Small engine Onan/Cummins blues

[daveshoe]

I have two Wheel horse tractors and the engines have problems. The 20 horsepower Onan[made by Cummins] was running on one cylinder,so I cleaned the intake valve. I thought it looked a little ''warm'' around the stem,and sure enough,it dumps raw fuel out the exhaust,indicating a big intake leak. Cleaning the thing simply made a small leak bigger.

The minimum repair would be lap the valve,as I can access only the bad one without removing the engine. I wonder if I should just pull the engine and replace the guides,valves,and lap them in. Opinions?

Engine #2 is a 16 HP that has a knock,I really hope it is just a carbon knock that can be fixed by pulling the heads and cleaning.

These engines are on Toro-Wheel Horse 520 and 416 mowers. I love these tractors,but the grass just keeps growing.

 

[seemore]

Not trying to be a smart butt, but did you try replacing the plugs, the Onans I worked on years ago were notorious for fouling one plug & running ok on the other.

 

[rbattelle]

I'd lap the one known bad valve and put the engine back in service. I assume you want to get this thing up and running again quick.

Hopefully, that will buy you enough time to finish this season. Then, if you decide to, you can do a more complete job in the fall.

Ryan

 

[B.G. Smith]

I have two Wheel horse tractors and the engines have problems. The 20 horsepower Onan[made by Cummins] was running on one cylinder,so I cleaned the intake valve. I thought it looked a little ''warm'' around the stem,and sure enough,it dumps raw fuel out the exhaust,indicating a big intake leak. Cleaning the thing simply made a small leak bigger.

The minimum repair would be lap the valve,as I can access only the bad one without removing the engine. I wonder if I should just pull the engine and replace the guides,valves,and lap them in. Opinions?

Engine #2 is a 16 HP that has a knock,I really hope it is just a carbon knock that can be fixed by pulling the heads and cleaning.

These engines are on Toro-Wheel Horse 520 and 416 mowers. I love these tractors,but the grass just keeps growing.

This is a flat head, valve in block design?? Please lead me through the process in which a leakng intake valve will dump raw fuel into the exhaust if that cylinder is firing?? Not trying to be funny, just don't understand. Thanks. bg

 

[daveshoe]

This is a flat head, valve in block design?? Please lead me through the process in which a leakng intake valve will dump raw fuel into the exhaust if that cylinder is firing?? Not trying to be funny, just don't understand. Thanks. bg

It dumps some raw fuel in during cranking,before it starts. Upon starting,white smoke appears. After the smoke clears,it runs ''normally,but weakly'' once it warms up. it has enough power to mow. This is worse than it ran just prior to losing #1 cylinder and me cleaning off the valve deposits. This suggests to me that the valve deposits did a better job of sealing up the partial leak in the intake valve than the clean valve surface. When I first examined the unit,the intake valve was fully stuck open.

As for the spark plugs,they both fire fine,but I can certainly replace them. This engine is a L-Head horizontal shaft design with the valves on top of the engine. I have found out that these engines are no longer in production,so I am considering ordering new valves,valve guides, ect. [while I still can] so I can keep them going into the future.

I appreciate the suggestions,and will try everything you guys suggest. The alternative is a new Kawasaki engine kit for $1250,but the gauges on the tractor would not operate. With pressure lubrication and an automotive type oil filter,I think the old engines have plenty of life in them. I will keep you posted

 

[B.G. Smith]

Ok, thanks for the explanation. Sounds like you are on the right track, low compression or no compression which is probably the valves. The white smoke on starting would indicate oil in the cylinder. You might pull the little plate over the valve spring area and see if the oil return hole is open. Don't know if these engines have adjustable valves or if you have to grind he end of the stem. Bet they don't have removable guides but new valves might cure any clearance problems with the stems. Good luck. bg

 

[RussJ]

I am also working on Onans (16, 18 & 20 hp) but I'm having trouble finding parts. Does anyone know of any sites that sell internal engine parts? Starters, carb kits and mufflers are easy to find but rods, gaskets, valves, etc. ?

 

[daveshoe]

If you have a model that is used in a Toro-Wheelhorse tractor,you should be able to get parts. Some of the parts guys will not even talk to you unless you have the model and serial of the tractor and the model and serial of the engine. This could be trouble if your motor is a fixed RPM model like used in a welder or genset. If you need these numbers so the counter guy will talk to you,let me know.

I will check my local guy and see if valves are still available. If they are,I intend to stock up on them,as this appears to be the weakest link in these motors. I was able to get a valve cap-[goes between the stem and the lifter],valve cover gaskets the other day.