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Onan Generators Restoring, operating and maintaining vintage Onan generators.

Onan Generators

ONAN JB 7.5 new life


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  #41  
Old 03-15-2017, 09:45:04 AM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

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Originally Posted by len k View Post
Forgot about that, I have spec AA no hank crank switch on it. His is spec AD , might be same , don't know

I just re-read the thread , try dribbling some gasoline down carb throat then cranking it. You said you took carb bowl off but didn't say you took jets apart to clean them, nor blew out the carb. I've taken my JB carb and many others apart to clean them. I've never installed new parts and they all ran fine afterwards.

When I first worked on my JB the carb was clogged. I made up a squirt bottle (shampoo bottle) with a VERY fine tube to restrict flow of gas. I was able to keep engine running by SLOWLY trickling gasoline down carb throat for a test run, even though carb did not flow any gasoline.
I know nothing about a (hand crank switch),Although it did come with a transfer box which I know nothing about either and it has a hand crank switch on it.
It looks to be hooked up on the outside, I haven't got that far yet nor bothered any of the controls. Len, I don't think getting fuel is the issue with it not running. Because when it tries to run and I give it a shot of starting fluid it floods.I think its a electrical issue.I'm going Trace it out today. Could be a fuel problem, I don't know we'll see.

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Originally Posted by Leon N. View Post
This threads author describes a newer version of the JB and one with a YD back end. There must be a newer or more recent version of the proper Onan Operators Manual and Parts List than 967-320. Yes, much of it may apply but thee PLs and the magneciter info obviously does not. Similarly for the JB/JC major overhaul manual, 967-500.
I studied the 967-500 two hours last night. Seems to be the best match so far.
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  #42  
Old 03-15-2017, 11:49:40 AM
len k len k is offline
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Default Re: ONAN JB 7.5 new life

I wouldn't use starting fluid can dry out cylinders of oil. I always use a squirt bottle of gasoline.

I may be wrong but I thought you said had spark.
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  #43  
Old 03-15-2017, 01:39:21 PM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

Ok braving the cold and WIND I finally found the problem. It was the centrifugal switch. It was not closing while cranking over.Took it apart and wiped the stem off put it back. That solved the problem. I suppose it was just by chance that is happened after I added a fuel filter.
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  #44  
Old 03-15-2017, 02:46:26 PM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

Seems to still have a intermediate problem with the centrifugal switch . sometimes it closes like it's suppose to sometimes it dont.May be the relay, not sure.
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  #45  
Old 03-15-2017, 03:01:54 PM
len k len k is offline
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Default Re: ONAN JB 7.5 new life

I looked at schematic of my JB spec AA, your spec AD is likely same.

Centrifugal switch only has 2 leads, one is at ~ +12 all the time (even with engine off), other lead sends that power to a resistor that powers a relay coil (K12), that relay's contacts power spark when release start switch

1) check that you have +12V on one lead of centrifugal switch even with engine off.

2) Check for +12V ( with test light) on other lead of centrifugal switch ( lead to resistor /relaycoil). Light should come on when centrifugal switch is happy with rpm , if switch works.

Could try dragging a good grade ( black grit) of ~ 220 or 320 sandpaper thru the points to clean them.
.

Last edited by len k; 03-15-2017 at 03:32:19 PM. Reason: added , one lead of centrifugal switch is always +12V
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  #46  
Old 03-15-2017, 04:17:15 PM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

Quote:
Originally Posted by len k View Post
I looked at schematic of my JB spec AA, your spec AD is likely same.

Centrifugal switch only has 2 leads, one is at ~ +12 all the time (even with engine off), other lead sends that power to a resistor that powers a relay coil (K12), that relay's contacts power spark when release start switch

1) check that you have +12V on one lead of centrifugal switch even with engine off.

2) Check for +12V ( with test light) on other lead of centrifugal switch ( lead to resistor /relaycoil). Light should come on when centrifugal switch is happy with rpm , if switch works.

Could try dragging a good grade ( black grit) of ~ 220 or 320 sandpaper thru the points to clean them.
.
Is this the resistor you are talking about?
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  #47  
Old 03-15-2017, 04:23:16 PM
len k len k is offline
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Default Re: ONAN JB 7.5 new life

Your cooling sheet metal looks different than mine. Got a feeling resistor in your picture is the series resistor for 6V spark coil, not relay coil. Follow the wires from centrifugal switch. From schematic looks like the resistor I was talking about is inside control box.

--------

Yes it is inside control box, I just looked at pics of mine, link attached
Not greatest pic, but on mine it's in center of control box, standing up, and is located between relay and a terminal strip (TB12).
https://www.smokstak.com/forum/attac...4&d=1446087117

Last edited by len k; 03-15-2017 at 04:46:58 PM. Reason: added link and better description
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  #48  
Old 03-15-2017, 04:30:13 PM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

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Originally Posted by len k View Post
Your cooling sheet metal looks different than mine. Got a feeling resistor in picture is the series resistor for the 6V spark coil, not relay coil. Follow the wires from centrifugal switch. From schematic looks like the resistor I was talking is inside the control box.


Yes it is inside control box, I just looked at pics of mine. On mine it stands up , and is located between relay and a terminal strip inside control box.
Will check it out.
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  #49  
Old 03-15-2017, 05:40:54 PM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

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Originally Posted by fisherman81654 View Post
Will check it out.
Its a little too blustery out there to go deeper today.
What I have found though is if I crank it over and hold in on the centrifugal switch while I'm cranking it over it starts. Then I turn it off and it starts agin normally most of the time. But sometimes I have to hold in on the centrifugal switch again.
I did clean the contacts some on the relay K2. It was hard to get to do a good job.
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  #50  
Old 03-15-2017, 06:09:13 PM
JohnnyC JohnnyC is online now
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Default Re: ONAN JB 7.5 new life

Fisherman -The resistor you point out is for the coil. Len, his sheet metal is different than ours because his cooling is set up of vacuflow (or what ever its called) and ours is pressure cooling. His setup is easy to work on because everything is exposed and ours is behind sheet metal tins.

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  #51  
Old 03-15-2017, 06:32:30 PM
len k len k is offline
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Default Re: ONAN JB 7.5 new life

Figured difference was vacuflow vs pressure cooling, just didn't know which one I had. Hadn't got that far with mine. All my tins are in a plastic box outside waiting for painting in summer.

If engine is coming up to speed and you can't see centrifugal switch contacts close then maybe the switch is sticking.
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  #52  
Old 03-15-2017, 06:55:06 PM
squidtrap squidtrap is offline
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Default Re: ONAN JB 7.5 new life

Did u check gap on switch?? I believe its .020
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  #53  
Old 03-15-2017, 07:58:14 PM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

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Did u check gap on switch?? I believe its .020
yes It was out I adjusted it to .020 earlier.
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  #54  
Old 03-15-2017, 08:47:24 PM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

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Originally Posted by len k View Post
Figured difference was vacuflow vs pressure cooling, just didn't know which one I had. Hadn't got that far with mine. All my tins are in a plastic box outside waiting for painting in summer.

If engine is coming up to speed and you can't see centrifugal switch contacts close then maybe the switch is sticking.
I'm not sure what switch you are talking about.

I do not no exactly how the centrifugal switch works.
I know it has to get up to 900 RPMS before it works, I don't know how that works.
There is a rod about an inch long that goes into a hole.
The other end rests on the centrifugal switch.

Does the rod move inward as the RPMS reach 900 and allow the contacts on the centrifugal switch to close.? And if that's the case why is it sticking? And how does the relay play into the sequence?

If I manually hold the contacts in on the centrifugal switch I can hear the relay click in the control box.(without trying to start it)
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  #55  
Old 03-15-2017, 09:10:28 PM
len k len k is offline
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Default Re: ONAN JB 7.5 new life

Good sign that pushing centrifugal switch points closed makes relay in control box click, means at least that part of circuit is working. Next test would be to use a test light between frame and +12V lead of spark coil and confirm that you have power at coil when you manually close the centrifugal switch. From schematic looks like it should do that even if engine is not running and you don't touch start/stop switch.

I haven't had to take my centrifugal switch apart. But I'm guessing some weights spin and somehow push a rod outward to close the centrifugal points. IF engine is coming up to speed that switch trips at ( ~ 900rpm) and rod doesn't move then sounds like the rod is sticking. Also check gap.

If gen is making AC, 900 rpm corresponds to 30 hz. Most people here use a $22 kill-a-watt meter to measure hz, which is directly proportional to rpm.

I'ld say read the manual for details on how centrifugal switch works, I haven't.

Last edited by len k; 03-15-2017 at 09:23:41 PM.
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  #56  
Old 03-15-2017, 09:12:32 PM
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Default Re: ONAN JB 7.5 new life

I did not read all the postings in this thread, so excuse me if this was already asked..... Is you JB cranking fast enough to reach 900 rpms consistently with each start? Do you have a GOOOOOOOOD fully charged battery? You don't need a big Fooking battery for a JB, but it is imperative that you got a battery with all good cells to produce min of 12 vdc at a reasonable CCA.

JohnnyC
New Jersey
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  #57  
Old 03-15-2017, 09:15:52 PM
Leon N. Leon N. is offline
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Default Re: ONAN JB 7.5 new life

By your last statement in post 54, I would surmise you do not have a relay problem but maybe an inoperative centrifugal switch. It could be the pin is sticking or there is something amiss inside that activates the mechanical switch. What is the situation with your lube oil? Does this machine show evidence of poor lube oil maintenance? You might try very carefully spraying with a straw something that may clean the centrifugal switch activating pin.

I had a case with my JB several years ago where by the points plunger was sticking and the points would not close during cold start ups. Turned out it was a stiff rubber diaphragm that acts as an oil seal. When Imswitched to a low viscosity synthetic lube oil, the problem went away. I do not know, but maybe you have a similar situation with that centrifugal switch.

Last edited by Leon N.; 03-15-2017 at 09:17:26 PM. Reason: Clarification
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  #58  
Old 03-15-2017, 09:18:08 PM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

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Originally Posted by JohnnyC View Post
I did not read all the postings in this thread, so excuse me if this was already asked..... Is you JB cranking fast enough to reach 900 rpms consistently with each start? Do you have a GOOOOOOOOD fully charged battery? You don't need a big Fooking battery for a JB, but it is imperative that you got a battery with all good cells to produce min of 12 vdc at a reasonable CCA.

JohnnyC
New Jersey
yes it cranks over very fast, put a new fully charged deep cycle marine high cranking amps in today.
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  #59  
Old 03-15-2017, 09:24:56 PM
len k len k is offline
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Default Re: ONAN JB 7.5 new life

Try some penetrating oil on rod. If that works oil with some marval mystery oil maybe. Penetrating oil dries up in a week or two.
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  #60  
Old 03-15-2017, 09:35:07 PM
fisherman81654 fisherman81654 is offline
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Default Re: ONAN JB 7.5 new life

Quote:
Originally Posted by Leon N. View Post
By your last statement in post 54, I would surmise you do not have a relay problem but maybe an inoperative centrifugal switch. It could be the pin is sticking or there is something amiss inside that activates the mechanical switch. What is the situation with your lube oil? Does this machine show evidence of poor lube oil maintenance? You might try very carefully spraying with a straw something that may clean the centrifugal switch activating pin.

I had a case with my JB several years ago where by the points plunger was sticking and the points would not close during cold start ups. Turned out it was a stiff rubber diaphragm that acts as an oil seal. When Imswitched to a low viscosity synthetic lube oil, the problem went away. I do not know, but maybe you have a similar situation with that centrifugal switch.
This JB has been maintained very well. I use the same oil you use. May be that the rubber diaphragm seal is too stiff on the centrifugal switch , I know its clean and lubricated well. Something amiss inside that activates the mechanical switch, My thoughts also.\
\
Quote:
Originally Posted by len k View Post
Good sign that pushing centrifugal switch points closed makes relay in control box click, means at least that part of circuit is working. Next test would be to use a test light between frame and +12V lead of spark coil and confirm that you have power at coil when you manually close the centrifugal switch. From schematic looks like it should do that even if engine is not running and you don't touch start/stop switch.

I haven't had to take my centrifugal switch apart. But I'm guessing some weights spin and somehow push a rod outward to close the centrifugal points. IF engine is coming up to speed that switch trips at ( ~ 900rpm) and rod doesn't move then sounds like the rod is sticking. Also check gap.

If gen is making AC, 900 rpm corresponds to 30 hz. Most people here use a $22 kill-a-watt meter to measure hz, which is directly proportional to rpm.

I'ld say read the manual for details on how centrifugal switch works, I haven't.
I didn't see much on how the centrifugal switch actually works.
I adjusted the gap properly.
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