Follow up to Loss of Power in a Small Engine

The 23 horse Onan 2 cylinder air cooled engine in my Deere skid steer has been losing power. A few days ago, it would stall when going up a hill or turning sharply. One exhaust pipe would glow red hot and blue flames would be present in that cylinders muffler. The loss of power has gotten worse. Now it stalls when put in gear.

I posted a message on here and followed the advice. Once advice was to check for leaks in the manifold. I sprayed it with carb cleaner with the engine running. The rpm's never changed, so a leak in the manifold is probably ruled out.

I was able to put a pressure gauge on the cylinder that has the fiery exhaust. It showed zero psi. The pressure gauge went on the other cylinder, it showed 15 psi.

The next thing I'll do is remove the engine. I have an owners manual that shows how to do that. A service manual has been ordered and is in the mail.

But until that service manual gets here, should I go ahead and get to the valves in both cylinders?

My thought on this is that the valves have no clearance. Before you remove the engine, pull the valve covers and check for stem clearance. Don't know the exact spec, but should probably be in the .012 " range to get you running. The cylinder with zero compression is very suspect to the valve stem clearance.

Gene

Intake is 0.005
Exhaust is 0.013
Cold.

I don't think your readings are correct. Very hard to start and run on 15 pounds.

Point gap is 0.016
Compression 100-120
Timing is 20 BTC

Sorry forgot to add that.

I agree, those readings must be faulty.

Strongly suggest a leakdown test before going into the engine.

You might find a New replacement engone (Onan) at:

Glenn's Surplus, Shelby, Ohio.
Better known as "The Shelby Mall"...
They have an unbelievable inventory...!!
Prices are pretty hard to beat.
Care for a Deuce&aHalf..?? Take your choice of Engine, Trans and Dump Bed..!!
They even had a Turbo-Diesel Generator, last time I was there..!

I'm not going to go as far as saying what your readings show is wrong, but being a mechanic by trade I'd say they are questionable at best. If the engine was running at all then at least one cylinder had to have enough compression to do so. In other words 15 psi isn't enough to run, and 0 psi definately won't fire at all. That said Onans have a reputation for running when you think they would never be able to run. Over the years I've seen two cylinder Onans running on one cylinder with the other rod hanging out of the block, with the cam broken in half, with no plug in the other side, etc, etc, etc. Funny thing is when doing this they seemed to be perfectly good one cylinder engines and simply didn't have the same power they did when running on two cylinders.

Now, some Onans had a compression release that would keep the valves open to help the engine spin faster when starting, and as such, act as a starting aid. Basically the faster it got up to speed the easier it would start. Often that device will cause faulty compression readings if the engine isn't spun fast enough when checking it. This could very well explain the reading you got of 15 psi on the one cylinder. The only way to get a true compression reading is to have the engine running, if at all possible. Unfortunately on a two cylinder engine this is all but impossible to do so your kind of stuck with what you've got in that case.

As far as the problem your seeing when running it sounds to me like you've got one good cylinder and a burned valve on the other. With the burned valve not seating properly it would cause the reading your getting of zero when just spinning the engine over, compression release or not. Now, when the engine is running on the good cylinder, the bad cylinder is moving fast enough that it could/would be sucking in and compressing the air fast enough to partially overcome the amount of 'air' being let out by the valve not sealing. As a result the cylinder would be firing and as as it fired it would be sending hot, still burning exhaust gas right out past the valve. This would cause the flames and red hot muffler your seeing on the one side.

In the end I'd do like the others have suggested and at least check the valve clearances before going any further. If needed pull the head and check for a burned valve before tearing into the engine even further. If the seat isn't hurt you can get a new valve and just lap it in, but if the seat is burned also then you'll need to pull and disassemble the engine to have a replacement seat installed. Good luck.

You have to have the throttle wide open when doing the compression test and it takes 5 to 6 or more strokes to get the pressure. Choke has to be open also. (without engine running)

All good points. The NHC does not have a compression release. So you would have one less thing to check. Burned valves would be a good thing to look for.Also as I said bad valve seats were common.

Thanks Wayne and Billy. The situation you are describing in pretty much what I suspected. Now, if I can just figure out how to get to the valves without removing the engine from the skid steer.