PotterRich 0 #1 Posted May 10, 2018 (edited) I have a 520-H with a 20 horse motor - P-220-G. When I use the mower deck it loses power. When I put a spark tester between the plug and the distributor, one cylinder seems to be weaker than the other. The brightness of the bulb is less on one cylinder. Could this be the distributor (spark sending unit)? I Guess this is the coil/battery sending unit. It is pretty costly so I could use some re-assurance that this would restore the power I know it should have. Edited May 10, 2018 by PotterRich Add clarity Share this post Link to post Share on other sites
Irv 115 #2 Posted May 11, 2018 Probably the dreaded valve seat come loose on the rear cylinder of that motor. 1 Share this post Link to post Share on other sites
lynnmor 6,762 #3 Posted May 11, 2018 Always, always check the compression first thing. Continuing to run an Onan with a bad valve seat can damage it beyond repair. Not sure that is the problem, but you must check. Report back. Share this post Link to post Share on other sites
953 nut 51,505 #4 Posted May 11, 2018 22 hours ago, PotterRich said: The brightness of the bulb is less on one cylinder. Since the indication is a cylinder getting leSs spark than the other I would suspect a BAD PLUG WIRE. The same coil feeds both cylinders and you can swap plug wires from cylinder to the other or substitute another known good plug wire to see if it is just the wire. 1 Share this post Link to post Share on other sites
PotterRich 0 #5 Posted May 14, 2018 Thanks all! I will check the compression then the spark wires. Rich Share this post Link to post Share on other sites
