Topic: 1978 CB750F clutch & engine issue

[Leemata]

So my '78 CB750F has been sitting since last season unridden.  Needed parts. license, insurance, etc.  Money's tight so I parked it (it's been killing me).  I've been regularly starting it up and running it until warm and it runs great, but it smokes quite a bit.  Last year it did that after starting, but would quickly go away as I went down the road.  More on this in a minute.  Anyway, last night after I fired her up, and decided to take it for a spin around the neighborhood.  Pulled the clutch lever in and shift into 1st gear and the bike lurches forward and stalls.  HUH?  Try again with the same result.  It's like all of a sudden I have no clutch.  So I decide to ride anyway and was able to get going with a rolling start.  The bike smoked all the way around the block.  Why?  Also, the clutch never did work.  Why?  The clutch has always worked great on this bike.  When I get back to the garage I see this rod laying on the floor where my bike was parked (see pictures).   Could this possibly be the clutch push shaft?? If so, How did it get out?  I'm baffled here as to what the heck is going on.  I could use some help.  Thanks in advance!

Terry

[JohnG]

If you go to BikeBandit.com you can look at exploded parts diagrams and decide if anything fell out...

I have had 750s sit for some time and then the plates are stuck together. Not unheard of. You can perhaps tell this with the bike
on the center stand, motor running seeing if the rear wheel is still turning as if in gear.  Put the rear brake on and see how much resistance you get.  Sometimes this will break it loose.

Worse comes to worse, take the clutch out and patiently
separate the plates one by one.

Good luck with it!!!

       john

[BVCB650]

I think your smoking history on start up is valves guide leaking oil past. They may have hardened even more from parking it. This is something that may go back to the way it was if you ride it more, but since they were smoking before you parked it, no snake oil in the world will help them out. But, you can ride it for many, many miles like that. As long as they don't get any worse and let your valves tip.

[TwoTired]

I'm not sure what your fallen part is.  But, for the smoking bit... You'd better sit down... ready?
The 77-78 model 750F models had a valve train like no other 750.  The design was changed for more power, but consequently the valve guides would wear rapidly.  No valve guide seal can last long with a wobbly valve stem going on.  And the adjusters wear and damage both themselves and the valve stem end.

The "fix" by Honda was hardened valve guides (often done under warranty).  But, it never did match the longevity of the earlier 750s and the K models of the same era.

The repair you are likely looking at is new valve guides, a valve job (seats at the very least).  If the valve faces are still ok, then the stems have to be milled and lash caps installed to restore valve height, along with new tappet adjusters.

If you are strapped for money, you are probably crying by now.  And, it is often cheaper to find a 77-78 K motor that will drop into your frame.  This would also cure your clutch issues, what ever they are.  You can paint the K engine black and ignore the F models higher redline mark on the tach.  No one else needs to know.

[Tower]

I don't know what that part is (perhaps its a cam chain tensioner rod?), but its definately not part of the clutch assembly.

[MRieck]

I'm not sure what your fallen part is.  But, for the smoking bit... You'd better sit down... ready?
The 77-78 model 750F models had a valve train like no other 750.  The design was changed for more power, but consequently the valve guides would wear rapidly.  No valve guide seal can last long with a wobbly valve stem going on.  And the adjusters wear and damage both themselves and the valve stem end.

The "fix" by Honda was hardened valve guides (often done under warranty).  But, it never did match the longevity of the earlier 750s and the K models of the same era.

The repair you are likely looking at is new valve guides, a valve job (seats at the very least).  If the valve faces are still ok, then the stems have to be milled and lash caps installed to restore valve height, along with new tappet adjusters.

If you are strapped for money, you are probably crying by now.  And, it is often cheaper to find a 77-78 K motor that will drop into your frame.  This would also cure your clutch issues, what ever they are.  You can paint the K engine black and ignore the F models higher redline mark on the tach.  No one else needs to know.

I agree with TT about the guides etc. The only thing I'd add is the valve face is junk more often than not because of the excessive valve movement. The good thing is the F2 valves are now available through Dynoman and maybe APE.

[TwoTired]

I agree with TT about the guides etc.

Good thing, 'cause most of the info I got from you.

 

[bikebitzofvt]

TT is likely right on the mark regarding your bike's valve train woes.  Been there, done that, way back in 91 on the same model.  That is, if it's oil smoke.  Is the smoke white or gray in color?  Could be carburation trouble, too.  What's the mileage on the bike?  Has a leak down test been done?
I looked at the part you found on the floor pretty closely.  I'm trying to determine the size of the pin, however it doesn't look like any part of the clutch release mechanism.  Is the cable moving freely at both ends?  You can remove the adjustment cover (the chrome part of the clutch cover) and check for proper operation of the mechanism.
Good luck & keep us posted!
Tom