Compression Test on an Onan - John Deere 318 or 420

How I do a compression test on an Onan engine found in a John Deere 318, 316, or 420.

[@SF-fm7ov]

I just bought a rough '86 420 that has not run for 2 years for $350. I got it running. Cold compression is 85 & 90. So I will probably have to rebuild someday. Doesn't feel like the carb is right. Would you recommend buying a new one or rebuilding the original. If new do you recommend one. Any advice is well appreciated.

[@tes997]

If you have 120 psi compression, can you still have bad headgasket?

[@mattbrennan8088]

nice

[@mikejoyce3782]

5:47 HAHA Your brain was so focused on remembering that your righty-tighty got delayed.

[@mmrbeef]

My brain struggles with the righty-tighty concept a lot if you have not noticed... this one was really bad!

[@jeffridgeway201]

Silly question but what some signs of a bad spark plug? See I have a little white smoke on initial start up but like you once hot it will Start again and purr.. recently tonight engine was bogging down? You think bad fuel? Spark plug bad? Or carb needs cleaned??

[@mmrbeef]

A bogging engine can be lots of things.... clogged filter, trash in the carb, air flow restriction, and the list goes on. Generally unless there is a huge build up of carbon or oil on the plug and the spark is nice and bright, I would start with the simple fuel and air inspections first.

[@RyanTitcomb]

You should have both spark plugs removed when doing a compression test

[@mmrbeef]

That is a great point and something I forgot to mention. Some manuals will say remove both, some don't specify, and some have the user remove one at the time like I did. On Onan's I've tried it both ways and it doesn't seem to matter... at least in the few test runs I have done it hasn't mattered. The compression did build faster with both plugs out same with the choke and throttle wide open but the overall high compression was identical across several runs. I do not know if this same principle will hold true on other engine designs. Now you have me thinking!! Thanks for the comment.

[@RyanTitcomb]

@@mmrbeef I'm an automotive technician by trade and have always been taught all plugs removed and throttle wide open during a compression test on any engine. Thanks for taking the time to reply

[@aidenwebb9466]

My 318 smokes when it gets hot after mowing for a half hour or more. Do you think there may be something wrong with it.

[@mmrbeef]

There could be, but do some test and that will help you figure out what you might need to do. The first question I might ask is what weight oil? Sometimes the multi-weight oils will smoke once they get nice and warm, but 30 mins seems a little long for that time frame. If you have multi-weight try dropping some straight 30 wt oil in it and see if that fixes the issue or at least lessens the amount of smoke.

[@aidenwebb9466]

mmrbeef thank you, I normally put 10-30 in it I believe. I will try straight 30 on this next oil change.

[@ThePadutch]

For those of you who need to get an accurate reading from a compression test: You need to remove ALL the spark plugs, in this case both plugs, make sure the choke is off and the throttle is wide open. You can't get compression if it can't draw in air ...

[@mmrbeef]

Please see my reply to RP Titcomb from two years ago that addresses this comment. In the two years since this video was posted I have tried Onan's with and without plugs and I have yet to see a difference in the ultimate compression value with the plugs in or out of the other cylinder. I DO think plugs being removed in multi-cylinder (3+ cylinders) engines becomes more important.

[@williamriley2528]

Yep, the theory is you want the least engine resistance possible during your compression test, EXCEPT for the cylinder compression of the specific engine cylinder you intend to rest. capish...???

[@drelliott2833]

Pull both plugs before doing a comp test!!!

[@mmrbeef]

Good point. I cannot remember if I also mentioned that the throttle should be wide open. However, both spark plugs being removed has little to no effect on the compression reading (at least on the Onan's that I have tested). By removing the other spark plug the engine will turn over faster with the starter since there is less resistance. This yields a faster result... but the result, at least in the times I have tested it, are the same. The only difference that I have ever picked up is on a cold engine - less that 50 degrees with 30 wt oil might have a 2-3 psi lower reading with the other plug in.

[@williamriley2528]

13mm std Spark Plug Thresd...

[@robertduncan2742]

I could use the hood and grille.

[@mmrbeef]

Sadly that machine was gone long ago. I might have a 420 grill somewhere.

[@robertduncan2742]

@@mmrbeef check on the grill and let me know. I have to pull the seat and cover from mine to check the 2 filters in the tank and hope their plugged since it doesnt run properly. A Honda repower with 400 hours.