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Onan 7.5 JB-3CR running issue


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  #1  
Old 03-16-2017, 04:29:50 PM
mheltunen mheltunen is offline
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Default Onan 7.5 JB-3CR running issue

Hello everyone and here is my problem. I recently acquired a 7.5 JB. Owner had no idea if it ran or not. I was able to get the propane hooked up and got it to start but it immediately shuts down after firing. After some research I discovered that the points cover and the centrifugal switch is missing and the wires leading to it are cut. They did have a crude on/off switch wired in to bypass it but I removed it. I also discovered that that how they had wired the bypass had heated up and ruined the ignition coil as well. Does anyone have a source for these items? Sorry for the long post. Any help is appreciated. My email is mheltunen@yahoo.com

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  #2  
Old 03-16-2017, 04:43:26 PM
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Billy J Shafer Billy J Shafer is offline
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Default Re: 7.5 Onan JB-3CR running issue

Just give everyone time to show up. Some one will have the parts.
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Old 03-16-2017, 04:46:54 PM
Max Thompson Max Thompson is offline
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Default Re: 7.5 Onan JB-3CR running issue

I do not have either of the parts needed. There is a thread titled "Parting Out Thread" https://www.smokstak.com/forum/showt...=99191&page=14 below and that may be a good place to start with a parts need. Onan Parts is a sponsor here at The Stak and he may be able to help also. Otherwise flea-bay may be the next stop.

If you do not have a parts manual, you can find it here http://www.twinslan.net/~n0nas/manuals/onan/ #967-0223 should be the parts manual for your set. You also need the vibration isolators that go under the "feet" on your set.

You can safely run your set with a switch to bypass the centrifugal switch, some of the older sets had a "Hand Crank" switch that did about the same thing. You just need to be SURE to shut it off after the set starts.
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Old 03-16-2017, 04:56:34 PM
mheltunen mheltunen is offline
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Default Re: 7.5 Onan JB-3CR running issue

Max, do you know of a schematic/diagram to wire a bypass switch properly? And how my picture shows is all the parts I have, so no way to operate the plunger.

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Old 03-16-2017, 06:28:39 PM
Max Thompson Max Thompson is offline
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Default Re: 7.5 Onan JB-3CR running issue

What I am thinking is just wire the 2 wires that go to the centrifugal switch to a single pole single throw switch. The switch would be closed (on) to start the set, then turned off.

One of the smart guys (Leon, where are you) will be along and let you know if I am leading you astray.

In the JB / JC service manual #0967-0500 on page 82 you will find a diagram for the older spec. P set with a "hand crank" switch. I think this will allow you to start & run your set. The centrifugal switch is only "necessary " when remote starting the set.

If Yellowlister does not have cover for the points box, I think I have an extra somewhere.
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Old 03-16-2017, 07:30:53 PM
Ray Lynch Ray Lynch is offline
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Default Re: 7.5 Onan JB-3CR running issue

Welcome to the Stak.
Make sure your cooling tins are all in place once you get the set up and running.
The set can overheat in a few minutes without the covers in place.
JB 's are nice sets. Good luck with it.
Ray
Try this vendor. He may have a centrifugal. switch as well.

http://m.ebay.com/itm/ONAN-JB-IGNITI...220?nav=SEARCH

Last edited by Ray Lynch; 03-16-2017 at 07:41:22 PM.
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Old 03-16-2017, 07:49:20 PM
len k len k is offline
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Default Re: 7.5 Onan JB-3CR running issue

Problem with manual switch bypassing the centrifugal switch is that applies power to coil all the time. If gen ran out of fuel power would still be applied to coil and it would eventually burn out again.

Want some type of auto shut down for coil power. Even if it was as simple as oil pressure switch driving a relay, so when oil pressure drops coil power stops.
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Old 03-16-2017, 07:57:22 PM
Dennis19508 Dennis19508 is offline
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Default Re: 7.5 Onan JB-3CR running issue

Actually if you wire a switch in place of the centrifugal run switch it will need to be closed to run and turned off when stopped. This will be fine for testing but should not be left that way. That circuit is used for the LP gas valve and the ignition. If it is left on when the engine stalls, LP could leak. Plus the ignition coil will be energized if the points are closed and it would probably fry again.
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Old 03-16-2017, 08:53:57 PM
Leon N. Leon N. is offline
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Default Re: 7.5 Onan JB-3CR running issue

Max I agree with your thinking in post #5. However, not knowing what this specific JB was built too, I would suggest someone provide the factory build sheet to see if the actual control wiring diagram can be identified.

Another side point about that hand-crank feature. If someone is going to hand crank the JB to start it, the physical exertion required to start the JB is practically impossible unless one is either using gasoline or the Garrestson has a primer button.I have no primer button, and I could never hand crank sufficiently to start it when using NG.

On the other hand ( no pun intended) I have used the hand crank to start the JB when the 12 volt battery was discharged, that is could only spin the JB for 1 or 2 seconds OR when the ignition start relay was not working.
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Old 03-16-2017, 09:09:38 PM
len k len k is offline
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Default Re: 7.5 Onan JB-3CR running issue

Agree hand crank might be OK with a semi dead bat helping.
Were all getting older, few winters ago my back didn't like pull starting just a 7 hp snowblower that had issues.
JB is ~980 CC engine, ouch.

Last edited by len k; 03-17-2017 at 12:46:46 PM.
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