312Hydro 474 #1 Posted May 31, 2011 Hi, I need information on an Onan 20 hp . It has low compression in the rear cylinder but seems to run well after warming up. The last sentance is the OP's. I'm looking at buying it but want to know what I'm getting into. I did a search in the RS Engine forum and from what I found out , the rear seats come loose on these. Also ran across info about the valves carboning up and staying open. Could any of you chime in hear with your opinion on this? I'm thinking about buying the but not if the motor is going to require alot to repair. My understanding is that it would be about $180 for just an oversized seat and machine work? I know without taking it apart it's not easy to diagnose. It's in a 90's 520h. What do you think would be a fair offer if it's repairable? I would appreciate any and all opinions . Thanks,Rich Share this post Link to post Share on other sites
mavfreak 11 #2 Posted May 31, 2011 It has been said on here that the rear cylinder overheats and the vavle seat can come lose. Once warmed up the seat and block swell and it can be back in place until it cools and is lose again. The easy way you can find out and cheapest is to just pull the head and have a peek! If thats not it your out a head gasget but you can decarbon the head while apart. Share this post Link to post Share on other sites
312Hydro 474 #3 Posted May 31, 2011 It has been said on here that the rear cylinder overheats and the vavle seat can come lose. Once warmed up the seat and block swell and it can be back in place until it cools and is lose again. Thanks,mavfreak. I guess the longer it runs like that the chances of enlarging the block from the loose seat increases? Am I close on the material and machine work estimate? Anyone have this done recently? I don't mind getting the engine repaired but before I make an offer I'd like to know the approximate cost on my end for the repair. Share this post Link to post Share on other sites
mavfreak 11 #4 Posted May 31, 2011 That I don't know but I'm sure someone will be along who does. Good luck Share this post Link to post Share on other sites
312Hydro 474 #5 Posted May 31, 2011 Thanks, I called a shop and they are familiar with the Onan seats. The fella told me that the block would need stripped down to basically a short block before they can machine it. Also said that a valve job on both cylinders would be a good thing to do . Share this post Link to post Share on other sites
shorts 182 #6 Posted June 1, 2011 Agreed pull the heads and prep for a complete valve job including guides if necessary, also check the cylinders for wear, if they are out of specs now is as good a time as ever to freshen up the engine completely. Share this post Link to post Share on other sites
