)
4-make sure both cams are turning
Good luck and let us know what you find out!
Locked
