Onan not running right, weird problem

[multihobbyguy 35]

This is a P220 in a 91 520 that has about 200hrs on it. The tractor runs rough until I take the front spark plug wire off and then it smooths out.  The tractor has 125 compression front and rear. I took the heads off and de-carboned and checked the valves. They were in spec so I did not adjust the valves. Put it back together and it was running rough and popping until I pulled the front plug wire then it smooths out and runs really nice. I changed the spark plug and the wire. I switched the plug wires on the coil and it still pops and shakes. I figured the valve adjustment was out after all but I pulled the cover for the front cylinder and checked them again with feeler gauges and they come out as still .005 for intake and .013 for the exhaust. It will not run on the front cylinder alone but will run on the rear cylinder alone. I have tried cleaning up electrical plugs and connections and even replaced the 9 pin connector with a new one, I am out of ideas and I was hoping that someone else would have an idea as what I should check next. If you have an idea I would appreciate your input.

 

Thank you, Chris.

[daveoman1966 3,624]

I had a similar issue with an 89 520 Onan and the problem was a blown intake manifold gasket.  Replaced gasket....still runnning strong today with <500 hours. 

[WH nut 553]

have you determined if it has spark on both cylinders

[squonk 39,008]

I think Dave is right Vacuum leak is causing too lean a mixture to fire right. I've seen this exact thing on cars a lot.

[sorekiwi 761]

If it is a vacuum leak in the intake, you can sometimes pinpoint it by waving the nozzle of a propane torch (don't light it!!) around the intake manifold.

 

The Onan intake manifolds are made from 2 pieces, sealed and riveted together, and can leak if the sealer deteriorates.  I think Martin has shown pics of resealing one.

 

When you switched the plug wires around, was it still the front cylinder that was running rough?

[AMC RULES 36,954]

Geez...it's got like no hours on it. 

[wheeledhorseman 573]

My experience with having a leak somewhere on the manifold itself (made in two halves as Mike has mentioned) was that it ran fine when cold but started hunting when hot. It was a 16 hp but principle is the same.

 

I'd go for gaskets first - but if you have to do the manifold you may find the post I did on it useful. You can find it here:

 

Hope you find a simple solution, if not then this may be helpful to you. Well worth the effort of putting them right.

 

Andy

[leeave96 487]

So if you pull the head - are loose valve seats self evident?  Can a blind bat like myself see that there is a problem with the valve seats?  I've got two of these Onan's I've got to troubleshoot this spring - just looking ahead.  Sounds like if everything else is in order, then the manifold might be leaking.

[multihobbyguy 35]

I had a similar issue with an 89 520 Onan and the problem was a blown intake manifold gasket.  Replaced gasket....still runnning strong today with <500 hours. 

I took the exhaust and intake off today to get to the other set of valves and when I replaced them I wiped them with a gasket maker type stuff that makes sure you have a even seal. Still no luck.

 

have you determined if it has spark on both cylinders

It does have spark on both cylinders. Very nice spark.

 

I think Dave is right Vacuum leak is causing too lean a mixture to fire right. I've seen this exact thing on cars a lot.

I checked for leaks and haven't found any yet. I think there maybe something else that I am not figuring out because it smells like raw gas something terrible when it runs and it almost seems like I am getting blow back through the carb. I checked the valves again today and they seem to be spot on when I rotate them and check them with the feeler gauges.

 

If it is a vacuum leak in the intake, you can sometimes pinpoint it by waving the nozzle of a propane torch (don't light it!!) around the intake manifold.

 

The Onan intake manifolds are made from 2 pieces, sealed and riveted together, and can leak if the sealer deteriorates.  I think Martin has shown pics of resealing one.

 

When you switched the plug wires around, was it still the front cylinder that was running rough?

I went to Home Depot today and bought a propane cylinder and nozzle to try this. All I had was mapp gas and wasn't sure what might happen. Unfortunately I turned it on full blast and and ran it all over the when the manifold bolts to the tractor, all along the sides of the manifold and around the carb mounting and where the carb splits and the tractor didn't seem to notice one bit.

 

Geez...it's got like no hours on it. 

The engine doesn't, but the tractor does. The tractor has 900+ on the meter but the rear seat went out of it. My father found me a P220 out of a Sears Craftsman and I swapped it over. The Sears tractor only had 200 hrs on it but had set out side it's whole life. So I took all the sheet metal and swapped it out and went ahead and decarboned the head while it was out. I used the carb and manifold from the 520's original engine along with the electronics(coil and wires). This engine had not ran in a couple of years when I got it.

 

My experience with having a leak somewhere on the manifold itself (made in two halves as Mike has mentioned) was that it ran fine when cold but started hunting when hot. It was a 16 hp but principle is the same.

 

I'd go for gaskets first - but if you have to do the manifold you may find the post I did on it useful. You can find it here:

 

Hope you find a simple solution, if not then this may be helpful to you. Well worth the effort of putting them right.

 

Andy

Thanks for the link Andy. I read it and may need to try it eventually. I am thinking after waving the propane torch all over the engine and the engine seeming to blow back through the carb I have something else wrong.

 

So if you pull the head - are loose valve seats self evident?  Can a blind bat like myself see that there is a problem with the valve seats?  I've got two of these Onan's I've got to troubleshoot this spring - just looking ahead.  Sounds like if everything else is in order, then the manifold might be leaking.

On the original engine I didn't see it until i rotated the engine and then when the valve was open I could wiggle/ remove the seat from the block. The engine also had low compression on that cylinder.

[multihobbyguy 35]

So this is where I am at now. I tried the propane. I checked the valve clearances again. I see that the engine seems to be blowing fuel out the carb. I am going to head back to the garage and pull the front head as that seems to be the cylinder I am having trouble with. I am confused that the engine seems to be blowing gas back through the carb but the valves seem to be correct and I am putting out about 120psi on front and read cylinders. I would think that I would have a hard time building that kind of pressure if the valve was stuck open for there to be blow by. I have had Kohlers with stuck valves that blew gas out the carb but they wouldn't build much compression. If you have any thoughts or suggestions I would love to hear them. I am running out of ideas.

 

Thanks again, Chris.

[WH nut 553]

Ok Does it seem to run ok when first started then starts missing. Blowing out the carb indicates and open valve, Stuck exhaust valve, maybe worn cam not opening the valve all the way, or plugged muffler

[multihobbyguy 35]

Ok Does it seem to run ok when first started then starts missing. Blowing out the carb indicates and open valve, Stuck exhaust valve, maybe worn cam not opening the valve all the way, or plugged muffler

It does not run well at start up. I just started it again and it is definitely blowing fuel out the carb. I never did think about a plugged muffler. I will start it up one more time to check exhaust and then i will pull the head. It almost has to be a valve problem but I am still confused as to why I have great compression.

[WH nut 553]

It does not run well at start up. I just started it again and it is definitely blowing fuel out the carb. I never did think about a plugged muffler. I will start it up one more time to check exhaust and then i will pull the head. It almost has to be a valve problem but I am still confused as to why I have great compression.

Cam not opening exhaust valve far enough, Or exhaust

Edited February 20, 2014 by WH nut

[multihobbyguy 35]

Both valves seem to open and close correctly. Both valve seats seem to be set and do not seem to move.

[leeave96 487]

Wild guess here, but this engine came off a sears? You changed the manifold and carb. Other than those things, is the engine otherwise the same, is it a P220?

I've got an Onan on a 520H and on start- up, it will flood crazy easy. I've come home from work and my boys have tried to start it and flooded it bad. Once I get it to fire it runs rough for a bit and I looks to me like it's spewing gas out the exhaust!

I've also got a IH Cub cadet and one time the carb was way to rich and it would spit a little gas out the carb. It also ran ruff as a cob!

I say all the above to say this - is your carb to rich in its setting? If the engine is different in some way vs the original, the carb might need to be adjusted to this new set up.

Good luck,

Bill

[multihobbyguy 35]

Wild guess here, but this engine came off a sears? You changed the manifold and carb. Other than those things, is the engine otherwise the same, is it a P220?

I've got an Onan on a 520H and on start- up, it will flood crazy easy. I've come home from work and my boys have tried to start it and flooded it bad. Once I get it to fire it runs rough for a bit and I looks to me like it's spewing gas out the exhaust!

I've also got a IH Cub cadet and one time the carb was way to rich and it would spit a little gas out the carb. It also ran ruff as a cob!

I say all the above to say this - is your carb to rich in its setting? If the engine is different in some way vs the original, the carb might need to be adjusted to this new set up.

Good luck,

Bill

Yes the engine is the same. There is only one adjustment other than the throttle stop on the carb. It is the low idle I believe.

[multihobbyguy 35]

Ok Does it seem to run ok when first started then starts missing. Blowing out the carb indicates and open valve, Stuck exhaust valve, maybe worn cam not opening the valve all the way, or plugged muffler

Ok, so earlier I started the engine and put my hand in front of the muffler discharge and it was pushing warm air but was kind of pulsing. I attributed that to the miss in the front cylinder. So I just got done taking the top off the motor again(exhaust and manifold) and I wasn't finding anything that looked off. So I was sitting there and decided to put my mouth on the muffler and give it a blow. The rear blew nice and easy. I put my mouth on the front hole and blew through it and about popped a vein in my eye trying to blow through it. You can blow through it but it is under a lot of pressure. So I am going to reassemble with the other muffler and try it out. I am not sure what happened to the muffler to make it so hard to blow through but I am really hoping this is my problem.

 

Thank you everyone for the help so far. I will update the thread once I reassemble and try it again. Chris.

[JackC 616]

I would clean the carburetor and put a new float needle in it.  Those engines are very sensitive to any issues at all with the carburetor.  Maybe the float needle is not working properly and it is dumping too much fuel into the front cylinder to fire properly which may also explain fuel backing up through the carburetor.  You can check to see if the plug is wet.  

 

Bugs or animals may have plugged the exhaust.  You might try running it without the exhaust on it to see if that is the problem. I would still overhaul the carburetor.  In my book, that is the number one most important procedure when dealing with an engine that is not starting or running properly.

[wheeledhorseman 573]

Following this with great interest Chris so do let us know.

 

I noticed a distinct change in the sound from the exhaust on my Onan a few weeks back that was definitely linked to one cylinder and thought one of the baffles must be rotting out inside the can. What you've found has got me thinking I need to check mine out as maybe bits that have rotted off inside can actually block the passage.

 

Andy

[multihobbyguy 35]

Ok Does it seem to run ok when first started then starts missing. Blowing out the carb indicates and open valve, Stuck exhaust valve, maybe worn cam not opening the valve all the way, or plugged muffler

We have a winner. I just got it somewhat back together with the Sears strait pipe and it runs smooth (although loud). Thank you everyone for your help. Now I have to figure out what to do about putting a muffler on the tractor. I have seen posts on Red Square about how much they have been selling for lately so I will have to decide how much I want to spend. Once again thank you so much! Chris.

[WH nut 553]

Cool, glad to hear

[boovuc 1,090]

Glad to hear this and I'm putting this tidbit of knowledge in the colander that is what's left of my memory.  

[multihobbyguy 35]

I think I will buy a used muffler or build something else. Has anyone ever cut an Onan muffler up to see what was inside? I checked partstree and they wanted

21 Toro Consumer 116471
MUFFLER-EXHAUST Your price: $652.04

That is a lot of money for a muffler.

[sorekiwi 761]

"Wheelbearing" had some pics of the innards in his restoration thread a couple of years back.

[wheeledhorseman 573]

Great news Chris and well done WH nut for suggesting a plugged muffler. This is certainly one for all Onan owners to note!

 

I say this because in looking for answers to the symptoms I did come across a thread elsewhere where a guy's 18hp went lame with the exact same symptoms but whilst mowing. Following the thread the usual checks were suggested and made to no avail but then the thread stopped so no solution was ever reported.

 

As I mentioned earlier, a few weeks back I noticed a distinct change in the sound from the muffler which has lost that distinctive Onan sound on one cylinder. I'll need to take it off and check now that it's not signalling the start of a major blockage.  Given the really sweet sound these normally make and the cost of a replacement I'm wondering if there's not some wadding in there somewhere that gets let loose when the internals begin to disintegrate. Well worth an autopsy I'd say.

 

Andy