Onan Generator Carbon Knock

[brett3533]

Mark, You might try some sea foam from napa. Start gen, and pour it into carb and stall out the gen. Alloy to set for between 20 and 30 minutes then restart. It will smoke alot but may loosen up and clean out your carbon deposits.

[dennis dunlap]

Mark ,I,ve used the GM upper cyl lube for many years ,worked for us. Sprayed it in an let it set , i used a long flexable hose to get in further.
Spin it over spray, let it set, repeat .I always had the headgaskets but never did pull the heads .Running generater with no load on it seems to be the enimey.....

[Mark Ruset]

Thanks to all who replied, I'm going to try the GM Top Engine Cleaner. I let you all know how it went

[Darrin Christen]

I had the same problem. I thought is was a bad rod bearing or broken piston. When I took the heads off I found a smashed chunk of carbon on top of one of the pistons. I probably could have sprayed something through the spark plug hole to dissolve it but I already had the generator out and apart. I would try some carbon cleaner before taking it apart. You can try a marine dealer who sells outboard engines. They carry several different types of carbon cleaners you spray into the engine while running.

[Mark Ruset]

Thanks Darrin
I'm feeling alot more confident that carbon is my culprit especially since the generator has not seen any real use in the past 3 years. I"m going to try a spray through the carb first on a hot engine and if that fails I'll try going through the spark plug hole and fill the cylinders up with a cleaner and trying to dissolve the carbon

[Rich Biebel]

The Onan brand, 4 C cleaner is a good product but it will never clean the carbon as well as I do when I do one of these machines. Removing the heads is the only way to really get the job done properly. It is a recommended service by Onan every 500 hours. Doesn't have to be done that often but will need that service at some point in it's life.

The carbon buildup is very bad on these units and especially on the rear cylinder. Removing the heads.....clean them to like new and I also do the pistons and the valves and seats. The backs of the intake valves will have heavy carbon deposits.

They can get so bad the engine will not turn over! Every time you shut it down the remaining fuel solidifys and builds up on one side of the piston due to it being a horizontal engine......The carbon piles up so high it will stop the engine from turning over and that is also the carbon knock you hear.

We ( me and Matt Schurman) removed one of these units from a Salvation Army canteen vehicle at the World Trade Center site just after 9/11......and that one was the first time I saw one so bad it would not turn over.....I fixed it up and it ran for years after that....

[Tom Goldman]

Rich is right on about removing the heads as opposed to using carbon cleaner.
Most carbon cleaners are a shadow of what they used to be before the EPA rules forced manufactures to tame them down or eliminate them completely, since the mostly used methlyene chloride.
My old Onan was so bad , I thought for sure it broke a piston or worse, but after removing the heads I found that there was absolutly no where for any more carbon to form, it was packed solid!
Cleaned it up and its still running for the current owner with over 6000 hours on it!

[Mark Ruset]

Well I pulled the heads off today and found a massive amount of carbon build up.Additionally found the intake valve stem seals were leaking causing the carbon. I'm sure no amount of spray or Top Engine cleaner could have saved me from pulling the heads. Thanks to all with their suggestions.Not sure if there is a remedy to keep the carbon from returning.