Onan Generator Carbon Knock

[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.

[Bob Mulry]

In the old days GM had a fix for deposits on the back of valves and in the combustion chamber........

The fix was to bead blast the back of the valve and the combustion chamber with walnut shells.......

Combustion chamber and the runners got vacuumed out and the manifolds & spark plugs reinstalled.....

It was the last attempted fix before cylinder head removal....

It seemed to work....

Since it appears that these generators suffer from the same type of problem it might be worth trying first before removing the head???????????

[Rick Bailey]

I was under the impression that Onan made a chemical that you could mix in with gas in a 5gal container and then run the pick up hose to the container.

I did once start the gen and had a knock, not that it was too smart ??? but I just held the throttle at a high RPM for a minute or two and bang the knock was gone (2 seasons ago) . From time to time I will give it the ole High Rpm treatment.............?

Works for me ! lol

[Rich Biebel]

An old trick that does work........WATER......While the engine is running feed it some water thru the carbs air horn......Steam cleans it....and it does work.

I worked in an engine machine shop for many years. If you see how hard the carbon deposits are to remove from heads of all kinds you'd know there is no magic chemical that is going to remove this stuff..

I rebuilt many Mack cylinder heads......The carbon buildup in a diesel intake port is so heavy it can close the port down almost completely shut. The old hot tank does not cut thru this stuff since it's so thick.....

I literally chipped it out with a hammer and chisel before hot tanking the heads....


The carbon buildup in an Onan flathead is nowhere near that bad but it's still the same stuff....unburned hydrocarbon.....COAL....
Superman could make it into a diamond.....LOL
I am sure of it....I saw him do it on TV.......

[Tom Goldman]

Originally Posted by Rich Biebel

An old trick that does work........WATER......While the engine is running feed it some water thru the carbs air horn......Steam cleans it....and it does work.

I worked in an engine machine shop for many years. If you see how hard the carbon deposits are to remove from heads of all kinds you'd know there is no magic chemical that is going to remove this stuff..

I rebuilt many Mack cylinder heads......The carbon buildup in a diesel intake port is so heavy it can close the port down almost completely shut. The old hot tank does not cut thru this stuff since it's so thick.....

I literally chipped it out with a hammer and chisel before hot tanking the heads....


The carbon buildup in an Onan flathead is nowhere near that bad but it's still the same stuff....unburned hydrocarbon.....COAL....
Superman could make it into a diamond.....LOL
I am sure of it....I saw him do it on TV.......

Rich, you are OLD to remember that!!

[Rich Biebel]

Tommy I am old but still pretty spry as you well know......
Who else my age rides a bike for hours on end when racing NHRA events.....Nobody thats who......LOL

I loved Superman.....The dude squished the rock and made a diamond and put in some statue to keep the natives happy......Well thats how I remember it anyway...... .....

I Might run the S/C series this year.....Matt's workin on me...I feel the need for speed......LOL

See ya soon my friend.....Rich

[Peter Ash]

And if they glaze the rings from too much idling/low load time, a little bon-ami down the intake will rescuff the rings and cylinder walls. Used to be supplied by Caterpillar with a part number. Also a long time ago.