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Possible new toy Onan 7.5 JB


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  #261  
Old 08-13-2017, 10:07:57 AM
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Jim McIntyre Jim McIntyre is offline
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Default Re: Possible new toy Onan 7.5 JB

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Originally Posted by squidtrap View Post
...Need to figure what makes the hunting go away, more or less sensitive, they say less???
To make hunting go away, you need less sensitivity.

Check your main mixture setting.
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  #262  
Old 08-13-2017, 05:52:17 PM
squidtrap squidtrap is offline
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Default Re: Possible new toy Onan 7.5 JB

OK, first thing i did was change regulators, no change, both are fine. Then i did what you said Jim, opened idle screw almost all the way and adjusted governor till i was at 63HZ, runs like a top. Put about 3K on it and hunting starts!!!!!!! screwed w/ that governor ratchet till the hunting went away but the HZ droped to 59-60 and it sounds like it is laboring. Messed w/ gas adjustment and still no luck in getting it to smooth out (purr) but the hunting is gone.

Anyhow, got tired of sweating and being on my knees so need more time, might just get propane tank to confirm a fuel issue. CCK and NH run fine loaded on nat gas would figure the JB would, going to check t-150 to see the specks.
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  #263  
Old 08-13-2017, 06:44:59 PM
JohnnyC JohnnyC is offline
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Default Re: Possible new toy Onan 7.5 JB

Squidtrap, I wish I had an answer for your problem, but unfortunately I don't. I never ran into a problem as you described, but had other perplexing problems. In the end, all my problems were simple to fix.

JohnnyC
New Jersey

P.s. - I need to go back and reread earlier postings, but are you running on propane? And if so is it a single BBQ tank? And what is the rating of the tank's regulator?
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  #264  
Old 08-13-2017, 07:24:32 PM
squidtrap squidtrap is offline
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Default Re: Possible new toy Onan 7.5 JB

JohnnyC- natural gas. thats the thing, runs the CCK and NH no problem, and the NH actually needs more than the JB????
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  #265  
Old 08-13-2017, 07:30:11 PM
len k len k is offline
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Default Re: Possible new toy Onan 7.5 JB

If you think it's starving for propane to carb, maybe supplement the propane with a TINY stream of gasoline.
On my JB at no-load, on 100% gasoline fuel, a steady ~ 1/32 inch dia stream of gasoline from a squirt bottle down the intake made it run normal, carb was plugged so I couldn't run fuel thru it.

I ran my JB no-load using BBQ grill reg and 20# tank at no-load. BUT it was an OLD BBQ reg, it had an aluminum cap to adjust the outlet pressure and seemed to flow more gas than 1 or 2 of the more modern sealed BBQ regs. Could Tee together output of 2 tanks and regs.....

Also wondering if someone changed your gov spring.
My JB spec AA tri-fuel gov spring is made of .091 inch dia wire , wound into 8 coils, whose OD is .642 inch. It does have rust on it so wire gauge may be off a little.

Last edited by len k; 08-13-2017 at 08:32:32 PM.
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  #266  
Old 08-13-2017, 08:06:11 PM
JohnnyC JohnnyC is offline
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Default Re: Possible new toy Onan 7.5 JB

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Originally Posted by squidtrap View Post
JohnnyC- natural gas. thats the thing, runs the CCK and NH no problem, and the NH actually needs more than the JB????
JB needs more gas than the CCK and NH.

When surging, does the throttle go wide open throttle (w.o.t) then back off significantly then goes back to w.o.t. in a repetitive cycle?

JohnnyC
New Jersey
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  #267  
Old 08-13-2017, 09:12:28 PM
Leon N. Leon N. is offline
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Default Re: Possible new toy Onan 7.5 JB

From my understanding, and earlier (years ago) Stak comments, if someone installed a different governor spring, all bets are off. You will have a very hard time getting the JB to run correctly.
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  #268  
Old 08-13-2017, 09:14:46 PM
RETCPO RETCPO is offline
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Default Re: Possible new toy Onan 7.5 JB

With everything you have posted and adjusted I do not recall if you have checked your timing on this set. You posted the you made adjustments and it was running like a top but at 63 HZ and when you slowed it back down that it sounded labored.

Let your ear be your guide and try advancing the timing until it sounds good. this is what I did with a new to me 5 CCK on propane with the timing set to factory spec it sounded labored after bumping up the timing I can drop 3500 watts and it barley grunts. Same when the load is removed less than .4 (yes point 4) HZ swing.

Just my opinion
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  #269  
Old 08-13-2017, 09:15:45 PM
len k len k is offline
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Default Re: Possible new toy Onan 7.5 JB

Different spring wire dia, OD , or # of coils would make spring stiffness different. Could get rpm right at no-load, but as kw load changed resulting new throttle position would not be optimum (factory).

Looking in wire guage chart looks like 16 gauge ( .095 inch dia) was likely the standard size spring was originally wound from from. I measured .091 inch dia, rusted.

Spring rate varies by 4-th power of wire dia and 1/( 3-rd power of spring diameter)
So my .004" difference in rusted wire dia gives a ~ 19% reduction in spring rate !!!! WOW .
Doesn't take much to make a large change

Thinking about it bouncing hz at no-load sounds like something else. . Check that you don't have play/slop in throttle linkage . particularly the "ball joints". My rototiller engine has throttle rod just going thru a hole in carb arm, the small pre-load spring to pre-load the play out of linkage was disconnected, and it hunted a lot till I hooked it up, 7NHM has same.

Last edited by len k; 08-14-2017 at 10:45:06 AM.
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  #270  
Old 08-14-2017, 02:34:29 AM
Ray Lynch Ray Lynch is offline
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Default Re: Possible new toy Onan 7.5 JB

Is the throttle stop screw adjustment correct?
Ray
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