Topic: Possibly caused my cam chain to skip a tooth

[zabzob]

I've been giving my 75 750K some attention after a few good riding seasons. Cam chain noise has always been a problem, so I thought I'd see if I could sort that out. I did the tension as per the book -- 15 degrees past TDC on cylinder 1. Still had some noise. Stopped the bike and decided to remove the tensioner and test the spring. Upon trying to put it back on the bike, I found I couldn't push it back into the bike. I rotated the engine to the proper 15 degrees past TDC without the tensioner on the bike, which might have been a huge mistake. I put the tensioner back on and released the spring, hearing it snap into place. Upon firing up the bike I heard the most awful racket. Not an intermittent sound like a chain slapping around, but more of a consistent super loud ticking. I redid the tension as per the book to be sure, and I've still got the terrible noise. When I rotate the engine with the bolt under the points cover, it resists going to the 15 degree past TDC position, then flies past it...

Is it possible that I skipped a tooth, and the only way to fix this is to pull the engine?

[PeWe]

Is the cam chain old and never replaced? It might be time to replace it anyway. Pull engine remove head, cyl block open cases..... Not perfect time of the year doing that.
When replacing cam chain with its tensioner, replace primary chains with the tensioner too when they must be old too.

This will cost some upon the mentioned parts like complete gasket set, complete sealer set, hondabond sealer (cases) and Gasgacinch sealer or similar under the rubber pucks under the cam towers.

When head is off check if seats are OK. Old head will most likely need to get valve seats recut and maybe some valves need to be replaced. Old steel guides might be OK, if not this will be added.
The valve guide sealer is a part of the complete gasket kit.
Clutch with springs might be worn or just old too. This can be replaced when engine in frame. The clutch nut and lock washer with tool is a must ato purchase.

Do not forget that this is a 40 years old where sealers, gaskets, o-rings, tensioners become old and need to be replaced.  My CB750 engine got all these stuff replaced 2012-2013 despite I did it all back in 1984.

[madmtnmotors]

Remove the spark plugs when rotating the engine. You are reaching the top of the compression stroke on one cylinder as you reach the mark, then as the piston passes TDC the compression wants to push the piston back down, thus the "fly past" feeling. Removing the spark plugs will alleviate this.

I seriously doubt you have "jumped a tooth". I'm guessing you have verified free movement of the tensioner? I will usually fully compress the tensioner and lock it down in the fully compressed position prior to install. Once installed then the lock down bolt can be loosened for the spring to release and exert tension on the chain. Lock it back down and you're done.

[zabzob]

@pewe: Hmm... sounds like pretty much a rebuild, which I was hoping to avoid. If I do end up pulling the engine I think it'll be just the cam chain and checking for bent valves. I've already spent my motorcycle budget for the year.

@madmtnmotors: I did as you described: pulled the tensioner all the way back, remounted it, and let is slam forward as described. I ensured that it's locked forward. I hope you're right and that I haven't jumped a tooth, but I'm not sure what else it could be. The only other thing that changed is that I had a fuel bottle setup for carb syncing that drained into the carbs overnight due to a lack of petcock. I don't know... filled a cylinder now it's making a horrible sound? Pull the plugs and let it dry up? I'm grasping at straws here hoping not to pull my engine!

[madmtnmotors]

Okay, I must not have read quite the entire post. I was under the impression that you were unable to reinstall the tensioner at all, but I see that you were able to get it reinstalled. Before pulling the engine, I would at least check valve clearances and make sure one of the adjusters hasn't come loose (they make terrible racket when shook loose).

Also, just for the sake of covering all the bases, you did re-tighten the cam chain adjuster bolt after install/adjustment, correct?

[zabzob]

I did tighten the cam chain adjuster bolt after the install, yes. I'll check on those valve clearances when I get home tonight.

[zabzob]

None of the valve adjusters are loose. Looks like I'm pulling the engine, unless somebody's got a better idea.

[madmtnmotors]

How many miles on the engine?

[Retro Rocket]

If you emptied a container of fuel into a cylinder then its also in your crankcase, I wouldn't run that engine till i had changed the oil, you don't want fuel in your oil at all...

[zabzob]

@madmtnmotors: About 34,000 miles.

@Retro Rocket: Good call. I didn't think about that but it's probably thinned out my fresh clean oil. Have to drain it to pull the engine anyway.

[zabzob]

Also @madmtnmotors: I removed the spark plugs and rotated the engine, and still had the hang at 1/4 TDC and then fly past. Possible valves touching the piston head?

[madmtnmotors]

Also @madmtnmotors: I removed the spark plugs and rotated the engine, and still had the hang at 1/4 TDC and then fly past. Possible valves touching the piston head?

There is still a tendency to rock past TDC when the piston reaches the top of the stroke, it should be much less pronounced with the sparkplugs removed though.

[zabzob]

Pulled the engine and suspicions were confirmed. Just a hair away from 1/4 TDC and look how far out the timing is. I removed the cam chain tensioner as part of getting ready to remove the head, then thought I'd rotate the engine once more to check something -- and I think it skipped again. Is that typical? Should you just not rotate it with the tensioner removed or you're going to have a bad time?