Re: NEED HELP!! '81 cb750c wont fire!!

Posted by mallen14 on May 02, 2012; 2:57pm
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/NEED-HELP-81-cb750c-wont-fire-tp3950861p3955219.html

Bought a compression tester and came home to check it out.

Ok so results are in and problems that I know about are...

1) Pulse generator wire was fragile and broke while inspecting it. Will try to solder the wires back together later but right now that's why cylinders 2&3 are not firing.

The main problem...
2) Compression test shows the following: Cyl 1=60psi Cyl 2=58psi Cyl 3=0psi Cyl 4=56psi. Also put in small amount of oil in cylinder and pressure shoots up to 150+psi which means new rings right?? Or could the valves somehow cause this?? Also Im guessing cylinder 3 has a bad exhaust valve(maybe stuck open allowing no pressure to build up).

After the compression test and oil in the cylinders(to add some pressure), I squirted some starting fluid into the engine, replaced plugs and wires, and tried to start it again. Of course it didn't start because only two cylinders are running, but it was firing and blowing smoke out the tail pipes. This makes me think now so much a spark issue as compression issue.

I cannot find what the compression should be if the engine is cold.

Also I pulled the valve cover yesterday and checked clearance and all were "within manual specs"(.002-.008) but I also reached down and felt of the cam/crank chain and it felt really loose on the side of the chain that feed straight down in between the two camshafts. Like it was flopping around down inside the engine. The teeth were still in place on the crankshaft but it had to be dangling pretty bad. It supposed to be that loose???

But I think I have decided to do a top end rebuild including honing cylinders, rings, and checking valves to replace as necessary. Would I need a new timing cam chain as well??

The manual gives me a step by step on tear down and rebuild, but anybody have any advise?? Where can I get new valves and springs?? Piston rings??