1987 JD 420 compression issue
Hello all. This is my first post and I thank you in advance for any advice you can offer. My JD 420 (just over 1,200 hours) Started acting funny this past summer in regards to some loss of power when mowing through tall grass which was never an issue before. Additionally, it became increasingly difficult to get the tractor started this summer and by the end of summer it was taking several minutes to get the tractor started and then it would only run at about half power until the tractor warmed up for 5 minutes. Once the tractor did warm up it ran rather well and did a fine job cutting my 1.5 acres. At one point my negative battery cable melted resulting in the need for a new cable and battery. After the last cut for the summer I was unable to get the tractor started at all so I trailered it to a local JD mechanic. The mechanic did a compression test and told me I had almost 0 compression and one cylinder and about 40 in the other. He told me it is a valve issue and that the old Onan P220G engines are notorious for valve issues due to high heat. His advice was to re-power with a vanguard 23 hp due to the Onan being an L block engine. He told me a valve job would be so labor-intensive I wouldn’t save that much money and then quoted me an estimated cost of $2500 to replace the engine with the Vanguard.
I put almost $2000 in two new tires, all new filters, oils and plugs along with having the mower deck patched, needle scaled, sandblasted, primered and painted so I do intend to keep this tractor for a long time.
Based on all the reading and research I have been doing it does sound like valve issues are common with the engine I have. I have had no issues with burning oil, just the compression issue so I was wondering:
Could this possibly be a coil issue, or a head gasket issue, or could the valves just need cleaned up and adjusted? A friend of my family it is a quality auto mechanic and Crew chief for a nationally sponsored drag racing team and he is willing to help me out as a favor. Does anybody have any advice on where to start in the effort to get the engine running properly again. I would very much like to avoid re-powering at this time if at all possible.
Thanks for your help!