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Discussion Starter · #1 · (Edited)
Recently acquired a 1981 CM200T with 2,600mi that has sat in indoor storage and hadn’t been started since 1989.

Initially the bike would not turn over, but after a few days of soaking the cylinders/top end in penetrating oil she freed up.

Next steps included:
• Replaced both plugs (NGK C7HSA).
• Drained & replaced case oil (10W-40).
• Replaced the air filter.
• Flushed the gas tank.
• Removed and rebuilt the carb.

After installing the re-built carb and replacing the fuel lines she fired right up after a few kicks, but after a few minutes I could tell something wasn’t quite right - The left exhaust pipe was cold, so cylinder #1 wasn’t firing.

My 1st though was spark – but both plugs were giving strong spark.

Then I checked compression – Cylinder #1 had zero compression (thumb method, not gauge... (EDIT: ~140psi, measured by gauge). I then put a little bit of engine oil down the spark plug hole and replicated the test on cylinder #1 to determine if compression failure was being caused by failed rings or cylinder scoring – but there was still zero compression.
At this point removed the left exhaust, valve cover and put cylinder #1 in TDC with both valves in the closed position. I then and placed an air hose into the spark plug thread. With both valves in the closed position and applying pressure, I was hearing and feeling the air coming directly out of the exhaust port of the head.

I also measured the valve/tappet clearances. All valves measured ~.005mm except for the exhaust valve on cylinder #1, which was >0.60mm. (see picture below).

Valve Tappet Gap.jpg



I sent a bore scope down the exhaust port and could see quite a bit of rust and corrosion.

E.Port Image.jpg

Bore Image.jpg

This tells me that the one of several things could be happening:

1. Rust/corrosion on the valve stem that is interfering with the guide and preventing the valve from its full range of travel, thus not sealing properly against the cylinder head
2. Valve & cylinder head sealing surfaces have built up rust/corrosion, that is preventing the valve some fully seating, and not sealing.
3. Valve springs are out of spec or broken, and not able to bring the valve back to the fully seated position.

Any other thoughts on things I should be checking?

Would it be possible to replace the valve assembly, or should I start looking for a new head?
 

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My guess would be the valve was open when parked and the stem is rusty, stuck open. The engine may not have to come out of the frame on that model to do the top end, but the top end would have to be disassembled and the valve at least cleaned up and re-lapped, possibly replaced based on how much rust is on the stem and in the guide. Since you'll be there you might as well do the whole top end based on condition, probably rust rings in the cylinders as well even if it was in good running condition when parked
 

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Discussion Starter · #4 ·
Gentlemen, thanks for the direction - looks like I've got a little engine out project to keep me busy over the winter!

I'll make sure to take pictures and document my experience periodically.
 

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Discussion Starter · #5 ·
So I pulled the head, and removed the valves - 2 of 4 slipped right out, and the other 2 required oil, Punch and a hammer. The valves that required a hammer had easy travel during their normal travel, but required additional encouragement once they got about half way out.

Head 1.jpg



1st question: Is this concerning? Did I damage the valves or head by forcing them out?

For the valves that required additional force to remove, I feel like I should replace the valve guides - any idea on how to remove these and replace?

valve guide1.jpg


Second question - How do I address all of the rust in the valve area - is the head shot?

Valve rust 1.jpg



Thanks in advance for the views and responses!
 

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The valve guides will have to be replaced, driving the valves out has likely scored the inner wall. A machine shop is the best choice for this, they have the knowledge and tools. Have them bead blast the head at the same time to clean up the rust, carbon, etc.
 

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Discussion Starter · #7 · (Edited)
Final Update

Update:

It's been a while, but wanted to close out the rebuild and share a few pictures that other may find helpful.

As pointed out by Jim, the valves/head needed to be worked on (thanks for the input!), which meant the engine had to come out. This was easily accomplished following the FSM, with the aid of an impact wrench and floor jack.

Engine out with valve cover removed:

IMG_1635.JPG


Once the motor was out, the tear down was quite simple again, following the FSM. The biggest recommendation during these steps is to take your time, and don't drop anything down the cam timing chain access area. I did find that the head was somewhat difficult to separate from the cylinder jug as the head gasket was holding the assembly together quite well. Time and patience was key in separating the two elements while lightly tapping using a small hammer and wooden dowel (I also spent a lot of time removing the old head gasket prior to installation of the new head gasket during reassembly).

Camshaft holders removed:

IMG_1638.JPG


Cam shaft and sprocket removed (take care not to lose any small parts down the timing chain access!):

IMG_1641.JPG


Cylinder head removed:

IMG_1643.JPG

IMG_1645.JPG



At this point i decided to take a gamble and not go any further into my tear-down, as the cylinder walls actually looks to be in decent share (2600 miles).

IMG_1647.JPG

IMG_1648.JPG


I did have to replace a few metal dowels that were stuck/bent during removal, and required vise grips to remove. Luckily these are still commonly used and easy to find at local Honda dealers:

IMG_1650B.JPG


Cylinder head back from the Machine shop:

IMG_1687.JPG

IMG_1690.JPG


Once ready for re-assembly, i though it would be a good time to remove all the oil sludge buildup in the bottom of the case and inspect the oil screen filter. I also had to remove and clean the friction plates and steel's of the clutch assembly as they had locked up from sitting compressed for close to 30 years.

IMG_1695.JPG

IMG_1697.JPG

IMG_1699.JPG


The re-assembly went pretty smoothly, again following the process outlined in the FSM paying attention to torque values and order of operations. Installing the camshaft sprocket was probably the most critical part ensuring the camshaft, cam sprocket, timing chain and crankshaft were all in the correct positions. take your time and follow the FSM, it's laid out pretty well. Also, make sure to oil up the cam holders and shaft before installation to protect the camshaft.

Once the engine was put back together and put back into the frame (which was more difficult than taking it out..), I took the opportunity to deep clean the bike before installing all of the components.

IMG_1630.JPG


New tires, break adjustments, oil fill and a few other small things it was ready to go!

Finished rebuild:

IMG_1708.JPG
 

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Another job well done. I'm guessing that from the numerous "take care not to lose any small parts down the timing chain access!" that you did so. Been there and done that :D
 
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