Evenin' folks, need some advise

[Archie B 6]

I have what we were told is a 1989 520H 20hp with a 60 inch deck. Been a great machine going into our second summer, when just a day ago, it started stalling, with black smoke puffing out the exhaust. I took the breather off, when i noticed fresh oil in the breather. Out of curiosity, i started with breather of and it started right up without hesitation, but i could see what looks to be oil misting from the carburetor. Any suggestions or ideas of the cause would be very heplful, thanks 

[Pullstart 66,747]

 

I am not too familiar with the 20 hp Onan in that tractor, but I have heard that too much cylinder heat on especially the rear cylinder is prone to taking out the valve seats.  It could easily cause a vacuum and pull oil through the combustion chamber.  Why it mists from the carb is beyond me, but possibly related.  
 

 

[lynnmor 7,817]

 

Have you replaced the air filter and pre-filter?  Start with that.

 

The carburetor may need cleaning or repair, smoking may be gasoline flooding from a problem in the carburetor.

 

It is always a good idea to check the compression to get an idea of the overall health of the engine.  How many hours on the engine?

[Ed Kennell 41,974]

to the   Archie.

 

     The twin should run on one cylinder.    To determine if the problem is related to a cylinder,  pull the plug wire off the rear cylinder and see if the engine will run on the front cyl only.

Then reverse the procedure.    If it is cylinder related, a compression check must be done.

[echris 1,425]

15 minutes ago, lynnmor said:

The carburetor may need cleaning or repair, smoking may be gasoline flooding from a problem in the carburetor.

 

 

I agree.
My first job, at 13, was at a Wheel Horse dealership.

The first rule they taught me:
If it's a fuel problem, remove, disassemble and clean the carburetor.
Do NOT waste your time spraying carb cleaner and revving the throttle.
Remove the carb, disassemble and clean.
It takes about 15 minutes.

[Lee1977 7,208]

You might want to check the crank case breather to see if it's stopped up. It's in a rubber tube clamped to the top of the engine with a hose clamp. 

There is a Onan owners manual in the Manuel section of Redsquare.

Edited June 8, 2020 by Lee1977

[SylvanLakeWH 28,400]

[squonk 43,819]

8 hours ago, Ed Kennell said:

to the   Archie.

 

     The twin should run on one cylinder.    To determine if the problem is related to a cylinder,  pull the plug wire off the rear cylinder and see if the engine will run on the front cyl only.

Then reverse the procedure.    If it is cylinder related, a compression check must be done.

I believe this engine uses "waste spark technology. Which means both plugs fire at the same time. Spark voltage leaves one coil, jumps the plug gap on that cyl to ground. Then it leaves ground on the opposite cyl. jumps the plug gap and returns to the opposite coil terminal completing  a circuit. Removing 1 plug wire and just letting it hang may kill both cylinders. You may need to ground the plug wire you unhook. 

 

I do not know if this is true for the Onan ignition system. but every car I worked on with a single coil for 2 cylinders works this way.

[Ed Kennell 41,974]

20 minutes ago, squonk said:

 Which means both plugs fire at the same time. 

Correct, but I believe there is only ONE coil firing both cylinders at exactly the same time.      One cylinder being in the power stroke and one cylinder being in the exhaust stroke.

I don't see how removing one plug wire could cause any damage as the single coil always has a path to ground thru the connected plug wire when it fires.   

Does this sound correct Mike?

I can't say I have ever done this on an Onan, but I have used this technique on Kohler KT 17s without any damage.

 

[The Tuul Crib 7,339]

T😎 

[squonk 43,819]

1 hour ago, Ed Kennell said:

Correct, but I believe there is only ONE coil firing both cylinders at exactly the same time.      One cylinder being in the power stroke and one cylinder being in the exhaust stroke.

I don't see how removing one plug wire could cause any damage as the single coil always has a path to ground thru the connected plug wire when it fires.   

Does this sound correct Mike?

I can't say I have ever done this on an Onan, but I have used this technique on Kohler KT 17s without any damage.

 

You won't damage anything Ed. But as an example: When I worked at GM garages on the 4 and 6 bangers they had 1 coil firing 2 cylinders. If you pulled a coil wire off you killed both cylinders to that coil. If a car came in with a skip we used a 12Volt test light. We would slip the probe between the wire boot and the wire on each cylinder. When the engine skip didn't change,we found the bad cylinder. We provided the ground thru the test light so the circuit was completed. At least on the GM DIS ignition the spark traveled as I mentioned. And I was taught that's how waste spark technology works. I'm sure it was all figured out in the ign module. It was pretty neat and simple to diagnose. Perhaps kohler and Onan modules separate the circuits and allow you to pull a wire off and not effect the other cylinder.

Edited June 8, 2020 by squonk

[Archie B 6]

Mornin' everyone. i am very thankful for such quick replies. i was standing in the shop scratching my head, wondering where to start. My son said, "check you forum you joined. i smiled and said, they ain't gonna answer that fast. but to humor him, Welp, here i am. Got to admit, was very surprised when i logged in. I'm gonna get back at it with quite a few ideas now. I'll let everyone know what we find. Thanks again, and y'all have a good day and a better one tomorrow.