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Onan 12.5JC Sorting it Out


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  #1  
Old 02-08-2018, 10:28:56 PM
consolenut consolenut is offline
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Default Onan 12.5JC Sorting it Out

Hello stakers

I picked up the 12.5JC. It started right up no smoke no knocking. Engine wise good to go. It doea have some weirdness going on im trying make sense of. It appears to be a gaseous unit. However here is wherenit gets crazy. The PO took a thick piece of massive rubber hose. What i assume was a propane line . put a nipple in the end of the hose and dropped it in a gas can and it started. Below that massive line is a plugged line. Which o assume to be for a fuel gasoline line. No fuel pump on machine. I did drop the fuel bowl this evening and no float in the bowl. So that leads me to my next question. Can i convert this carb to run gasoline. Also the wiring in the box looks new. No mouse damage etc. The wiriing in the box best i can tell is wiried for 240. The leads are tied togther as follows below

T3, T9.
T2, T8
T1, T7

Everything else is tied to the ground lug at the bottom of the box.

Any help or suggestions appreciated
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  #2  
Old 02-08-2018, 10:35:01 PM
consolenut consolenut is offline
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Default Re: Onan 12.5JC sorting it out

Better pic of wiring diagram
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Old 02-08-2018, 10:39:16 PM
Steve Dawkins Steve Dawkins is offline
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Default Re: Onan 12.5JC sorting it out

The generator output is currently configured for 120/208 volts, 3 phase.
Line 1 = 1 & 7
Line 2 = 2 & 8
Line 3 = 3 & 9
Neutral = 4,5,6,10,11,12

I can't answer regarding the carb question.
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Old 02-08-2018, 11:03:19 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan 12.5JC sorting it out

You want to wire the way the second diaghram down on the right says, The two diamonds one on top of the other for 120/240 - 60HZ Output will have L-1 L-0 & L-2, All the L-0 wires tied to the junction box.

Last edited by nothingbutdarts; 02-08-2018 at 11:06:04 PM. Reason: added info.
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Old 02-08-2018, 11:16:34 PM
Leon N. Leon N. is offline
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Default Re: Onan 12.5JC sorting it out

To answer your question, in my opinion, yes, it could be converted to run on gasoline. However, special precautions would have to be taken and keep in mind it was not designed to run on gasoline. Onan tri-fueled plants are of the low compression design. In addition to changing the carburetor, you would have to find the proper ignition timing point to preclude engine knocking(pre ignition) so as not to damage the internals?
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Old 02-08-2018, 11:34:33 PM
JohnnyC JohnnyC is offline
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Default Re: Onan 12.5JC Sorting it Out

Yes, you can convert it to run on gas-gasoline. As Leon stated it is best that your JC is setup for low compression. Can you do a compression test on the cylinders and post back with the results? From the readings you post, we can tell you if your set is high or low compression.

JohnnyC
New Jersey
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Old 02-08-2018, 11:41:01 PM
consolenut consolenut is offline
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Default Re: Onan 12.5JC Sorting it Out

To counter pre-ignition or detonation in normal car engines. You would you just run a better grade or higher octane fuel to prevent it. As im sure the compression ratio is probably higher.

Higher compression or more iginition timjng means higher octane fuel. Fuel i dont mind buyjng. I drive a 8.1L gas engine powered chevrolet kodiak. 8mpg is the norm loaded or unloaded every day. And it lives on premiim fuel.

---------- Post added at 11:41:01 PM ---------- Previous post was at 11:37:04 PM ----------

This genset more then likely will get its fuel supply. From the 53 gallon tank which is almostt always stocked with 93 octane fuel. Or non ethanol same octane rating.
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Old 02-08-2018, 11:42:39 PM
JohnnyC JohnnyC is offline
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Default Re: Onan 12.5JC Sorting it Out

As for what you need to run tri-fuel, look at the picture below. This is all you need and you could bolt it up in 30 minutes and be running gas and gasoline. Just need to check timing and spark plug gap as per your manual. The big question is your compression. High or low?

JohnnyC
New Jersey
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Old 02-08-2018, 11:48:39 PM
consolenut consolenut is offline
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Default Re: Onan 12.5JC Sorting it Out

JohnnyC. The side of my carb looks like yours. Instead of the gas line from the fuel.pump. it just has a threaded pipe plug there. The large line above it is where that big piece of rubber hose is.
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Old 02-08-2018, 11:58:11 PM
JohnnyC JohnnyC is offline
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Default Re: Onan 12.5JC Sorting it Out

Right... But the internals are missing for gasoline such as the float, needle valve and seat. I've seen tri-fuel carbs for sale on Ebay from time to time. Prices vary. You may want a low pressure electric fuel pump rather than the OEM Onan mechanical fuel pump. That may set you back about $75-$100. Maybe you can find a gasoline carb for parts???

JohnnyC
New Jersey
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