1978 JD 312 burning oil and blowing smoke...HOW???

The cam lobes on solid lifters engines like the K301 are ground at a very slight angle so the lifters or cam followers rotate as they rub against the camshaft. That's so the wear between the camshaft and lifter is distributed to the whole surface of the lifter.

I suspect You have a stuck piston ring which is causing the oil consumption/smoking. Pretty common problem on engines that have sat for several years.

I'd clean away as much of the carbon deposits as You can, replace the head gasket, maybe even try to get some sort of solvent down between the piston & cyl. bore to dissolve whatever is sticking the ring. Run the engine for a short time, change the oil and run Your favorite "Overhaul-in-a-can" in the gas and maybe the oil also. A brand many people have used is "Seafoam" and there are many others.

thanks Dennis. makes perfect sense about the valve lifters. I've already ran a whole can of seafoam through it now...well about half a can in the tank and I drizzled the other half in the carb with it running. It smoked badly before I did it and it still smokes pretty bad now. I've heard that the seafoam can make it smoke bad too, while it's burning it with gas.

Do you think I would have good compression with a stuck ring? Oh and I think it sounds like it's knocking too?

thanks again,

Ryan

I just did some measurements of the cylinder. The cylinder is bored for a .010 piston. All of my measurements came up pretty close to the tolerances, but the piston is really loose and floppy in the cylinder. There is almost a 1mm gap between the top of the piston and the upper part of the cylinder wall. It looks like a complete rebuild? It's just kindof confusing that it's already been rebuilt once and everything looks perfect, even the cylinder measurements. But the piston flop and the smoke...

might want to clean or replace the mesh in the crankcase breather/vent too, insure the parts inside are
orientated correctly

david,

that was one if the first things I did, replaced the "filter" and the gaskets, and made sure I put it together right....

I guess I'll break it all down and take more measurements...piston, ring clearance, ect.....

thanks!

Ok, ring gap dimensions and orientation in bore as well as having the rings installed in the proper order on the piston? Getting the piston out and measuring it too would be the next step and like you say checking the valve guides. Not many places left to check.

thanks!

Ryan, I think you will find the oil ring is stuck, lost tension, or worn out, but the compression rings will probably check good. Lots of carbon means it likely has plenty of run time since the rebuild. A burn or melted look at the top of the piston nearest the valves will also indicate several hours use.
The very top of the piston is considerably smaller than the rest of it. When you get it out you will see they are not round nor the same size top to bottom.

tommyhawk

Thanks for all the help everyone! Tom, all that makes perfect sense!!! I'll hopefully have this baby apart tonight to take some real measurements!

well, I removed the piston and took some measurements. One thing I don't get is the service manual's cylinder bore specs. It says something like 3.38 inches new, but then says that the out of round and taper of the bore has to be within .005 The bore spec doesn't even go into the thousands place??? Again, the cylinder is .010 over and the crank journal is .010 undersize. But back to the measurements. Nothing is way out of wack, the bore measures almost perfect. The piston isn't that bad either. The rings look great too. The only thing is what Tom said about the oil ring. It doesn't spring out like the compression rings at all. What should a good one look/measure like?

thanks again guys!

Ryan

Ryan, you can download a free manual from www.kohlerengines.com All the specs will be there. Look under "Classic Engines" then "K" series. You will get all the standard sizes with tolerances into tenths of a thousandth.
Just for fun, I usually measure the rings with dial calipers, I.D to O.D. Wear will usually show more at the ends. Nothing really accurate about that, but if you have a set of new ones, it's a good comparison. Probably the best check is measuring the end gap of the ring installed in the bore. The book will list minimum clearance for the compression rings, may not for the oil ring, especially the 3 piece kind. Don't know a good way to check for lost tension except comparing to a new one.
Could possibly have std. rings in that +.010 bore. Wear in the piston ring grooves?
Rarely would I reuse a rod, piston, or rings in a "K" series without knowing it's history. But if everything measures up to snuff.....

tommyhawk

thanks Tom. That helps a lot! I think i have a bad oil control ring "spring"...