Topic: CB500 Valve Tappet Clearance Issues

[fantino]

Hi all,

I've been restoring a 73 CB500 I inherited from my brother over this last year and finally got the engine back together. New valves (lapped and sealing well), cylinders bored, new rings, gaskets etc. etc. Anyways, after putting the cam-cover on (I took all the tappet screws out first), I installed the tappets for 1 and then 4 at the 1-4T mark, then 2 then 3 to 002 intake and 003 exhaust in the same order my clymer manual showed.

When I crank the engine over by hand, the valves seem to get stuck open, and then sort of go "ka-ping!" about half a crankshaft revolution later when they snap shut. Just about every valve seems to do this with vary degrees of regularity\intermittence. If I back all the tappet screws out about 1/8"-1/4" and turn the engine over, they all open and close smooth as butter, the second I tighten them back down again they start sticking open and and popping closed. I've double checked that the camshaft mark lines up horizontally right at the 1-4T mark, and reinstalled the head and it still does this. I tried a spare cam cover I had and it still had the same issue.

Does anyone have any advice to offer? I appreciate any help :-)

[Frostyboy]

Anyways, after putting the cam-cover on (I took all the tappet screws out first),

Although you removed the screws, are you certain that there was no interference with the rockers to the valve stems during installation?
It sounds as if the valve stems are bent. With the tappets set to spec, the stem might be jamming in the top of the guide. When you increase the clearance the bend in the valve stem is staying clear of the guide.
I think you might have to pull the head again.
You'll know for sure if you can't get the valves out of the head that the stems are bent.
Good luck.

[bradweingartner]

Take the tappet covers off and see if the adjusters are snagging on the edge of the valve stem or if they are loose when it gets stuck in the open position. You either have bent valves or your rockers are snagging the valve.

[fantino]

Bummer, looks like it's probably bent valves then... #$%*ty lesson learned the hard way (of course is there any other way? Haha). Looks like I was wrong about it being all of them though, I've narrowed it down to three or four... Luckily I have five or six extra valves of each sort in fair condition, so hopefully it won't have to be a terribly expensive lesson...

Thanks a ton for the advice!

[Fritz]

I've narrowed it down to three or four... Luckily I have five or six extra valves of each sort in fair condition, so hopefully it won't have to be a terribly expensive lesson...

Don't want to discourage you, but I had the same problem.
If your valve stems are bent so badly that they are sticking, you'll have to bend them straight before you can extract them. I bet that the guides are also bent/cracked, so you'll have to have them checked and/or replaced. Possibly the valve seats have to be reground before you can use new valves. Unfortunately, the valve guides are not cheap. And you cannot replace them by yourself if you do not have the right tools (valve guide reamer, valve seat grinder, dial micrometer, etc...)

I had only one valve sticking after I missaligned a rocker during head cover installation. The result was not only a bent valve stem but also a broken valve guide. The head had to go to a shop where I had the valve guide replaced and the seat machined. Alltogether it cost me about $150 - including resurfacing the head.

You might get away without the machine shop, but even if the valve guides are not cracked, you should check the valve stem clearances as shown in the shop manual.

Good Luck!

[SteveD CB500F]

Moral of the story:  back ALL tappets fully out before bolting down the cover.

Then set them with the T mark.

[fantino]

Ugh, this is so embarrassing I don't even want to admit it. I pulled the head and popped off all the valves springs, and it's pretty much trashed (valves and guides). Major bummer. I had just gotten to the point where I finally felt like significant progress was being made, looks like I'm back to finding a new cylinder head to refurbish. A little depressing and certainly a pain in the wallet. On the plus side I've learned to never make that mistake again? 

This is my first time putting a motorcycle engine together (and I probably couldn't have started with more of a basket case to learn on, haha), so progress has been slow, exhausting and filled with constant correction of (usually) minor mistakes. I feel like I'm learning an awful lot though, even more so when I mess up and have to spend the money to fix a mistake... Luckily (or unluckily depending on your perspective) I'm so emotionally invested in this bike I will put as much time and money into this project as it takes, as I can afford it. So no giving up, press on ever forward to success! 

Thanks again for the help. I'll probably be using\searching the hell out of these forums from here on out as I continue to work on this project...

[Frostyboy]

So no giving up, press on ever forward to success! 

That's the spirit mate, hang in there.
Slow & steady wins the race. It will be worth it in the end.

[dave500]

yeah those assemblies can be a little tricky,,never just wind down on bolts to close something up.

[Fritz]

Ugh, this is so embarrassing I don't even want to admit it.

No error - no progress

[fantino]

Thanks to Frostyboy, I found a replacement head for $25 after shipping, so the project can once again continue :-) I also got some welding done on my frame today and it's off to the powder coaters tomorrow, so yey. As long as everything works out with the new head it looks like I'll be working on putting everything in the frame and battling the electrical system next...

Thanks to everyone for being so helpful, I can't believe I've been working on this bike for the better part of the year and haven't spent any time on the forums here yet!

[Frostyboy]

Sometimes ebay just works for ya hey?
You're welcome fantino, I hope it's a good one.