john deere 318 help, i think the Onan is done for good?

ok, i dont know if anybody is still on this link, if i don't get any responsed i will post a new link. The tractor is running...there's the good news. Now the bad news. It's definantly not running right. I now finally got to mow the lawn with it "fixed" and she certainly wasn't running like she used to. First off, the tractor is much louder, and rougher sounding. 2nd, it's vibrating like crazy everything is vibrating, motor steering wheel even the fender. 3rd. It doesn't seem to have the same smooth throttle that it used to. When you would increase the throttle it would move up at a nice even pace, now it seems more choppy. Finally the big one, it definantly doesn't have the same power when it is "working". I couldn't notice before when just testing the machine out, but now when i have a deck on, the pto engaged, it doesn't have near the power it used to. It's bogging down, even dying on hills, and it just seems like its working really really hard to do what it used to do effortlessly. Again, i don't know where to go with this, does anybody have any suggestions? The only things different than when it ran great originally are these.....I put on a used genuine Onan starter instead of the cheap knockoff (would seem irrelevant to it running bad). During one of the many times pulling the motor, i ripped the wire out of the condenser, but i had a used spare condenser so i put that on (plausible that this could maybe have something to do with it?) 3rd, new plugs, they are AC Delco R44LTS. I do have new plugs coming in the mail NGK TR5. my dad got the delco's, but doing research online i'm not 100% that is a compatible plug. Could any of these things be a problem? Also, when starting the thing, It's starts up fine, virtually right away HOWEVER, it still doesnt sound like it used to when you turn the key. It used to sound real smooth, now its loud. My guesses, and they are pure guesses are these, something is still wrong electrically causing the bad starting noise and maybe uneven voltage?? or that something is misaligned somewhere, or finally that my pistons are going, which would mean low compression and possibly the vibration? any thoughts? Also, would a video of the thing being started and ran be of any use? I could probly swing that if people think it would be helpful.

Yes, it sounds like that twin is now running mono. Probably that nice "plinka-plinka- plinka" that it used to do at idle has now turned into a "pop-pop-pop". Do you have a compression gauge? Or, pull both plugs and crank with with your finger in the plug holes ( one at a time is OK. Do you have compression? It should be good in the one good cylinder and maybe low or none in the bad one. If no compression, then you might have a valve stuck open. Low compression could show a valve not closing all the way because of carbon buildup on the stem or something worse better left unmentioned for now (like a bad piston ring - oops,I said it) . Pull the valve cover and see if they move properly while cranking (no plugs installed). If compression is good, then put a plug on the wire, ground the plug shell and see if you get a nice fat spark when you crank. If not, a bad ignition lead could be the problem. Check the connection at the coil. Don't have resistor ignition wires with the resistor spark plugs. It reduces the spark.
Good luck and please give us some progress reports when you can.

A brilliant idea! Thank you Leine! Why didn't I think of that? It is running on one cylinder. Pulled the plug wire on the left side and it died. I pulled the plug wire on the right and nothing happened. So the problem is finally diagnosed.

So what can be the possible causes for a dead cylinder?

I pulled the plug on the bad cylinder. It was wet with fuel which makes sense. However, i dried it off and started the machine with the plug out. I put the plug close to the block and i most definantly have spark.

That leads me to believe that the plug, plug wire, and ignition coils are ok. Am i correct in this assumption? If so, then the what other possible cause could there be?

Seems to me the only other problem would be bad compression which would mean
1. Bad piston rings
2. Stuck valve (i can only hope)

Any others possibilties? Correct me if I am wrong but i don't think it could be a bent crankshaft or connecting rod as this would cause problems for the other cylinder as well and it shouldn't be able to run at all.

I think my dad has a compression gauge, so im gonna go grab that this afternoon if I can find it to make sure compression is my problem.

My final question is this, What would be the cause of the rings going bad if that is what it is?
Is there a root cause that needs to be fixed to prevent this from happening again?

Also, at this point is it worth pulling the engine apart to replace the rings, is it likely that the other cylinder would go soon too? If one set goes bad, does this mean that I should just overhaul the whole thing?

The engine has 745 hours on it, seems to me most people around here say an Onan should last to at least 1500 hrs. or so which makes me think it might be worthwhile to do.

ok, so i don't remember exactly how i posted a picture so if there is a few random posts with a jumbled mess i am sorry. Here's all the new information, and there is alot.

I started breaking the engine down. I got to the valves and saw both were moving so i new i had to pull off the cylinder head.

I pulled off the cylinder head and saw some not so good news.
1. Cylinder is not moving when i turn the engine over
2. When i pulled off the head water came out.....yes, water. looked like water, smelled like water, and i even soaked it up with a paper towel and tried to light it on fire but couldn't water. Also, if the picture comes up you can see surface rust starting on the top of the sleeve. indicating water. The water must have been from the previous owner, because i store it inside, and it was outside when i bought it. I'm suprised that it took that long to surface as a problem. Again i used it all fall with no issues.

So i have stopped here to see what everyone thinks i should do next. It seems to me that the answer to the problem is indeed a broken rod? Or are there other possibilities? I do have 2 extra rods and piston heads that look ok in my spare parts. So i could get the rod replaced cheap.

I'm assuming at this point i would have to do a total teardown, polishing the cylinder sleeves etc. If i can get the broken part for nothing how much would a teardown/rebuild roughly cost? Is there anything else you would need besides new gaskets? a polishing tool and the tool to get valve stems off the spring (the few remaining things ive retained from high school shop when we rebuilt a small engine)?

Finally what next? do i have to get that cylinder out next? and how do i do that? Pulling the cylinder head is easy, and i can get my hands on a torque wrench to put that back on, and thanks to alphie i have the specs . But from a quick glance the cylinder sleeve wasn't so obvious on how to take off, and i'm not even 100% sure that is what the next step even is.

At the very least it's been a huge learning experience for me.