Onan Head De-Carbing

[MarkPalmer 81]

The valve seat problem wasn't due to how the engines were made as it was done by the industry standard, the same way everyone else who used aluminum blocks/heads did it. (If it were a design fault, the problem would occur much sooner than it typically does.) Those engines need to have their valve lash adjusted at the proper intervals as the best way to prevent the problem. The trouble occurs when the valves start to wear a little the lash begins to decrease and the valves progressively start getting hotter during operation. Re-adjusting the lash compensates for this, yet it so often gets ignored. If not done, the excessive heat starts softening the aluminum block material, and the seat first begins to pound in to the block and then starts getting loose. The repair is expensive, too expensive IMHO to not take the time to adjust the valves. One of those procedures that takes some time to learn the first time around, but is easy thereafter. It is better to adjust the valves a little loose and hear some noise rather than have them set too tight. If you run an Onan engine, I would at least check the valve lash every 250-300 hours of running.

-Mark-

[mattd860 162]

The motor can be easily rotated on the frame so you can access the rear head. You have two options - you can remove the (4) bolts that secure the motor mounting plate to the frame or you can remove the (4) bolts that secure the motor to the motor mounting plate. The (4) bolts that bolt the motor to the plate are A LOT easier to access and remove/re-install. Once the bolts are out, all you need to do is remove the drive belt and the fuel line (possible the throttle and choke cables but I can't remember). All this only takes 15-30 minutes tops.

To clean the heads, I used carb cleaner and a scotchbrite dish-washing sponge. I also used a fine stainless-steel wire wheel on the end of my Dremel to clean the difficult areas like the spark plug hole. You can also use a plastic paint scraper if needed.

Someone correct me if I'm wrong but I don't believe you have to clean ALL the carbon off the heads. If you risk damaging the soft aluminum head to remove the carbon that's caked on over the years, I would say leave it there and just remove what you can.

[MarkPalmer 81]

The main reason behind removing the carbon is it creates "hot spots" that can cause detonation. No, you don't have to remove it all and make the engine kitchen clean just remove it within reason. I soak it with carb cleaner then use a scouring pad. In fact today's detergent gasolines produce very little carbon buildup compared to those in the past.

-Mark-

[SAM58 30]

GREAT CHANCE FOR A STEP BY STEP THREAD!!!!

TAKE A LOT OF PICTURES!

[MarkPalmer 81]

If the valve seat problem makes anyone feel bad, it could be a lot worse. This exact same seat loosening problem occurs often with big truck diesels where the valve lash is neglected too long- and those are cast iron heads that the hardened valve seats come loose in! It was about a $4000 job to put a new head on a Detroit Series 60, yet only a few hundred bucks to adjust the valve lash. You can adjust your Onan engine valves for free just by reading the service manual.

-Mark-

[zeedubbya 18]

Just as a quick update--I got home later than expected last night and was only able to adjust the valve lash. Both of my intake valves were way off with the rear intake valve being totally frozen. Exhaust valves were pretty bad too. At TDC the rear intake lifter was so tight it was hard to adjust with a wrench! After adjustment and replacing the gaskets it runs perfect. I do plan on doing the decarbon in the coming weeks an I'll take pictures during the process. Thanks for the input as always!!