Topic: '78 cb 750 k meltdown

[bellygunner]

I've got a '78 Honda cb750 k that I got a year ago.  Last summer I had the carbs rebuilt, new sprockets and drive chain installed, the front brake caliper rebuilt, the seat recovered and I bought new side covers and sanded and repainted the tank and side covers.  It went from looking like junk to a really nice bike.  It ran well last July-October.  It developed an oil leak at the front of the engine in October and the oil would blow back on to my shoes.  I didn't do anything with the bike until last Sunday.  I cleaned the oil off the engine and tightened a couple bolts, put a new battery in it and let it run for about 10 minutes as I inspected the engine for the leak.  I couldn't determine where the leak was, in fact, it didn't seem to be leaking any more.  I did see wisps of smoke coming up from the engine.  They seemed to originate from the number 4 cylinder.  I decided to take it for a short ride of about 4 miles but it was running good and it was a great day so I extended it to about 20 miles.  Five miles from home I heard a "bing" and noticed that the tach needle was reading 0.  I figured that the tach cable broke but the bike soon started to lose power and within a mile it quit running. 

When I put my hand over the end of the exhaust and turn it over I can't feel anything.  Also, I pulled the no. 4 plug and put my finger in its place and again, when I turned it over, I couldn't feel anything.  I does have spark and it has oil.

Does anyone know what might be wrong and what might need to be done to fix it?

Thanks!

[bryanj]

You either have a few concurrent faults or you lost oil feed to the cam and it siezed and snapped

[bellygunner]

Thanks for the quick response.  I'm not a mechanic but I have done tune-ups on the CL 350 I had when I was in college many many moons ago.  I've never tore an engine apart.  Is removing the engine and taking it apart to try to see what the problem is a job best left to a professional mechanic?

[honda750k]

No, its thoroughly possible with some patience and knowhow.  I have an 86 kawi POS that has had the motor out of it more than I'd care to admit.  by no means am I a pro mechanic, but I've rebuilt my own motors.   you cant do it if you're not mechanically inclined though. I'll say that much for sure.

[Gordon]

You either have a few concurrent faults or you lost oil feed to the cam and it siezed and snapped

Yep, all signs point to a broken camshaft. 

[cookindaddy]

Something like this happened to me last year. The oil leak was from a little rubber plug that had cooked under the cam support post that was overheating. Mine looked like it was leaking into the #1 spark plug hole and my pant cuff got soaked with oil too. My cam cooked also, probably from lack of oil like was suggested but it didn't break. The journal on the cam at the #1 end and the support post was a mess when I finally got it out.

I didn't tear the whole engine down, but I had to take it out of the frame to get the valve cover off to get at the cam and supports. I don't think you can do this with the engine in the frame.

Getting the engine out of the frame is a bit of an adventure but you'll find lots of helpful information here as I did. I used the "tip the bike on it's right side and lift the frame off the engine" method. Taking the wheels off and having a couple guys to help once you get everything disconnected helps, but there are other ways to get the engine out.

Good luck with it.

[bellygunner]

Thanks for the information everyone.  I think I'll have a go at it and see if I can figure it out.  The bike has a Windjammer fairing on it so I'll remove that item also.  I'll just take my time and if I run into an issue a put a questions up.  Thanks again.

[MCRider]

Thanks for the information everyone.  I think I'll have a go at it and see if I can figure it out.  The bike has a Windjammer fairing on it so I'll remove that item also.  I'll just take my time and if I run into an issue a put a questions up.  Thanks again.

yes that is the classic case of twisting a cam in half, right down to the "bing" which i have heard as well. Very distinct, you'll never forget it. If you can imagine half of that thick stick of steel being twisted by 50+ horsepower while the other half is seized into its easy chair from lack of oil.

I have never known this to happen to an engine that was never opened. Has your engine been worked on? Its usually a piece of debris (silicone gasket sealer very common) getting stuck in one of 2 very small oil delivery holes in the head.  One hole for each half of the cam. There is really no reason why both holes couldn't get plugged, but I've never seen/heard of it, always one or the other. And usually the 3-4 right side which is why you lost your tachometer as well, it drives off that side of the cam.

It can also happen from running a big bump cam and the the cam towers will twist up and the oil pressure will spit out the oring that seals one of those holes. This is much more rare as it involves a big bump cam and overly aggressive valve springs.

Several threads here on this, so you're not alone. Proceed with the tear down. Someone here will help for sure. While you can rebuild the top without pulling the head, or splitting the cases, you'll definitly want to pull the oil pan and clean the oil pump screen, I'll bet its coated with flecks of someone else's assembly debris. A shop vac works well.