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GS550 Top End Refresh

sam000lee

sam000lee

Forum Mentor
My friend's 1977 GS550 was having trouble starting up towards the end of last season. We noticed that the engine was super easy to turn by hand on the kickstarter so I thought low compression. Pulled the plugs and 1 and 4 were both oil fowled and crusty.

The cold compression was

1: ~80
2. ~110
3. ~110
4. ~80

Which matches the results of the plugs. After putting a bit of oil in the 1 + 4 cylinders, compression readings immediately went up to ~180psi so it seems like the rings are worn.

We've pulled the cylinders + pistons. The cylinders look great and I can still see nice cross hatching on them. The engine has less than 20k miles which seems low to me for just the rings to have worn. What would cause this??

Obviously, we're going to replace the rings and gaskets now and are also deciding what else to do while it's apart. We have no reason to suspect any leaking through the cylinder head - is it still worth taking apart and replacing seals, replapping, etc. ?
 
Did you guys adjust the valves before busting down the engine? Hope so anyway because the clearance drops with mileage and the valves will hang open and not seat after a while, which obviously drops compression.

Anyway, I'd pull the cylinder and replace all gaskets and seals up from the crankcase on up. OEM gaskets only if you value oil tightness.
 
you're in this deep, i'd reckon it wouldn't hurt to lap the valves. at a minimum i'd recommend checking for leaking at the valve seat, and i do this without the valve springs in place. flip the head upside down with valves seated (held by gravity only,no springs) and fill the combustion chamber with any light oil of your choice, let sit overnight. you'll know which, if any, valves need to be lapped.

along with looking for a nice seating area after lapping valves to their seats, this is how i check for leaks when lapping valves.
 
Replace valve stem seals for sure. It's not worth all the work and skip a critical step like this.
 
Thanks all. Valves clearances have been regularly checked and adjusted and were in spec. We'll lap in the new valves and put in the new seals. Is there anything else to replace while the head is apart?
 
Cam chain tensioner seals. There is a tutorial on this. Tach drive seal and O-ring. Basically all the various seals and gaskets throughout the top end. You may need to go page by page through the parts fishe to make sure you don't miss anything.
 
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