Topic: Advice Needed: CB550 Exhaust Valves Snapping Shut

[justinfinite]

Hey everyone! Looking for advice on a recent top end rebuild.


Firstly, I would like to thank everyone on this forum as I've learned SO MUCH from browsing through post over the past few years.

For the past few years I’ve been working on a 1974 Honda CB550 that I picked up in ROUGH condition. This is my first bike and it has been a great escape for me to step away from the computer for my job and enjoy working with my hands again.  When I got the bike, the motor ran well however was spilling a considerable amount of oil from the head and base gasket. While the motor was out of the frame, I disassembled the top end to replace the gaskets and found that the previous owner used both RTV and paper gasket material to seal the top end, which led to countless hours carefully scrapping gasket material. During this process, I never disassembled the head or valves, and only focused on cleaning the surfaces and any carbon deposits. Everything went well, except for installing the rocker/cam cover. I didn’t realize but I did not loosen or lift the tappets when bolting down the cover. Being a novice when it comes to working in an engine, I finally wrestled it into place I didn’t think anything of it.

A few months have gone by and I finally had the bike in a position to reassemble the entire bike and finally ride it after multiple years of work. I put oil in the motor and began to rotate the motor by hand with with crank. Everything was smooth until it got close to Top Dead Center and rotating the motor got really tight and eventually made a loud “clank” sound. I started trying to diagnose what the sound was coming from.and after several hours, I found that the sound was coming from the exhaust tappets/valves snapping closed when the motor was close to top dead center. I have determined that all 4 exhaust tappets/valves snap shut versus smoothly closing. I double/triple checked the valve clearances, removed/reinstalled the rocker cover and confirmed that the cam timing was spot on per the Clymer’s Manual. Each exhaust tappet/valve is still snapping closed.

Does anyone have any advice on what may be happening? My first thought is that I may have bent a valve but for all 4 exhaust valves behaving this way makes me think otherwise.

Huge thanks for reading this far and any help!

PS - Here’s a couple photos of the bike from the day I rescued it compared to now.

[bryanj]

If you did not heed the warnings to hold up the rockets you have bent the valves and also possibly cracked the valve guides, if so cheapest fix is a second hand head(500 and550 are all the same ) and this time read the Honda manual CAREFYLLY before reassembling

[justinfinite]

Thanks for your reply bryanj!

I followed the Clymers manual which mentioned lifting them to prevent improper seating, which I did by hand when originally installing the cover, however I have just recently learned of the rubber band trick that most people use.  I just wasn't sure if there are any other situations that would cause all 4 exhaust valves/tappets to hang and snap shut at TDC, such as timing or otherwise.

[bryanj]

Nothing i can think of

[calj737]

Cam sprocket/cam chain slipping? Tensioner not seated? Has to be something with that assembly.

The rocker cover can bend a valve but it’s not a guaranteed outcome. And very unlikely you’ve bent all four and are causing this issue.

[justinfinite]

Hey Cal, Cam chain and sprocket seem to be fine, however I just noticed that my tensioner is NOT springing back when attempting to adjust it. The tensioner adjustment screw appears to freely spin about 1/2 turn with absolutely no tension.

Any possibility that this could lead to the issue with the valves snapping back?

Thanks and I very much appreciate the advice!

[calj737]

If the tensioner is wonky, the valves will operate unreliably as they are all interconnected.

Best course of action is to remove the tappet inspection covers and observe the rockers movement. If the rocker is indeed “snapping” shut as you believe, remove the rocker cover altogether. Then rotate the motor using the alternator side in the correct direction, and observe the cam chain.

Nothing can be assessed as “seem to be fine”. Everything needs a thorough inspection to confirm it is correct, per the Honda manual. Assumptions here will cause you much pain and anguish.

A common error during reassembly is not seating the tensioner correctly, or in fact, installing it upside down. Either way, the tensioner should have some “spring to it”.

[justinfinite]

Thanks Cal,

I should have been specific in my last post. I have entirely removed the cam cover and have turned the motor over by hand.  This is when I discovered the tensioner screw was not springing back, as it should be. I will go further into the motor to ensure the tensioner isn’t damaged and/or seated properly.

Thanks!

[scottly]

If the tensioner is wonky, the valves will operate unreliably as they are all interconnected.

WTF?? When a valve "snaps shut", it's because it's been stuck open from seizure in the guide, possibly due to a bent stem, and then "nudged" past the sticky point when the piston hits it at near TDC. A faulty tensioner will Not produce this symptom.

[754]

Thats what I am thinking,   seeing as it was apart I lean towards bent.
 Should be quick to find out with compression or leakdown test..

[scottly]

I had the "valves snapping shut" issue before, in that case on a 413 Mopar that had the guides rusted up from sitting. The initial cranking of the engine with the starter also bent several push-rods.

[Tracksnblades1]

I had the "valves snapping shut" issue before, in that case on a 413 Mopar that had the guides rusted up from sitting. The initial cranking of the engine with the starter also bent several push-rods.

There wasn't  a "fuel injected string ray" to blame the 413 on.....?

[bryanj]

I think you have two seperate distinct problems
1 bent valves and compression rest will not diagnose that neither will leak down only strip out and visual check, as i said before if guides are cracked a used head is cheaper
2 a tensioner mechanism with a shot rack and ponion for somebody forcing the bolt round with a screwdriver, Honda should never have put that slot in there!

[calj737]

If the tensioner is wonky, the valves will operate unreliably as they are all interconnected.

WTF?? When a valve "snaps shut", it's because it's been stuck open from seizure in the guide, possibly due to a bent stem, and then "nudged" past the sticky point when the piston hits it at near TDC. A faulty tensioner will Not produce this symptom.

On the contrary, Scottly, it is quite feasible that the cam chain having too much slack becomes bound up during a rotation of the engine (remember, he’s rotating by hand) and then suddenly slips past the binding, it can/will create a valve train reaction. I’m not declaring this to be the problem, I said check it.

I know all about bent valves and guide obstructions. But think analytically for a moment: what are the odds that he bent all four valves and only on the exhaust side at once? If installation of the rocker cover did in fact bend a valve, or multiple valves, they would not all be on the exhaust side. The rockers would be in a firing sequence due to cam position. Hence, my encouragement to inspect the cam chain assembly as a start.

You know a great deal for sure, but too often anyone who thinks they know everything has only stopped asking the right questions.

[Tracksnblades1]

If the tensioner is wonky, the valves will operate unreliably as they are all interconnected.

WTF?? When a valve "snaps shut", it's because it's been stuck open from seizure in the guide, possibly due to a bent stem, and then "nudged" past the sticky point when the piston hits it at near TDC. A faulty tensioner will Not produce this symptom.

On the contrary, Scottly, it is quite feasible that the cam chain having too much slack becomes bound up during a rotation of the engine (remember, he’s rotating by hand) and then suddenly slips past the binding, it can/will create a valve train reaction. I’m not declaring this to be the problem, I said check it.

I know all about bent valves and guide obstructions. But think analytically for a moment: what are the odds that he bent all four valves and only on the exhaust side at once? If installation of the rocker cover did in fact bend a valve, or multiple valves, they would not all be on the exhaust side. The rockers would be in a firing sequence due to cam position. Hence, my encouragement to inspect the cam chain assembly as a start.

You know a great deal for sure, but too often anyone who thinks they know everything has only stopped asking the right questions.

Unless the Honda Cb550 is the "exception to the rule", I assure you time the camshaft wrong, force two revolutions, and you shall have four bent exhaust valves...

Sometimes as with family, the truth just trickles out....

[justinfinite]

Much thanks to everyone who has replied!

Cal, as you mentioned, I found it odd that all four exhaust valves are behaving identically or that all four exhaust valves were bent and nothing effecting the intake side.  I just wasn't sure if anything else could be causing this behavior.  Now, noticing that the tensioner has no tension, I anticipate that calls for a replacement of the tensioner versus being repairable?

My experience isn't quite deep enough to fully understand what I am looking for when diagnosing possibly bent valves or guides, so a visit to the local shop may be in order.

[bryanj]

If you can take the head off you can fix the problem.

To find a bent valve all you do is remove the springs and spin the valve in the guide whilst watching the head, it should NOT move sideways also it should slide very easy up and down the guide.

Only special tool you need is valve spring compressor

[calj737]

If you can take the head off you can fix the problem.

To find a bent valve all you do is remove the springs and spin the valve in the guide whilst watching the head, it should NOT move sideways also it should slide very easy up and down the guide.

Only special tool you need is valve spring compressor

And this is almost necessary if there's a problem with the tensioner. Kill two birds with one stone. Inspect the valves/head, then confirm your tensioner.

[754]

Should add, they should be able to slide up and down easily and freely with spring off. Because running they must do it hundreds of times a minute.... and that is at idle..

[justinfinite]

I'll be ordering a spring compressor as Bryanj has mentioned.  Read through a few manuals on the process and it seems doable, it inspect.

Question: How obvious will it be to determine a bent valve or guide?  As I understand it, getting into the head is dealing with very tight tolerances and precision and I certainly don't want to just "assume" things are fine.

Also, there clearly seems to be an issue with the pinion on the tensioner.  Are these rebuildable or is sourcing a replacement for the entire cam chain tensioner assembly, my only option?

Thanks!
- Justin

[bryanj]

Depends on what is actualy wrong with the tensioner, i have repaired some it depends how bad.

Before you buy a compressor post a link as its easy to get it wrong

EDIT  ebay item 293573814661 appears to be in US.
 HINT dont just put it on and wind down to remove collets, with head on flat surface put a deep socket on the top collar and give a sharp rap to the end with a hammer, this loosens the taper which can get incredibly tight in operation

[754]

If they are bent, and guides are not cracked....... dont push out report back ,we can advise on how to get them out without ruining guides.

[ckahleer]

Can you watch the cam lobe, rocker, and and valve as you slowly turn the engine? As the exhaust lobe starts going down, does the exhaust valve stay open, then later snap shut?

[justinfinite]

Can you watch the cam lobe, rocker, and and valve as you slowly turn the engine? As the exhaust lobe starts going down, does the exhaust valve stay open, then later snap shut?

When I had rocker cover installed, I was able to watch the exhaust valve slowly open, then stay open as the crank got hard to turn over at TDC. Then with a little more pressure, the exhaust valve snapped shut and the engine spins smoothly again, until the next rotation when the same situation would happen to the next exhaust valve in sequence.

To answer your question regarding the cam lobe, I was unable to visually see how the lobe was contacting rocker during rotation.  I am unsure if the lobe was or wasn't making contact with the rocker at TDC when the valve snaps back.

[justinfinite]

Depends on what is actualy wrong with the tensioner, i have repaired some it depends how bad.

Before you buy a compressor post a link as its easy to get it wrong

EDIT  ebay item 293573814661 appears to be in US.
 HINT dont just put it on and wind down to remove collets, with head on flat surface put a deep socket on the top collar and give a sharp rap to the end with a hammer, this loosens the taper which can get incredibly tight in operation

I was looking at this valve spring compressor. Amazon can have it here by Saturday.

https://www.amazon.com/ABN-Compressor-Service-Motorcycle-Equipment/dp/B01M4J094J/ref=pb_allspark_session_sims_desktop_263_2/144-8428623-6980965?_encoding=UTF8&pd_rd_i=B01M4J094J&pd_rd_r=2eb80549-4c86-484e-a7fe-5b3a6acd4a67&pd_rd_w=ani0E&pd_rd_wg=SlPZw&pf_rd_p=e500004d-dce5-4973-9afd-bba519c83f08&pf_rd_r=CEH9DBBW6QSJ6BYWJW9V&psc=1&refRID=CEH9DBBW6QSJ6BYWJW9V