Running on 1 cylinder

[S416-8 332]

Got rid of the Try-Bilt GTX20 and picked up two 520 with the Onen. ONe runs fine while the other I got running but only on the inside cylinder. The other has good spark and when I pulled it it was wet and getting fuel, but if I connect the plug wire the machine runs for a min at a low RPM and then dies. When I dIsconnected that plug again it and it will stay running.. To top that off the carb is spitting fule back into the air box.. haven't pulled the head yet but planning on that soon.. any thoughts ? Thanks

[davem1111 2,063]

I'm no expert on engine problems but since no one has commented yet.... it sounds like a valve problem on that cylinder, to me.  Compression check first if you can?  Then I'd anticipate some valve work. These Onans also have issues with the split intake manifold leaking, but I don't think that would explain fuel blowback.

[S416-8 332]

Great thanks.. was just looking at a pressure tester on the Harbor freight site

[davem1111 2,063]

4 minutes ago, S416-8 said:

Great thanks.. was just looking at a pressure tester on the Harbor freight site

 

Yeah, something I don't have and should probably get one.  

[squonk 41,737]

If the intake seat has popped, then yes you will get blow back thru the carb. 

[Maxwell-8 4,366]

I once had two different sparkplugs on my Onan. it made it run on one cylinder. bought two new sparkplugs from the same type an it runs perfect

[S416-8 332]

Ok cool. I think like squonk about the blow back I should check the intake seat. 

[lynnmor 7,443]

They can run rather well with a loose valve seat.  A compression test may not be off with a loose seat.  But yes, the first order of business is a compression test.  You can swap spark plug wires at the coil to see if the fault swaps sides, both plugs fire at the same time every stroke.

[Bill D 2,037]

Consider making up some new solid core plug wires once you've verified that that you don't have a bad valve seat or broken rod/piston.  If the wires are original then they're probably 30 years old and may be dry rotted and "leaking".  

 

If you do find something catastrophicly wrong with the engine, consider swapping in a P216, much less likely to have major issues.  Save the P220 for parts.

 

 

Edited January 21 by Bill D

[S416-8 332]

I actually bought this for parts but decided to see if I could get it running. Not going to put much cash into it but will give it a try.. worse come to worse I have a ton of parts for the one that I am using. 

Edited January 21 by S416-8

[Bill D 2,037]

22 minutes ago, S416-8 said:

I actually bought this for parts but decided to see if I could get it running. Not going to put much cash into it but will give it a try.. worse come to worse I have a ton of parts for the one that I am using. 

Onan parts are getting scarce.  Definitely save what you can.  Personally, I won't touch the Onan 20.  Too many problems for 4 extra horsepower.

[S416-8 332]

will do thanks

 

[953 nut 56,342]

16 hours ago, Maxwell-8 said:

I once had two different sparkplugs on my Onan. it made it run on one cylinder. bought two new sparkplugs from the same type an it runs perfect

Start with the simple inexpensive maintenance items first. Your spark plugs will fit either cylinder, swap them out to see if the problem follows the plug.. Both cylinders get spark on every revaluation of the crankshaft, swap the plug wires from front to rear and see if the problem follows the plug wire.    

[Pullstart 63,272]

18 hours ago, S416-8 said:

Great thanks.. was just looking at a pressure tester on the Harbor freight site

My thumb has always worked pretty well for this.  

[Red-Bovine 324]

I had an Onan 520H that puked the valve seats several times. I repaired them by putting the seat back in place and staking it by using a pin punch to peen over the edge to make it fit tighter, Some owners have sent them to a machine shop to replace the seats. This was a common problem with the 520H Onan engine and other ones. I eventually repowered with a Honda V twin. 

[S416-8 332]

Thanks.. I did have one mahined a few years back.. Not sure what I am going to be doing with this one. Part it or try rebuilding it.

[Bill D 2,037]

Any updates?  Curious to know what you found.

[S416-8 332]

sorry not yet.. been well below 0 each day after work so have been avoiding all things outside.. pressure test kit should be in tomorrow and then I hope to do some digging this weekend.

[S416-8 332]

Testing kit in tomorrow so will see what it shows and update the thread. Then I just ended up picking up a barn find D180 and pick it up tomorrow. Now to find some D180 accessories. 

[S416-8 332]

Got a new toy !! Now to find a 3-point hitch and a plow

 

Edited Thursday at 11:50 PM by S416-8

[oliver2-44 9,968]

Your on a roll. 

[S416-8 332]

for the 180 I hope that I can find the 3-point hitch for it. if anyone has one for sale let me know.. 

[S416-8 332]

Amazon deliver is horrible now and they delayed the pressure tester a week.. will need to get one a harbor freight when it I can

[Beap52 961]

On 1/21/2025 at 7:19 AM, Pullstart said:

My thumb has always worked pretty well for this.  

Actually if a fellow was to  keep his thumb over the spark plug hole and hooked the spark plug wire to your pinky, it might fire up! 

[S416-8 332]

could be a shockingly semi-fun test if I can find sucker to give it a try