Topic: Engine won't crank

[greddm]

Nearing the end of my rebuild which included a complete tear down of the engine. Since reassembly, I have turned it over several times with no problems. This weekend I fitted the exhaust, installed the plugs, and put the carbs back on. I hit the starter and it sounded bound up. Tried the kick start and it doesn't budge! The bike is in neutral, what's going on?


1977 CB750

[greddm]

Just to be clear, I'm not trying to start the bike, just turning it over.


1977 CB750

[HondaMan]

Often, the engine is very stiff until you feed oil into it. Remove the plugs, fill the oil tank, and spin the engine with the starter (no more than 10 second bursts, please) until the oil light goes out to show you got pressure: this takes a while. You will hear the cranking speed increase about 4-fold when the oil hits the crank bearings and cam.

[greddm]

Thanks Mark, I'll do that.


1977 CB750

[greddm]

Ok, took hondaman's advice and removed the plugs and put oil in the tank. Still no crank. I verified that the bike is in neutral and just for kicks, pulled the clutch, when I did that it turns over, no problem. What's going on?


1977 CB750

[Vinhead1957]

Seems more like the neutral is not neutral.  Does the bike roll in neutral?   Tranny problems?

[greddm]

Yeah, rolls fine and shifts through the gears, which is why I'm scratching my head. Could something be wrong with the clutch?


1977 CB750

[madmtnmotors]

Okay, sounds like a problem that has been encountered here before (myself included), but is rather obscure. The only question I have is:


Did you replace the clutch cover? I'm referring to the aluminum, primary clutch cover that bolts to the engine case, not the tin outer cover. The early models (70 thru 75 or 76) had shorter clutch packs and the early clutch covers protrude into the case an extra 1/4" in a couple spots, mainly at the back of the screw bosses where the outer (tin) cover attaches. The bolt patterns are the same (where they bolt to the cases) but the extra material on the early covers will interfere with the clutch hub, essentially "locking it up".

The later (77/78) 750's had a taller clutch pack and thus a slightly modified clutch cover. I learned this when I replaced my friction disks on my 78' 750 and had a chromed cover laying around and said "hey, that would look cool", only to find out form does not always equal function! Once I realized the problem, I was able to remove the offending material from the back of the clutch cover and all was well.

[greddm]

Okay, sounds like a problem that has been encountered here before (myself included), but is rather obscure. The only question I have is:


Did you replace the clutch cover? I'm referring to the aluminum, primary clutch cover that bolts to the engine case, not the tin outer cover. The early models (70 thru 75 or 76) had shorter clutch packs and the early clutch covers protrude into the case an extra 1/4" in a couple spots, mainly at the back of the screw bosses where the outer (tin) cover attaches. The bolt patterns are the same (where they bolt to the cases) but the extra material on the early covers will interfere with the clutch hub, essentially "locking it up".

The later (77/78) 750's had a taller clutch pack and thus a slightly modified clutch cover. I learned this when I replaced my friction disks on my 78' 750 and had a chromed cover laying around and said "hey, that would look cool", only to find out form does not always equal function! Once I realized the problem, I was able to remove the offending material from the back of the clutch cover and all was well.

Genius! thats exactly what must have happened. I had the one off my bike and one off a 74 which was in better shape and they appeared identical. So I put that one on instead. Good thing I kept the original. I'll take it off and see if that does the trick - Thanks!

[madmtnmotors]

That should fix it. Inspect the aluminum clutch hub for damage (cracks). You will likely see some dimples or scuffs where the clutch cover was touching the hub. I did not have any problems, and I was even rather enthusiastic with the kick lever, to the point that my clutch hub had some deep gouges where I had actually gotten the hub to turn a small amount, and I have not had any issues.

[flybox1]

Nice save mad....that was outta left field

...love this forum! 

[madmtnmotors]

Nice save mad....that was outta left field

...love this forum! 

If I hadn't done it myself I wouldn't have believed it! 

Fortunately for me, the only thing I did was replace the friction disks, and the only thing that was different was the clutch cover, so my troubleshooting was very narrowly focused when it happened to me.