Onan Spark Plug Problem

[Russell]

I did my Onan maintenance early because the Onan appeared to be surging slightly under load. I changed the spark plugs, fuel filter, air filter, oil
and oil filter. It appears to run fine now, but one of the spark plugs looks really bad. Here is a link to a picture of the two spark plugs
removed.

http://www.gmcmhphotos.com/photos/onan-generator-maintenance/p66482-champion-spark-plugs-out-of-the-onan.html

Could it be carbon build up causing one of the valves not to close completely? I did run some sea foam top engine cleaner spray through the carburetor
while it was running, according to the instructions, before I did the maintenance. My Onan manual recommends the "Onan 4C" spray engine cleaner,
which does not appear to be available any longer. I changed the spark plugs just 73 hours ago, and the removed plugs did not look as bad at that
time, although one appears slightly worse than the other.

Here is a link to a picture of the spark plugs I removed last time, only 73 hours ago.

http://www.gmcmhphotos.com/photos/onan-generator-maintenance/p63070-onan-spark-plugs.html

My Onan is not original, but it is 20 years old. Its an NHD series 6.5kW, Model No. 6.5NHDFB1L

Should I plan on removing the heads for carbon cleaning and valve cleaning?
Any other recommendations?

--
Russell Keith,
1978 E2 "Harry" 403 (still carbureted), Danny Dunn Tranny, Thorley, Stock Brakes w/Remote Vacuum Brake Booster, Quad Bags, Dakota Digital Dash, 6.5 kW
Onan, Dunedin, Florida

 

[jim miller]

> I did my Onan maintenance early because the Onan appeared to be surging slightly under load. I changed the spark plugs, fuel filter, air filter, oil and oil filter. It appears to run fine now, but one of the spark plugs looks really bad. Here is a link to a picture of the two spark plugs removed.
>
> http://www.gmcmhphotos.com/photos/onan-generator-maintenance/p66482-champion-spark-plugs-out-of-the-onan.html http://www.gmcmhphotos.com/photos/onan-generator-maintenance/p66482-champion-spark-plugs-out-of-the-onan.html

I’d recommend the principle of “ABC” - Anything But Champion. I have had an endless litany of problems with them in small engines over the past 25 years. Consider NGK.

As for the crud buildup: it is possible you have something going on with intake valve guide wear or perhaps piston ring issues in one cylinder that is allowing excess oil up into the combustion chamber at an inordinate rate but I don’t think I’d pull any heads off just yet. Have you done a proper carb mixture adjustment to ensure that you aren’t running with a really rich fuel/air mix?

Also: keep in mind that your NHD uses the same wasted spark system as our original NH and BF units. Both plugs are fired at the same time and since the plugs are fired with a DC circuit one plug will have the spark jumping from the center electrode to the tip and the other from the tip to the center electrode. Ask any TIG welder about the difference between DC Electrode Positive and DC Electrode Negative and they will tell you that one of them produces tremendous heat in their torch electrode and the other does not. The same thing happens with your Onan ignition - one plug runs hotter than the other one and thus a difference in appearance between the two is not out of the ordinary. Yours, based on the picture, is inordinately crudded up but then again it is a champion; it may have been your higher heat plug and didn’t handle it so well. But then again it is a champion - oh wait, I already said that.

If it were me I’d put in some NGK’s with the proper gap and run it 20 or 30 hours and re-evaluate the situation before I did anything else.

Also: If you have one of the early NHDs (spec A or B, I believe) which have breaker point ignition AND you don’t know the age of your condenser then I’d suggest replacing it and probably your points too. This doesn’t have anything to do with the problem with your plugs - but it is a good policy since the condenser is a source of a LOT of problems and a bad condenser can result in all sorts of engine misbehavior. Bad condensers also destroy points so that is why it is a good idea to change both at the same time.

HTH.

—Jim

Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH