NEED HELP!! '81 cb750c wont fire!!

[mallen14]

Sorry for the confusion but I HAD spark on cylinders 2&3 before I broke the wire running to the generator for those cylinders. I still have spark on cylinders 1&4 though. But I bought a compression tester and came home to use it.

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.

[Hellbilly429]

Now that I think about it, you said you had the carbs off and cleaned them, these carbs are so finicky that if they are not synced damn-near perfect the bike will not run. Also these bikes do not like to stay running even if they are just 1 cylinder down, so I am sure running it 2 cylinders down isn't helping at all. Also, these CV carbs are vacuum driven, so if you are running the bike without the stock air-box on then that probably isn't helping your situation either. If the plugs are wet though, I am really thinking the carbs are the culprit, especially since you said it was pissing gas out which could mean it's also flooding the engine with fuel. I'd check those carbs one last time before you start digging deeper into the bike and into your pockets.

[seestheday]

I haven't needed to do new rings yet, but that question about oil increasing pressure has been answered here a few times. A search should be able to help you.

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "mallen14 [via Honda CB750'S]" <[hidden email]>

Date: Tue, 1 May 2012 12:10:26 -0700 (PDT)

To: seestheday<[hidden email]>

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

Bought a compression tester and came home to use it.

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.

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[cdaiscool]

A motor needs approximately 100 psi to run. Your compression test indicates it's definitely rings. If they all shot up to 150+ then there's no way the valves are to blame; the oil coats the rings only, sealing them off.

[mallen14]

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??

[TOOLS1]

You could do a leak down test to find out if, you have a bad valve. However since, you all ready know, you need to do a rebuild there is not any need, for it. Most any good machine shop that works on motorcycle engines can source valves, guides, and seats if needed.
TOOLS

[mallen14]

Update:

Got the engine unmounted yesterday, the valve cover removed, both cams removed, and the head unbolted. Everything so far is looking good as far as wear on the cams and shims. The only thing I have run into so far is a broke cam holder "A". Apparently this engine has been opened before because it had a make shift bracket fastened over the broken bracket to hold it in place.

The main goal is still getting to those valves and replacing what needs to be but I have looked just a little and can't find a cam holder to replace the broken one. I'll keep my eyes out for one but unless I luck up on one I might have to reinstall a broken one

[IMG]http://i200.photobucket.com/albums/aa154/mygsxrwillspanku/Motorcycle%20Stuff/IMG_0633.jpg[/IMG]
[IMG]http://i200.photobucket.com/albums/aa154/mygsxrwillspanku/Motorcycle%20Stuff/IMG_0634.jpg[/IMG]

[mallen14]

Just found a whole set of cam holders on ebay for $18!! Got lucky...

Just found that my cam chain tensioner is broke though, that's another $75 at least :(