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An Onan JB Performance Question


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  #101  
Old 08-24-2017, 09:25:45 AM
Leon N. Leon N. is offline
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Default Re: A JB Performance Question

From my compression readings I believe my JB internals are OK. So now I turn to looking at the running timing. If that is OK, then the problem points to the natural gas supply. I did try forcing more NG into the carburetor while it was slowly recovering but that did not help. I also tried fiddling with the enrichment adjust but that also did not help. And, the throttle is fully open during recovery.
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  #102  
Old 08-24-2017, 11:28:18 PM
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Motorhead Motorhead is offline
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Default Re: A JB Performance Question

Leon, Did you ever check the valve clearances? I am big on that like Len K is about checking the slip rings resistance on the BGE series sets. With the time you have on the set you have, it wouldn't hurt to check them, especially on an air cooled engine.
Other than checking the advance unit to see if it is working, check the points for proper gap before setting or checking the timing. Point gap, plus or minus to proper gap, can affect timing reading.
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  #103  
Old 08-25-2017, 05:32:56 AM
Leon N. Leon N. is offline
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Default Re: A JB Performance Question

Motörhead that would be a bigger undertaking than I want to do at this point. The compression pressure difference between cylinders is acceptable. I suspect the pressures I measured are reasonable, do not know for sure, you may have a point, but the machine purrs like a kitten. The next easiest thing for me to do is re check the point gap and timin. Thanks
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  #104  
Old 08-25-2017, 05:46:39 AM
JohnnyC JohnnyC is offline
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Default Re: A JB Performance Question

Hey, I got a stupid question.... Since your JB has been sitting in in its place and plumbed to NG for decades (since the 1970's), does your gas setup include a wye that has a mesh filter screen inside it as many commonly do? If so, is it possible that it may have accumulated enough rust particles or crap that allows enough gas to flow to let it run under a small load, but not enough for heavy loads?

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  #105  
Old 08-25-2017, 07:43:23 AM
Leon N. Leon N. is offline
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Default Re: A JB Performance Question

J ohnnyC good question. I can pull 7500 watts and keep within the 5 percent frequency tolerance, so wouldn't you think the strainer you mention is not the problem? It seems to m something is not responding fast enough. It is not the govenor because the throttle goes wide open immediately. Some how I think it is the igntion timing or that old KN2? Now that theweather has cooledoff a bit, I may investigate before too long.
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  #106  
Old 08-25-2017, 11:00:46 AM
len k len k is online now
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Default Re: A JB Performance Question

I think your demand reg has asmall capped off port to test inlet side pressure, mine does. Why don't you put a u-tube manometer on it and check pressure as you drop a large load on the JB. Very basic test for any upstream restrictions like a plugged screen. Pretty ease to do too, just some clear tubing and water.
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  #107  
Old 08-25-2017, 03:27:16 PM
Leon N. Leon N. is offline
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Default Re: A JB Performance Question

LenK I like that idea. I will go take a look at my Garretson.
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  #108  
Old 08-25-2017, 03:41:26 PM
len k len k is online now
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Default Re: A JB Performance Question

If you also have a larger dia clear pipe, or tall jar, you can fill it with water and drop the clear tube down it for an easier direct reading. Pressure is just distance from surface to water inside the tube. Make the outer tube/container larger dia and water level in it will change negligibly as water moves in the inner tube, makes measurement easier.

With less water in inner tube it should have a faster response time too, less water mass. IF your shooting for 8 inch WC , only put the tube end ~ 10 inches under water so it only has 2 inches of water in it. Less water mass= faster response time

I did something like this when I didn't have a clear tube handy, I just dropped a rubber hose underwater till it stopped bubbling. Then measured depth.

Last edited by len k; 08-25-2017 at 03:52:08 PM.
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  #109  
Old 08-25-2017, 05:02:16 PM
Leon N. Leon N. is offline
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Default Re: A JB Performance Question

len k good suggestions. Just came from testing the incoming gas pressure at the Garretson regulator using my home made manometer. flipped the JB to hand crank without starting and the recorded gas pressure was 5.5 inches of water. Started the JB and at idle the gas pressure increased just under a 6, or closer to maybe 5.5 inches. Dumped a 4 k load on the JB and the gas pressure did not change, just jiggled a bit.

What I did observed after that was the JB response time did improve a tiny bit, maybe my impression maybe not so, if I remove the air cleaner.

Anyways I now have an urgent bigger problem with the starter. Sometimes it takes several attempts to start because the starter whines and does not engage the flywheel on every attempt. Best way to start is to use the hand crank position to just kick it over and wha la.

As soon as I get my ambition back I am going to remove the starter once again after first checking the running timing.
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Old 08-29-2017, 03:56:56 PM
Leon N. Leon N. is offline
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Default Re: A JB Performance Question

Took down my JB this morning. Removed the starter to inspect the starter armature shaft spline and freedom of movement of the Bendix. Could not find anything amiss until I removed the shifting fork. The two pauls showed some wear but that would not cause the starter to sometimes whine and not engage so I was told this morning.

Took it to the local auto-electric shop only to learn the Bendix was ? bad. That is the clutch mechanism was close to total failure. After discussing my intermittent starter operation with you guys, I was under the impression that the armature shaft spline was not sliding freely. Recall some say no lubrication, some say use a dry graphite lube, etc, etc. Well after discussing with the auto-electric technician he noted the Bendix should lock in one direction of rotation and NOT, I repeat NOT spin freely in the other direction, but should turn with some resistance. Mine turned "almost" freely and I was not aware that turning or spinning freely without some resistance while locking in the opposite direction is a sign of failure of the un-repairable caged assembly.

So that failed Bendix lasted some 47 years! So I guess I have no grounds for complaining.

Next I checked the stopped timing. It was OK with the points opening up at 5 degrees BTC. That is correct IAW 967-500 for gaseous operation. Now I must find where I can borrow a timing light, sure do not want to fork out $45 for a one time use.

Also fixed the thermostatic exhaust air shutter top shutter which would not open. Turns out one side of the upper shutter pivot screw and lock washer must have got loose and fell out.
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