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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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Old 06-18-2017, 05:24:35 PM
EICBob EICBob is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Awesome information Leon!
Update, my gen set starts and runs, bad points and cruddy plugs were the cause of the no-start condition. I am getting 108 VAC to 110 VAC across each leg of the 220 vac wiring that this unit has been set to. After taking the nose apart and inspecting the starter motor for grinding at crank, I came to realize I need a new bendix gear for my starter motor and at the very least a new ring gear.I would prefer to locate a good used flywheel instead.
I tested the transfer switch, LTE30-3 1761A, and the contactors are very noisy, (hum) as they sit in standby mode. The exercise clock and return to utility power timer work as hoped.
A question for the moderators, as this thread coveres the generator, will it be better to start another thread for the transfer switch?
Thanks
More pictures to follow...hopefully they will post right side up!
-Bob
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Old 06-18-2017, 05:41:10 PM
MBB MBB is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Being they are to separate units one generator another switch it may be easier to follow having separate threads.
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Old 06-18-2017, 06:02:11 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

The ring gear is really that chewed up? That's a bummer!
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Old 06-18-2017, 06:44:05 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

EICBob funny you are having so TR with the starter. I had similar problems with my JB stater. My flywheel ring gear also had rounded of (worn) teeth on one side. I had starting problems like, I think, you are mentioning. THe JB would start to crank, catch and sputter a bit, then the bendix would briefly drop out, then re engage, grinding gears and just a horror show. That scenario was aggravated by a couple of things that I finally figured out after replacing the bendix. #1, the electrical contacts in the stater shifting solenoid were worn and making intermittent contact at times. This was aggravated by a relatively weak starting battery. #2, A less than perfectly adjusted Carburetor hi-speed natural gas jet and by a weak ignition condenser.

Some days it would start perfectly OK! But many times I had to play with it to get sufficient cranking speed, even with the controls set for hand crank.
Fineally after taking the shifting solenoid apart, and noticing the severely worn electrical contacts, worn to,the extent I could not turn them over and use the other side, I took the starter to an auto electric shop and they rebuilt it with a new shifting solenoid and it has worked pretty much OK since what 1975 or there a outs. I have some pictures if interested of the worn solenoid contacts.

NOw the way that starter solenoid shift drives the starter pinion gear into the ring gear with one loud positive clunk, you probably do not need to replace the ring gear. Good Luck.
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Old 07-03-2017, 08:50:13 AM
EICBob EICBob is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Update.....
I replaced the bendix gear on my starter, cleaned out all the dirt and gunk and re-lubed the solenoid fork and pivot points. Took a hand file to the ring gear and dressed up the teeth a bit and took down the curled high spots. Put it all back together and 90% of the grinding, thrashing and overall nastiness has ceased!
I am still on the lookout for a good flywheel with ring gear, but am happy with the intermediate outcome.
I had to replace, read create from various parts, the 4 rubber control box mounts so the box would stay put and not vibrate and bounce around. I used 4 automotive shock absorber rubbers and 4 1/4-20 "J" nuts to fashion new mounts. The control box is now happy and solidly mounted to the generator housing again.
I need to come up with a way to accurately measure the RPMs of this unit, to verify the governor settings and the carb settings, any ideas?
I have fashioned a monitor panel for testing and so far I like what I am seeing
I have a short video of the unit in operation, just need to post it some where for your viewing enjoyment.....
More later as this project continues!
Thank you all for your kind assistance!!

-Bob
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Old 07-03-2017, 08:56:30 AM
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Measure the output frequency as 60hz is 1800rpm on that set.
It
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Old 07-03-2017, 08:59:42 AM
Dale Russell Dale Russell is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Buy a Kill A Watt Meter which can be found at most of the big box stores or on line. It will show Hrz, Volts, Amps & ETC. for around $25.00. This is what mosthere use to set the Hrtz. Haves fun!!!
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Old 07-03-2017, 09:01:23 AM
pauls12 pauls12 is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

http://www.ebay.com/itm/LCD-Digital-...JWrH2T&vxp=mtr this is what I use on my panel.
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Old 07-04-2017, 07:06:40 PM
EICBob EICBob is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Here is a YouTube link to my son's account where my short video of the JB running is posted...
https://m.youtube.com/watch?feature=...&v=MZ3zHR13Ku0

Happy 4th!!!
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Old 08-14-2017, 10:52:42 AM
EICBob EICBob is offline
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Default Re: Onan 7.5 JB-18R/2731AA Followed me home!

Good Monday Morning
I spent the weekend finishing up some of the wiring on my monitor panel and some more testing of my JB.
I have come across an issue that has me a bit confused. Upon initial start up the voltages across the 2 legs will either be low, at around 90 VAC, or they will be high, in the area of 135 VAC. These voltages are with out any load on the generator. I see them at the monitoring panel as well as measuring them with a good DVOM at the output lines of the set.
My original thought was a cranky voltage regulator board. What has me a little less sure about the VR is this, if I stop the generator and restart it, 9 out of 10 times the voltages will then be at 120 to 122 VAC and remain there.
I ran a 90 minute load test with 2000 watts on each leg and the unit ran smoothly and the voltages never varied more than 2 or 3 volts.
Ideas?

Cheers!!
-Bob
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