520H misfiring

[stuppy62 58]

I have a 520 with the Onan engine with over a 1000 hrs. on it. I just got it this spring and I've had numerous problems that I've solved, but I can't get it to run right. When I got it, it ran on one cylinder, tuned it up, didn't help. Discovered after a compression check no compression on rear cylinder. Adjusted valves and seemed to fix problem, fired on both cylinders. The carb was dirty and someone had put the float & needle valve together wrong, now it runs great for 30-45 minutes. At this point it starts to misfire on rear cylinder & won't straighten out till it cools down completely. Also, now when it starts cold it's one cylinder for a couple seconds till the other kicks in. I've read a lot of post on here about the coil, but I don't want to spend $150-200 on a coil only to find it doesn't fix the problem. Any good advice out there?  

[Martin 2,131]

i could be totally wrong here, but it sounds like a valve seat issue to me. when you adjusted the valves you could have changed things in the combustion chamber a little. maybe heat and cooling in the block surrounding the valve seat is grabbing the seat sometimes and letting it move around when the block expands.

 

i think removal of the rear head is in order to investigate. it could just save you the block if it isn't already damaged.

[stuppy62 58]

If you are correct, what if it's tight when I get in there because it's cold.

[Martin 2,131]

if you remove the valve, i would think there would be tell tale signs the seat has been moving around in there.

 

another possibility is a stuck valve, if the engine sat for any length of time with a valve partially open and the normally covered part of the valve stem was exposed to moisture, you could have one sticking in the guide. best course of action is to remove the head and check it out.

[Forest Road 594]

Great tractor. Made better with a Honda!

[wrmhaste 235]

I fully agree with removing the head and checking the valve, however there are a couple easier things you could try first. Immediately check for spark on the rear cylinder after the 30-40 minute run time when it quits. Then immediately check your compression on that cylinder.

[km3h 543]

The first thing I would do is check the coil when it starts to misfire. Easiest way is to simply switch plug wires at the coil. If the front cylinder misses then you know that the coil fails when hot.

 

Not sure about the Onan, but on the Kohler twins I used a coil from a Harley Davidson Motorcycle on my 417-8. Lots of other guys have done it also and it works like a charm. You will need to make sure that the voltage feeding the coil is correct as there are two different voltages. I am sure there is someone here who can tell you which one to get. The Harley after market coils are very reasonable. Mine was about $32.00 new.  

 

If you have not already done so, you should remove all the tins from the engine and clean the fins thoroughly. The rear of that engine gets caked with debris and causes it to overheat. The maintenance manual states that the engine should be de-carbon every 500 hours. Bet this has not be done before. Heavy carbon can cause all kinds of problems.

 

So I think you should solve the coil issue first and then remove the engine no matter the coil is bad or not and give it a complete cleaning and get that carbon out of there. Here is what my engine looked like after 535 hours.

 

 

 

 

[diamondred 63]

When I bought my 520 , the fella couldn't get it running after doing  various things and changes (geesh), found out the condenser was on the wrong post of the coil.

 

Just saying, some condensers will last a lifetime, some will fail , some will be intermittent with temperature change.  

Edited August 18, 2014 by diamondred

[stuppy62 58]

I did do a compression check after it started acting up & it was the same as always 90 rear, 115 front cylinder. Does this sound about right? Also, the suggestion to switch the coil wires, can you do that?

[Martin 2,131]

both plugs fire at the same time, so yes you can swap wires.

 

same principle as a harley engine.

[km3h 543]

I did do a compression check after it started acting up & it was the same as always 90 rear, 115 front cylinder. Does this sound about right? Also, the suggestion to switch the coil wires, can you do that?

25 lbs different is a lot. Sounds like you may have a valve or ring problem. Best way is to do a leak down test. There is something wrong in that rear cylinder. But it should still start. That engine will run on one cylinder so the first thing you have to do is correct the electrical problem.

Edited August 19, 2014 by km3h

[stuppy62 58]

Ok, it does run on one cylinder, always has, it's just looses the cylinder after it gets hot. How would I do a leak down test? I'm trying to determine whether my problem is electrical or mechanical.

[km3h 543]

First did you switch wires on the coil? To do a leak down test you need a tester and air source. Auto Zone probably has one they will loan you. You will need an air compressor. Here is a link to how to do the test from UTube. Google leak down test and you will find dozens of sites with instructions. 

 

https://www.youtube.com/watch?v=jc3j4ShE-tk

[stuppy62 58]

I finally got it, it was the coil. thanks everyone for all your suggestions. Switching the wires on the coil proved the problem.

[km3h 543]

Glad the problem is solved. Hope you have a long and happy experience with that tractor.