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An Onan JB Performance Question


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  #1  
Old 08-05-2017, 06:10:26 PM
Leon N. Leon N. is online now
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Default An Onan JB Performance Question

I have been wondering for some time why my JB will not meet the 2 second response time until it is thoroughly warmed up? My JB has the tri-fuel carburetor but I alway burn the utility supplied 1000 BTU natural gas. From a cold start once up to speed dumping even with just a 4KW load on it all at once can take up two 4 seconds to return to a stable RPM. However, when fully warmed up by running for 10 minutes it will return to stable operation in less than 2 seconds applying the same 4 KW load in one step. All other specified parameters, voltage and frequency remain in the specified tolerances be it NL or with a FL of 32 amperes.

Has anyone else observed such behavior? Could this droop be due to the fuel and or that old Garretson regulator which I continue to use? I also note there is noticeable, hunting and occassional popping back through the carburetor until the unit warms up. Once fully warmed up, it behaves like a charm and the issue I describe here goes away.

To me it is no big deal, just curious if others have seen this and what if anything they did about it.
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Old 08-05-2017, 07:50:53 PM
Handyhiker1 Handyhiker1 is offline
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Default Re: A JB Performance Question

Just as a general engine acting like this, it sounds like it is a little lean on the fuel.
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Old 08-06-2017, 12:59:04 PM
len k len k is online now
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Default Re: A JB Performance Question

Leon..... I'ld open up starter and do some maintenance. My 7NHM will sometimes not engage and just spin. Does it more in cold weather than warm. Plastic stater gear assembly
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Old 08-05-2017, 07:56:56 PM
len k len k is online now
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Default Re: A JB Performance Question

Common for engines. Never gave it much thought why they run rough when cold.

On gasoline engines I would have guessed gasoline needed a warm engine to help evaporate the liquid, but your nat gas fueled so that theory is out the window. Would expect the propane to flow fast enough to quickly purge any air that back diffused into demand reg.

Guess that leaves oil wanting to warm up and thin out, less drag. Could be heat expands clearances and makes less drag too.

Last edited by len k; 08-05-2017 at 08:17:54 PM.
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Old 08-05-2017, 08:22:40 PM
dracer dracer is offline
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Default Re: A JB Performance Question

popping back threw the carb means it is running lean
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Old 08-05-2017, 08:38:08 PM
Leon N. Leon N. is online now
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Default Re: A JB Performance Question

Yes, it appears to be running lean when first started up. I can stop the popping by riching it up a bit. BUt, then once it has warmed up, it does not meet the Onan performance spec unless I lean it out a tad. So what is the likely hood it is the Garretson regulator?

Where are all the JB users tonight? Could they share their experience? I guess in order to comment one has to know what the Onan JB spec says. There has been lots of discussions in the past about performance in terms of frequency and voltage but little discussion about the Onan JB 2 second recovery requirement except to hear comments like my machine grunts a little bit but recovers. To me meeting this 2 second requirement is important WRT starting motor loads, like a central AC system which could be damaged, not to mention other connected loads.
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Old 08-05-2017, 08:55:11 PM
MBB MBB is offline
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Default Re: A JB Performance Question

Better to leave it run a little lean. Natural gas better to run lean than rich can burn valves and pistons if too rich. My JC does the same for about 2 minutes. It too runs on natural gas. I have yet to do a tuneup on it though since I bought it.

Last edited by MBB; 08-05-2017 at 09:31:00 PM.
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Old 08-06-2017, 12:02:11 AM
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Jim McIntyre Jim McIntyre is offline
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Default Re: A JB Performance Question

Doesn't seem right, Leon.

Here' how mine behaves. I had run the set for about 60 seconds before this video, so it was't really warmed up.

https://youtu.be/0vERaGEr6ac
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Old 08-06-2017, 01:28:38 AM
len k len k is online now
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Default Re: A JB Performance Question

Leon........ here's a test you can do to check demand reg, to eliminate variable of air diffused into nat gas in it. Energize nat gas solenoid and blow into vent of demand reg for 5-10 sec. That way propane flow will purge demand reg of all air. Then start engine and do your transient response load test.
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Old 08-05-2017, 10:52:35 PM
JohnnyC JohnnyC is online now
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Default Re: A JB Performance Question

Quote:
Originally Posted by Leon N. View Post
Yes, it appears to be running lean when first started up. I can stop the popping by riching it up a bit. BUt, then once it has warmed up, it does not meet the Onan performance spec unless I lean it out a tad. So what is the likely hood it is the Garretson regulator?

Where are all the JB users tonight?
I be here.... Just so happens I was "playing" in my garage with my JB, ATS and controller. I had a 2kw load going through the ATS to the Utility. I simulated a power outage by cutting the utility breaker inside the garage panel. The ATS kicked and started a cold JB (75 degrees outside) and immediately transferred the 2 kw load to the JB after the JB started (3 seconds of cranking and within 2 seconds once the JB kicked over and up to speed). There was no issues, no bogging, no abnormalities. I am running on propane and the JB was NOT started or warmed up prior to my test. If you want, I can redo the test with a 4 kw load, but I know the results will be the same.

Below are two pictures showing the specs I was running this evening on each of the 2 legs (A and B) single phase.

JohnnyC
New Jersey
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