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Onan JC - Problem With My “Very Reliable” 15JC-3R/3998L


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  #11  
Old 02-07-2018, 02:54:26 PM
Leon N. Leon N. is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

Len I do not think it is advisable to mess around with the Magneciter. I would leave it alone.
Really there is no reason to let the JC run at low RPM for any length of time. A minute or less is not going to harm it and it should become immediately apparent that something is wrong if it does not reach 1800 very quickly. Shut it down and make further adjustments and try repeatedly to start it. Going into the back end and disconnecting wires can lead to further complications which have nothing to do with the described problem.
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Old 02-07-2018, 03:17:59 PM
maineboy maineboy is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

Thanks for all of the comments, I really appreciate the help.
I’ll dive in tonight and see what I find, I’ll keep you posted.
I’m going to check compression on each cylinder, I doubt I lost compression on all of the cylinders all at once, but I’d like to know.
I’m going to double spark at the plug, if it’s weak I do have new points and a condenser to install. I also have a new distributor cap and rotor button.
I do not have a manometer, is there any other way to test the gas pressure?
I know I’m getting LP gas at the carb and the new KN reg has a prime button on it so I can give it a couple little bumps (and I have) to see if it would fire and at least run rough for a second or two.

Oh and here is a picture of the old KN diaphragm (25 years old), it was probably fine, but it looked stretched out, so I installed a new one.

Thanks again.
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  #13  
Old 02-07-2018, 03:23:59 PM
Kevin K Kevin K is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

How to make a manometer.

https://www.youtube.com/watch?v=QnlfQK-8qy4
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Old 02-07-2018, 04:06:08 PM
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BigBlockChev BigBlockChev is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

Ditto to Kevin on making a manometer, short length of clear plastic tubing and a ruler. Priming the KN may make the no firing problem worse, too much fuel and it won't light. You have to verify that the KN is not leaking by and that the pressure regulator is not set too high or too low. Ignition problems such as worn point rubbing block or sticking or dirty points or point gap too close also have to be checked and ruled out. Compression is easy to check but make sure you hold open the carb butterfly or you won't get a good reading. In my opinion the chance of a compression problem is very low but verifying everything is good practice , often you find something other than you were expecting. Checking valve clearance is another potential cause of poor running, tight valves can cause backfiring. I think this is low on the probabilities but to be thorough it might be done. The fact that this came on rather suddenly makes me think it is fuel or ignition related. Cheers Dan
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Old 02-07-2018, 04:12:03 PM
len k len k is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

even easier manometer ...... I duct taped off one end of a pipe (vacuum cleaner wand) filled it with water. Dropped a small diameter hose into water, then pull it out till it bubbled. Measured depth to end. Pressures into demand reg are typically only 7 inch WC for NG and ~ 11 inch for propane.

With opaque pipe and hose , doesn't let you watch pressure over time. But it's quick and easy to make from scrap you have laying around.
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Old 02-07-2018, 04:18:11 PM
Leon N. Leon N. is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

There should be no gas flowing to the carburetor until a slight vacuum is created by cranking the JC except when you hit the prime button. Do you have a so called " hand crank" switch if put on should open the gas valve immediately in fron of the KN regulator? No gas should flow from the KN regulator when the gas valve is on. I recall with my KN I can adjust it just to that point so that no gas flows into the carb until drawn by cranking the JC. There should be a fitting on the KN where you can attach the manometer. As I recall the pressure at that point is around 4-6 inches of water.

My first thought that comes to mind is a fuel problem. Too much or too little gas into the carburetor. I can get all sorts of erratic operation by miss adjusting the fuel supply. In 48 years of use, I have never had a need to re adjust the valves.
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Old 02-07-2018, 04:25:31 PM
len k len k is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

If no compression tester, Simple test for a piston hole. Take out spark plugs, shut off fuel, crank engine. Feel for suction and pressure from each plug hole with your thumb. Hopfully they will all be making about same pressure pulse and suction
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Old 02-07-2018, 05:24:26 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

While you in looking at the points don't forget to put a couple drops of engine oil on the wick under the rotor to keep the mechanical advance from rusting up.
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Old 02-07-2018, 05:54:15 PM
Leon N. Leon N. is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

Has anyone ever seen a hole burn't into a J-Line engine's piston? Really? If so I can think of several reasons how this might happen.
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Old 02-07-2018, 06:52:13 PM
JohnnyC JohnnyC is offline
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Default Re: Onan JC - Problem with my “very reliable” 15JC-3R/3998L

Quote:
Originally Posted by len k View Post
If no compression tester, Simple test for a piston hole. Take out spark plugs, shut off fuel, crank engine. Feel for suction and pressure from each plug hole with your thumb. Hopfully they will all be making about same pressure pulse and suction
Len, you can have a cracked piston and still feel pressure with your thumb, but you won't know what pressure in lbs really is.

If the dip stick and breather blew out of the block when it backfired, something leaked bigtime from the cylinder into the block. Usually a backfire will be noticed through the exhaust or a burp through the carburetor, not from within the block. I wonder if vapor built up in the block because of a cracked piston/ ring or a hole, then suddenly detonated when the spark plug fired in the cylinder that has the defect.

Now, if there is a problem with compression, he will need to find out if the problem is the piston, rings, or valve(s). If a ring or piston ring land cracked off, it could have messed up the cylinder and valve(s) if the piece that cracked off gets mushed between the top of the piston and head.

JohnnyC
New Jersey
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