[kmar]

This summer, my 316 started hunting severely. After many years of grass cutting, I decided to do a major rebuild on the entire engine and fix every other little thing that was wrong on the tractor. This engine had been short blocked by the PO, and about a thousand hours on it, but has been properly maintained as long as I've had it. Everything part in this engine was well within spec, and in excellent condition, so I did the rings, cylinder hone, a valve job, and all associated seals and gaskets. It's hunting at wide open throttle, so that leads me to believe that it's not a governor issue. No amount of adjusting of the springs makes any difference. The oil filter cylinder side is already full of carbon, unlike the opposite side which has a normal burn pattern, which leads me to believe that the intake valve is sticking from what I see. (It spits fuel up out of the carburetor on shutdown). The valves spin freely in the block, the oil gallery oils properly, the valve lash is set properly, and I believe this valve is hanging open only when the machine is running. No amount of rotating this engine by hand gives me any indication that anything is wrong. Could this be a week valve spring, or a valve seat that's moving during operation?
An initial bit of information, after the rebuild, on the first startup this machine ran beautifully, and in about 15 minutes it went back to its old hunting ways. My guess is when the assembly lubricant I used finally washed out, that's when it went back to hunting.
Any help on this will be greatly appreciated.

 

[FrankIII]

You sure it's not a carb issue?

 

[kmar]

You sure it's not a carb issue?

I put a new carb on it, did the standard reseal of the intake manifold, new ignition module, wires, coil, regulator, condenser, as part of the rebuild. Then I put the old carb back on, no change.

 

[mtoney]

Hunting is a lean condition with the carb. Was the new carb an OEM carb or one of the Chinese clones? I have had tons of issues with the clone carbs on all makes with running lean and surging. Doesn't matter if its Onan, Kohler or Briggs. The carb is lean and the gov senses this and give it more fuel to make up for it leading to the surging. Even a new carb could have debris in a passage or a badly drilled passage leading to lean running. I would take the old carb, properly rebuild it, removing ALL welch plugs and cleaning all passages. Another test you can do to verify you got a good reseal on the intake runner, with the tractor running, spray carb cleaner along the sides of the runner where the seal is, all the way from end to end. If the engine stumbles or reacts to that, you still have an intake runner leak.

 

[Joe Baring]

My p218g did the same thing after I decarboned the head. It cleaned the carb and thats all it took to fix it.

 

[kmar]

My p218g did the same thing after I decarboned the head. It cleaned the carb and thats all it took to fix it.

I still haven't figured it out yet, I'm bringing back that machine from our farm in a couple weeks to get it figured out. It also loses compression on that cylinder when it heats up, leading me to believe that it may have a weak valve spring. I rebuilt the original carb, tried a new replacement carb on it, rebuilt the entire engine (it was due anyway), all new electricals, sealed up intake plenum, so there's really not much left I haven't done. It acts the same now as it did before I did a full rebuild on it. Really, the only thing left is a sticky valve, or something crazy and very uncommon.

 

[extech]

i bet a galled valve stem. sticks when hot

 

[mtoney]

The P220 in my JD 420 that I bought last fall with 523 hours on the meter was very tempermental in starting, easy to flood. This is a huge red flag of valves being out of adjustment on an Onan engine. Since I had no plans to use the tractor thru the winter, she sat in the shed till a few weeks ago. I did the decarbon on the heads and piston tops and set the valves to factor spec. The exhaust were so tight that even my thinnest feeler gauge would not fit and I suspect the valves were being held open a few thousands. Intake were so wide that even the thicker exhaust spec feeler gauge was super loose. So not getting enough intake and exhaust being held slightly open(which is a fast route to a popped valve seat btw) Once set back to spec, she fires up on the coldest morning within 1 to 2 crankshaft revolutions. Keep those valves adjusted folks. I try to do mine every 200-250 hours to be on the safe side. Its a great time to fully inspect the engine for oil leaks from crankshaft seals, especially on the flywheel side and making sure ALL of your cooling fins are clean and free of any obstructions. Also, make sure you have the rubber grommet around the oil filter, you loose a lot of cooling air on that side if its missing(which is very common) and its only a $5 part.

 

[kmar]

i bet a galled valve stem. sticks when hot

Yes, could be. When I bring it back I'll have a good look at them.

 

[kmar]

The P220 in my JD 420 that I bought last fall with 523 hours on the meter was very tempermental in starting, easy to flood. This is a huge red flag of valves being out of adjustment on an Onan engine. Since I had no plans to use the tractor thru the winter, she sat in the shed till a few weeks ago. I did the decarbon on the heads and piston tops and set the valves to factor spec. The exhaust were so tight that even my thinnest feeler gauge would not fit and I suspect the valves were being held open a few thousands. Intake were so wide that even the thicker exhaust spec feeler gauge was super loose. So not getting enough intake and exhaust being held slightly open(which is a fast route to a popped valve seat btw) Once set back to spec, she fires up on the coldest morning within 1 to 2 crankshaft revolutions. Keep those valves adjusted folks. I try to do mine every 200-250 hours to be on the safe side. Its a great time to fully inspect the engine for oil leaks from crankshaft seals, especially on the flywheel side and making sure ALL of your cooling fins are clean and free of any obstructions. Also, make sure you have the rubber grommet around the oil filter, you loose a lot of cooling air on that side if its missing(which is very common) and its only a $5 part.

I'll recheck them to spec when I bring it home. Thanks!

 

[Georgia317]

I agree with it being a valve issue. Most of these replies are good information but I think the air and fuel spurting out of the carb on a shutdown would indicate a sticking valve. A bad carb would not produce a difference in the carbon or condition of the opposite cylinder as he mentioned. I would start at the intake valve of the cylinder that lacks compression paying special attention to the valve guide. It too could become loose in the block as the engine heats up. Certainly not uncommon with these engines.

Sometimes this can be diagnosed by tapping the hot cylinder with a hammer just after shutdown. If you hear an internal clank at the same time, it's a sticking valve for sure. Another more common diagnosis is to use an infrared temp gauge to see if one cylinder is running cooler than the other. That's by far the easiest and most accurate and it narrows the cause down to either lack of compression or bad ignition in a bad cylinder without any disassembly of the engine. (If you have one of these) A bad carburetor will not cause one cylinder to be cooler than the other. Nor will it cause air to flow back into the carb at shutdown. If it was the carb., some change would be noticed when switching them out. No two ever act exactly the same. And a dead cylinder can cause the governor to surge with a sticking valve. Good luck.

 

[kmar]

Great info you've given here, spot on to this machines symptoms. I'm looking forward to the hammer tap test, as I know that side is much hotter than the "good" side. I'm bringing that machine back from our hobbie farm in Susquehanna Pa in the next week or so, waiting for the 18" of snow to melt.