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Onan Generators Restoring, operating and maintaining vintage Onan generators.

Onan Generators

Onan 7.5 JB-18R/2731AA Followed me home!


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  #1  
Old 06-15-2017, 12:41:25 PM
EICBob EICBob is offline
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Default Onan 7.5 JB-18R/2731AA Followed me home!

As a new forum user, I have found a host of information concerning the care and feeding of all things Onan! I am looking for any information about the history of this unit. Specifically a build sheet. Serial number is 0574737568. I paid $175 for the generator and the transfer switch. I do know it is a LPG powered motor and the generator is a 12 wire 3 phase unit. There is a label on the transfer switch indicating it was for emergency lighting.
Currently the poor thing will not start but does turn over and develops oil pressure. I will post some pictures as I try to bring this neglected unit back to life.
Thank you everyone for all the information that is contained in these postings.
Cheers!
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Old 06-15-2017, 02:29:59 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Build date of May 1974. You might check the points for being corroded and not working if it has sat for very long.
Manuals here: http://www.twinslan.net/~n0nas/manuals/onan/

Guys will be along in a bit that have build sheet files.

Welcome to the green disease sight!!!
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Old 06-15-2017, 06:17:05 PM
Kevin K Kevin K is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

That's odd, the site would not let me upload the build sheet, which probably means it appears somewhere else on the site. Maybe Max or MBB can try to upload a copy.

Your wiring diagram is 612-4794. Here is a link to it: https://www.smokstak.com/forum/showp...35&postcount=8
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Old 06-15-2017, 07:53:10 PM
MBB MBB is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Lets try something else The .pdf wouldn't load.
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Old 06-16-2017, 09:51:27 AM
EICBob EICBob is offline
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Thumbs up Re: Onan 7.5 JB-18R/2731AA Followed me home!

I have a soft spot for these Green beasts!
Thank you for the information, I will be starting on the genset this weekend, I need to locate a muffler so when it does fire I won't irritate the wildlife too much!
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Old 06-16-2017, 11:13:36 AM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

If you are a hi tech sort of guy, why not look into the Maxim Mufflers? I have one on my JB tuned to provide the maximum attenuation at the exhaust frequency produced by the JB running at 1800 RPM. Couple that with inch and one half piping and it will purr like a kitten. Seriously, want more info just let me know.

Leon
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Old 06-17-2017, 04:48:08 PM
EICBob EICBob is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Hi Leon
I am interested in how you have your JB exhaust done, I just put a basic NAPA tractor style on and got the unit running.
Here as promised are some of the pictures of this unit.

-Bob
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Old 06-17-2017, 07:27:18 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

OK, here is what I did back in 1970 and it is still in operation today. Note, I housed the muffler inside the un-insulated shed to keep it dry. I used large diameter piping and cut the tail pie to 45 degrees to minimize back pressure. The flex section is 18 inch section of stainless steel. I recall the manufacturer warned not to expose this section to any torsional forces. There is some lateral motion as the JB rocks during operation. That is OK. I have at least a 12 inch clearance from any combustible surface. The thimble is a home made welded 1/4 steel plate with a welded, what 2 inch opening for the 1 1/2 inch tail pipe to slide through.
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Old 06-17-2017, 08:36:03 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Leon,
How long is the muffler? You have a very lucky generator, you really take care of it!!!
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Old 06-18-2017, 11:14:27 AM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

My JB muffler is 28 inches long. I purchased it from AMF Beaird. It is called a Maxim Silencer. Attached is the pertinent information from their Maxim catalog. I have also included the page describing there flex section accessory. I chose the inch and one-half size which is 24 inches long. It is the stainless steel bellows design. I have seen it glow cherry red in the dark under a heavy load.
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