Engine compression on JD 420 lawn tractor

[gschwab]

Hi I have a JD 420 lawn tractor onan engine its got a miss to it I have al ready changed plugs and all that dose any one know what compression it should have I have around 60 psi in both cylingers

 

[bertsmobile1]

60 lbs is way too low if it is a real reading which it can not be or the engine would take an hour to start and blow smoke signals.
Start by checking the mechanical stuff like valve lash and the iming key in the flywheel.
No need to remove the flywheel just look closely at the 2 halves of the keyway they should make a perfect square.

 

[Rivets]

As Bert said 60# is way to low, but the way Onan has their compression release I doubt you are getting an accurate reading. Which Onan engine do you have? Many of the older ones had a points and condenser ignition, which were prone to corrode and get out of time, plus the coils were know to get weak and fail. It could be valve clearance, but on the L-head engines this was very uncommon.

 

[John R]

Onan engines are famous for dropping a valve seat, this is a known problem.

 

[Rivets]

Highly unlikely to loose a valve seat on both cylinders at the same time, and this was most commonly caused by overheating. Would really like to know which model Onan we are talking about, what make it easier to post a good reply and not quess.

 

[Modeere318]

Compression test on a new engine should be near 120 pounds. When you get to about 70 pounds, it is time for a rebuild. B48G engines were not known to drop a valve seat, it was the P220 which would have a valve seat loosen and that was often associated with heating issues. It would usually happen on the same side of the engines. I would check the key way like has been suggested. That would more likely be the cause of the miss.