I have a Coleman Powermate (Model PM0525303) with a 10 HP gas engine (Tecumseh Engine Model HM80-100). It starts right up and runs fine for about 3 or 4 minutes. Then the engine starts to pulsate (i.e. RPMs go up and down) and subsequently shuts down almost like it ran out of gas. Sometimes, the engine might backfire shortly before shuting down. I use a fuel stabilizer (Stabil) , but I did noticed the plug had carbon deposits on it. So, I replaced the plug, drained the gas and replace it with new fuel (including "Sea Foam" fuel stabilizer/cleaner this time instead of Stabil). This did not solve the problem. The engine starts right up again when I set the choke and pull the start cord.....but the it always shuts down around the five minute mark. I tried this several times to get the new fuel and cleaner into the engine, but it always shuts down around the five minute mark. Help!
I have an HM100 on a 5500 watt gen with the same problem. This motor calls for a J17 spark plug, but I found using a J19 (hotter) plug solved the problem. I think the J17 doesn't get hot enough for a complete combustion burn so it act like fuel starvation thus resulting in the black sooty and wet plug.
The carb may have to have a new kit installed. or theres a good chance the coil is heating up and may need to be replaced. also look for a low oil pressure switch on the engine there bad about going bad. when you find it disconnect the wire to it and try it and see if it runs longer.
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