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I'm sure most of you haven't followed my escapades as I'm just a newbie, but heres the quick rundown:

82 XS400J Maxim Motorcycle (yamaha). Thought it was an ignition issue that was keeping it from running. I had thought there was compression. So, today I replaced a blown diode, hooked up a brand new and charged battery, sprayed some ether in the carbs and tried to start her. Nothing.

So I think to myself, lets start from the beginning.
Check Spark, Yep, good and blue.
No Compression guage, but put my thumb over the hole. Hrm, odd, no pressure. Called the wife in.. Had her feel, still no pressure. Had her feel while turing the back wheel in gear, very faint. Put a ziptie down the hole and it moves when hitting hte starter/moving rear wheel in gear, so pistons are moving.
Check other side, some compression, but still very little.

So, its not an ignition problem, I just aint' got no compression. Before I start tearing into the engie, does anyone have any advice on what/where to check? I'm thinking rings, head gasket, valve adjustment.

Are there other options beside dropping the engine?

I would suspect the head/valves first. Stuck open/really out of ajustment valves will cause such syptoms. If it was rings, you would get somewhat more pressure build-up than you seem to be getting. A hole in the piston is a different story.....

Is it the same in both cylinders?

IT appers to be a little worse on the right side, but not much.

if you suspect rings or want to see if it helps try putting a small amount of oil into the cylinder and then cranking it over again to see if you are building compression

Today. I finally found a compression tester and tested, here are the results:

Right Side - 0 Compression
Left Side - ~90psi after ~10 trys on the starter

I put a few squirts of 10w40 in the plug holes and measured again:

Right Side - 0 Compression
Left Side - ~120 psi after ~10 trys on the starter.

Lookin like a combo of rings and valves. I'll be interested to see what it looks like in there. Any other sugestions?

-Adam

You have either a stuck vavle or a badly burned one. If it is just stuck, you can probably free it up with some WD-40 and working it up and down by hand to at least get it running.

If it's burnt... or bent or busted or the head is cracked or the piston is holed, well then you gotta dig in and replace the bad parts.

Your compression isn't that low. Unless you plan on keeping this bike for many years/miles, re-ringing it might not be worth it..... your call. I'd get it running first, then go deeper from there if I chose to.

Update:

Pulled the Engine out today and here is what I found on first inspection:

I pulled the valve cover and turned the engine by hand. The Cam Chain did not move.
I shined a light down the engine and turned again and it appears the chain is not connected
to anything below, weither or not it is broken or just off a sproket, I don't know.

Do I need to tear the head off now or is this more of a total write off?

Thanks for all your help!

-Adam


PS - This bike is a project that I am truly enjoying, and I intend to keep it for quite a while if it can be running, so I am willing to do a ring job and the such.

Your going to have to at least remove the cams and see what's up with the camchain. If it's broken, you'll need to change it. If it's jumped off the bottom sprocket, it's still FUBARed and needs changeing.

If you don't have a service manual in your posession first, I wouldn't recommend starting any of this.

Yea.. I got smart and got the service manual early!!

I'm going to tear down a bit more today. I'll update this with my progress.

you need to find out everything that needs replacing and get prices before you start piecing it together only to find something else as you go along.these things can turn into an endless money pit.been there done that.