20HP Onan

[flyboy1015 0]

I also have a P220G on my 520h. It sat for a few years and I am having trouble getting it going now. (It had belonged to a relative) I get spark to both cylinders, but after a lot of troubleshooting, I have found that it will run on the front cylinder and not the rear. I've pulled one plug at time and tried starting. It will crank and start on the front cylinder but will push up against compression and stop cranking on the rear cylinder. Seems very strange to me. Compression is still pretty good, though.

I believe that the spark is firing on both cylinders at the same time. I have pulled the plugs and watched the spark as I crank the engine. Even as the engine slows to a stop, it appears that they both fire simultaneously. On the few occasions that it will push on past compression, the engine will backfire. I believe this is the rear cylinder firing problem.

I hope someone can help me to diagnose the problem. Any takers? I'd love to get it going again.

[Sparky-(Admin) 23,186]

Might be a valve seat has come loose on the rear cylinder. Seems to be a common problem on the 20HP Onans. I do not know what the fix for it is tho.

Mike.....

I moved this to its own thread for you

[rickv1957 75]

Hello and Rick

[flyboy1015 0]

Thanks, Mike. I had that thought as well, but couldn't figure how the valve could stick shut. There are a couple of covers on the top side of the engine. I think I'll have the remove the muffler to get to them, but I'm guessing that the cams may be there. Probably a decent place to start looking.

[Roger from southern Iowa 1]

:omg: Wish you the best on your Onan.

[sorekiwi 761]

I dont quite understand what you are saying here:

It will crank and start on the front cylinder but will push up against compression and stop cranking on the rear cylinder.

You say you have checked the compression?

The Onans are a little prone to losing the inlet valve seat on the rear cylinder. This causes the valve to not seal. (low compression) It does not cause the valve to stick shut.

The two covers on the top of the engine are for access for setting the valve clearance. The camshaft is not visible from the top of the motor.

You can see part of the inlet valve seat through the inlet port with the manifold removed. Onan sells a slightly oversize seat to repair it, and Onanparts.com has a bigger oversize seat if that is needed. A machineshop would be required to install it.

I bought a tractor that wasnt running on the rear cylinder, I assumed the valve seat had come loose, but it turned out to have a broken rod. But it ran really well on just the front cylinder. Apart from no power you would have no idea that the motor had major issues.

[Kelly 1,033]

Not sure if the P series engines had this problem but the Case guy I've delt with said the B series engine had cam problems they broke in half and would only run on one cyl. I sold many good cams to them even had one broken one in a engine.

[flyboy1015 0]

I don't have an answer as to what is happening with the rear cylinder yet. I till try to get the head off this evening after work. I will probably take the exhaust off first to see what his happening with the valves.

I don't believe anything is broken since it was in good working condition before it sat. The symptom I was describing is that if I have the plug in the front cylinder only, it will crank and start. If I put the plug in the rear cylinder only, the starter can't crank past compression and it will backfire. Once compression leaks past the rings, the starter will turn it on around. It sounds to me like a stuck exhaust valve or possibly valve seat like Mike said.

I've never dealt with valve seats, so this is new to me. I have been into the cylinders, pistons, rods, etc., so I'm not new to engines. I've just never seen something like this.

I'll see what I find and report back. Sounds like if I'm having a problem with a valve seat, I'll have to pull the cylinder and take it to a machine shop along with a new seat, valve, possibly valve guide, etc. Does this sound about right?

Thanks for all the help so far. I really appreciate it.

[MalMac 1,332]

Flyboy1015, I have got two 520s with the Onan in them. I have only had carb trouble with them. Please keep posting as to what you find wrong. This is a learning factor for me and anything associated with the Onans would be most helpful. I know the Onan is famous for a bad valve seat on the rear cylinder. How many hours is on the tractor? I have heard that the seats can start going from about 700hrs on. I don't think that is wrote in stone. Thats just what a couple of dealers have told me. When you don't know much about the Onans you just have to gather any and all info. So again please post what you find. Hope its just a small problem for you.

Tim

[flyboy1015 0]

This one was my dad's. It has 1,000 hrs on it. Still ran good when he stopped using it. So now, you're probably wondering why. The pulley on the deck is worn and the belt rolls over after a few minutes of use. Not sure how much that will cost.

I guess my next question should be, "How much does an overhaul cost?" I had hoped to eek out a year or two before spending much money. I have 2 more months worth of student loans to pay for school. Will need to cut grass before then.

While I'm at this, do you want pictures too? I can do that to help document my work and findings, if you like.

Chad

[flyboy1015 0]

Well, no luck tonight either. I checked the rear cylinder and nothing wrong there. Turned the engine and valves open and close as expected. Put everything back together and got the same result. I'm at a loss. New starter, new condenser cap. Got spark, but the engine turns over way too hard with the plugs in. Can't get it to turn past the compression stroke with the starter. Won't start. Oil is dirty, but no metal shavings. Changed oil a second time and got the same--a little dirty, but otherwise "clean" of any metal shavings. Turns over smoothly with sparkplugs removed. No clanking or bad noises when cranking. Not sure where to go from here. I think I'm done--feeling pretty defeated. Anyone want to buy a tractor and deck?

[flyboy1015 0]

I feel a little better today, but I'm still baffled.

I guess I'll buy a service manual for the engine. They are available on Ebay.

I keep trying, but I just can't give up. I'll keep you posted as my mood swings from good to bad to whatever. Hopefully, I'll have some good news of some sort to report on this.

[flyboy1015 0]

I got the service manual and can't find anything that would make any sense as to the problem I'm having. I may have to take the cover off on the PTO side and see if there is anything to be seen inside the engine. Oil looked good, but can't think of anything else and no other clues. It's really too bad that no one else has had a problem like this. Would be great for someone to pop up who could say "Aha! I know the answer"

[Kelly 1,033]

Just tell me where to pick it up I'll figure it out,then test it for a year or so :omg:

[gwgdog66 23]

Hang in there Chad. A lot of us don't have experience with the Onan Engine's. Someone will be along directly that will know what's wrong with it.

How old is the battery? Fully charged? I've run across some batteries that were strong enough to turn the engine over with the plug out but not stout enough to get past the compression stroke. These were of course Kohler's.

[MalMac 1,332]

Very unlikely but maybe a clog or blocked off exhaust in the muffler. I have seen baffles come loose and close off exhaust before. The pressure would leak off then when the engine would turn over the exhaust stroke you could not turn it at all till the pressure would bleed off just enough. Anyway its just a thought. I am sure someone with alot of knowlegde on here with Onans will appear. Also I got your PM and replied back.

Tim

[Buzz 502]

It doesn't seem like we have very many Onan experts online right now so why don't you register with these groups/forums and ask there. It never hurts to have too many sources of information. Most of the members of the second forum are professional small engine mechanics.

http://groups.yahoo.com/group/Onan_Engine_Club

http://www.perr.com/phpBB3/viewforum.php?f=14

[gwgdog66 23]

Good advice on the second link Buzz. A bunch of knowledge there also.

I'm going to print this topic and take to work tomorrow. The Maintenance shop foreman used to be the service manager at a Cummings / Onan dealer. I'll see what he has to offer.

[flyboy1015 0]

I appreciate the encouragement guys. It's really frustrating when you can usually find something to point the finger at, fix that thing, and then see an improvement. Unfortunately, that isn't the case here. I'll try one of the forums you suggested. I hadn't thought of that.

Regarding the battery, it's dead ~ 3-4 volts. I've been using jumper cables and my truck, sometimes running, at that. I'm 100% sure it's not a battery problem. I checked the ground, and positive cable. I've even connected the jumper cables directly to the engine and positive post on the starter, just to rule out the chassis wiring. No change.

This will probably sound strange, but since I don't get a really hot spark during cranking -- due to the fact that all of the current is being used by the starter, I've considered using two batteries. They would be connected as two separate circuits. One would be connected to the chassis wiring and the other would only be used to hit the starter post. The grounds of both batteries would be connected to the chassis. This would mean that the power supply for the ignition isn't being dragged down by the starter. I know it probably sounds like a crazy idea, but has anyone ever tried this before?

In case you're wondering, I'm a mechanical engineer. If I come up with some off-the-wall idea, bear with me. There really is a method to my madness. I'm not an academic whiz, but I did well enough to make it. My strength is the "hands on" part of things. That's why this problem is so frustrating.

Have a good day everyone. Thanks again.

Chad

[flyboy1015 0]

Kelly,

Be careful what you wish for, I may show up at your house one day if frustration continues to build.

Chad

[gwgdog66 23]

I showed this thread to the boss guy at work today, that used to be the Cummings/Onan service manager. He is very familiar with the Onan Generators, and said they have a tendency to carbon up the cylinder heads.

I let him know this was on a Wheel Horse.

He said if it will run on one cylinder then there are 4 possible cause's for the other cylinder. Carbon has come loose in the cylinder, valve seat loose, intake or exhaust valve stuck closed, or a broke cam.

He did ask if and how bad was it knocking while running on the one cylinder? Was there any compression in the bad cylinder?

If it is knocking don't run it, take the head off and see what the cause is.

If not knocking, he stated that the ignition system could have some issues with the pick up coil (He lost me there?). We got interrupted with a call about a problem child Hyster forklift.

I hope this helps some. I'll pick his brain some tomorrow also.

[flyboy1015 0]

Dog,

Tell your boss that I'm sorry to disappoint him, but I got it running and it wasn't a broken part.

That crazy idea of mine worked! I connected two batteries to the tractor. One was connected to the starter. I pulled the small red wire from the starter and connected it to battery #2. I had two circuits, one for the starter and the other for the ignition system. This way I wasn't robbing the ignition coil of the voltage it needed while the started cranked. A short time later, it finally came to life. Hallelujah!

I let it run for about 5 minutes or so and warm up. It seemed to loosen up a bit and started much more easily in subsequent starts. I checked the oil and don't see any glitter, so that's a good sign. I'll get a battery for it and then order belts and a pulley for the deck. Looks like maybe Rick is the go-to guy for that?

Thanks for all the encouragement on this. I was gonna cry uncle if I didn't get something to happen tonight--sorry Kelly.

Everyone have a great evening!

Chad

[Roger from southern Iowa 1]

Glad you got going and it wasn't internal. :omg:

[Raider12 13]

Tell your boss that I'm sorry to disappoint him, but I got it running and it wasn't a broken part.

I bet that felt good when that engine fired up! Iv been there done that on some projects in the past. It sure is a relief to hear it run. Way to hang in there and be persistent. It paid off.

Denver

[gwgdog66 23]

Cool Deal!!