ONAN RS-12000 Overcrank

How many hours? Have you checked compression?

404 hours, compression is good.

Fuel/air ratio has gone out of adjustment.

What does the manual say?

Have you checked Onan's web site?

How would fuel/air ratio go out of adjustment? The plugs are nice and tan. It simply will not run with a filter on.

I have done everything the manual says, and could not find anything related to this issue on Onan's site.

I don't know, but the filter would change the F/A ratio to some degree.

Beyond that, I have no clue, but they need fuel, air and fire to run.

Will look into that, thanks.

Odd thing is, it will run fine if I start it without the air cover then put the cover back on after it fires up. Just won't start by itself with it on.

Seems a regulator failure would be that it is not getting enough fuel to start, whereas mine needs more air. Can a regulator problem allow too much fuel in, where it needs the extra air to fire it? Then when it is already running, and you close the cover, the mixture just gets rich but it keeps running?

What are the telltale signs of a natural gas engine running rich? As I said, plugs look great, and are now new. No smoke or rough running. Did notice after finally starting it with the air cover off that the exhaust was heavy with steam for the first minute. But then of course it was 0°.

Thanks for your thoughts.

Evan

I can follow that logic, until I shut it off and immediately try to start it again with the cover on. No time to build up excessive gas, just cranks away to the fault.

It may be the regulator that is supplying more than the less required.

As asked before, when a regulator fails, what does it do....less gas or more gas?

Evan

Regulators are usually fail safe(fail in the closed position)....No I would think that the shut off solenoid is not closing with a good seal....Isolate the line somehow and check for an accumulation of pressure when the unit is off....disconnect it if you can and put your finger over the opening....maybe it just closes slowly, is another possibility....or perhaps it opens too soon....is there a vacuum controlled advance?

Thanks SE.

No vacuum advance that I can see.

I am leaning toward your thinking of too much fuel, but cannot test until the weekend.

Does anyone know how much water vapor is in natural gas? The reason for my question is the huge amount of steam when she fires up, air cover off. If I were to go through a couple cycles of "overcrank", could that put condensed fuel/water vapor in my exhaust. I noticed one of the spark plugs had what appeared as rust on it. Could it be that a couple overcranks left fuel/water vapor in the exhaust with a valve open and rusted the plug?

I will pull the solenoid and regulator off this weekend. No sense in having an automatic backup power source if it won't start.

Evan

ANY hydrogen based fuel, when burnt, forms water as part of the chemical burning process.....water vapour is normal, but usually best seen on a cold day.......

Then what about making a telephone call to their Technical Helpline?

I agree with Lyn. The fact that it won't run with the filter cover on, confirms that the air flow is insufficient.

Something must have changed, and perhaps not mechanically...

eg altitude wrt sea level ?

I concur with Lyn's analysis, it may have something to do with the F/A ratio! It seems the filter when on is creating enough resistance to the air flow - like a choke that makes the initial mixture to be richer?

The reason why it runs w/o problem after it has started, is may be simply due to the extra negative pressure created by the engine.. That once running can sustain the combustion of extra fuel as well creating enough negative pressure that maintains combustion!

I would check the idling RPM to validate, it could have increased a bit, as compared to before this starting problem!

I wouldfirst check the inlet NG pressure and verify it to be correct with a manometer.

If this is ok, I would then check the coil/magneto voltage output.

If the spark voltage is too low it will not ignite rich fuel mixtures.

Well Mr. Eagle, I must apologize for being so cavalier in saying the compression is fine. That was based simply on a fat thumb on the plug hole. A gauge today said 110 on one and 100 on the other stone cold. Not great for only 400 hours, but functional I would think. Just looked and service manual says 75 to 115.

Found that the plastic oil cap had broken, snapped the dipstick off and blown some oil out of the fill tube. An approximate measurement showed it a quart low. So I changed the oil last week (10w30 Mobil 1), and ran it for an hour after. The next day, it started fine by the exerciser clock so I thought my troubles might be over. Not so fortunate.

This week it would not start for its exercise time, but now would not start even with the filter off. And now there was new clean oil in the air pan. I just pulled the air pan and PCV valve, screen and filter and they were soaked in oil. When trying to start it without the PCV in place, there was an incredible amount of crankcase blow coming out of that filter body. It is likely that crankcase pressure that blew my oil cap off. If within spec on compression, that can't be blow-by can it? If not, where is it coming from?

I have not bothered with the NG regulator yet, since it started and ran fine last week. To me, that would not seem to be something intermittent. Frustrated.

Evan

Determined that the only source of such crankcase air and oil driven into the PCV must be the crank spinning in oil. Drained a half quart of aerated oil, and the problem went away. So you cannot believe the oil capacity in the manual. I put 2.5 quarts in at the change without filter, and it only wanted 2.

Starts immediately now, so it appears the whole issue was oil level related.

Thanks for your shared thoughts.

Evan