SOHC Top End Rebuild Time! Veterans Needed

[TimmyT]

Compression test showed 100 psi on the left and 75 on the right. So I took it in my hands to tear the top end down. Set gas tank to PRI and remove (good to have a working petcock). Drained the float bowls and pulled the carbs. Removed ignition coils. Removed top end engine mounts. Then got into the top end. Followed the cross pattern for removal doing only fragments of a turn at a time until bolts were free, and finger loose. Each bolt was set in a cardboard jig with a co-responding number for reference. Removed the head, removed the cam chain tensioner/guides, removed the cam from the sprocket with a wire tied through the sprocket and chain to the frame. Everything came apart alright, the pistons may have came out too easy for my liking. The only thing is I need a "C" clamp to compress the valve springs for disassemble.

So all in all I have a good day of cleaning ahead of myself and another good day of rebuilding this sucker. In the interm I decided to start this forum to share photos and get pointers. There is alot of carbon deposits on the pistons and valves as expected after 20k miles. Nothing seems gouged or even scratched enough to cause a problem the only blue I see is on the piston wrist collars. what could cause my huge drop in compression. Piston rings? valve seals? I have a new engine gaskets waiting for the install. Pointers are appreciated, these measurements will take some time I guess Im hoping to find a shortcut. I was going into this with the assumption everything is still good on the inside but too much carbon is not allowing valves to seat properly. Sorry photos are too large for now I will upload after resize so they fit.

 

[bcware]

Carbon won't keep the valves from seating, but they should be lapped and you should wiggle them while they're free to check for play in the guides. The carbon doesn't build up on the seat because the valves smash the crap out of it.

Pull the rings off the pistons and insert them into the jugs; check the gap as per the manual to see if they're too worn. Keep them organized and oriented. The same rings go back on the same piston and they are directional. Same goes for the valves. If you remove them put them back in their original home.

A blown head gasket will also cause a loss of compression.

Best case scenario is a blown head gasket.

I used a c-clamp to remove my valves originally, but I had to make a spacer. I used a copper pipe fitting. I ended up buying a valve removal tool from sears and it still needed my crappy adapter. Valve removal is a bit sketchy as things can go flying when you use a ghetto rigged tool. I'm not sure what tool other people use, but standard ones seem to be problematic in terms of fitment out of the box/package.

 

[arfstrom]

VALVE SPRING COMPRESSOR 59. bucks +- s/h
http://www.motorcycle-superstore.co...alve-spring-compressor?WT.ac=RichAutoComplete


Sears has one as well, for 57.
http://www.sears.com/otc-large-valv...1x000001&kpid=00971032000&kispla=00971032000P

I went a local in home machinist, and he had one of these tools, took him no time flat, and the last time I seen him, He did the removal/re-install of the valves for free.

 

[salisride]

did you check valve lash before comp test and were you WOT ?

 

[wirehairs]

I'm doing the same thing you are, but with an 82 DOHC motor. Let me know if you find a good source for new piston rings.

Oh, and I guess it's standard procedure to replace the 8 valve guides, and those gaskets around the pin dowels. If you find a good source to any of those or the rings, please share.

 

[xschris]

I would try yamaha first. Ebay is also a good place to look for stuff. You can also Google the part numbers to see what comes up.