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Onan 7.5 JB Fuel Efficiency


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  #1  
Old 04-12-2018, 11:00:02 PM
ayumifanshawn ayumifanshawn is offline
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Default Onan 7.5 JB Fuel Efficiency

hi i have a
model no:7.5JB FL12903D
"""NOTE"""I have changed the stater to a 3rc 120/240 only for full outputs.
it is setup to run on lp only and has a gasues fuel only carb

my question is im not the best at understanding the timing could someone pleas explain a simple way to get the best fuel economy on this unit and the simplest way to achieve that i do have a timing gun and feeler gauges.

thanks:
shawn w
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Old 04-13-2018, 02:14:33 AM
Ray Lynch Ray Lynch is offline
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Default Re: Onan 7.5 JB Fuel Efficiency

Shawn
Manuals available at the attached link can give info on timing and settings for getting the most efficient operation from your set.
Ray

http://www.twinslan.net/~n0nas/manuals/onan/
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Old 04-13-2018, 07:31:15 AM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5 JB Fuel Efficiency

The service manual will explain how to set the timing with a test light (you can make one for a couple $). I have timed sets this way then checked with a timing light, it works well.

A set of feeler gauges is needed to set the points correctly. I am sure there are substitutes for a gauge, but again this is not an expensive item.

Likely just as (or more so) important is fuel mixture. I use a frequency meter (often a Kill-A-Watt meter) to adjust the fuel mixture while under full load. Adjusting the mixture to obtain the highest frequency (Hz.) without an over rich mixture. Using this method, it is very clear when you have the mixture correct and you can see the frequency start to drop when it is too rich.
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Old 04-13-2018, 09:01:45 AM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 JB Fuel Efficiency

You seem to be interested in the fuel efficiency of the JB. Also note Onan designed these machines back in the 1960's. They did publish the fuel consumption figures tabulated in the Onan tech bulletin, T-015.

The two manuals I use are the Owners Manual and PL, 967-320 and the JB/JC Major Overhaul Manual 967-500. The later tells you how to make these adjustments.
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Old 04-13-2018, 09:08:57 AM
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Billy J Shafer Billy J Shafer is offline
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Default Re: Onan 7.5 JB Fuel Efficiency

I don't think anyone was worried about fuel economy. When those were made. You might get a little better by testing what works the best. But those old units were made to run.
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Old 04-13-2018, 10:08:01 AM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 JB Fuel Efficiency

And run they very well do. Recently I ran my JB for 68 hours continuous with no problems. There will never be another JB made like that ever again. I just started up my JB a few minutes ago to put a household electric dryer load on it to give it a periodic workout and get it hot. Just thinking as I observed the mageciter NL voltage sitting at 250 volts, I have never touched/adjusted the magneciter except to replace the end bell bearing back in 1970. No one will ever know or appreciate what a gem this is except a few Stakers who understand these sorts of things.
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Old 04-13-2018, 10:22:39 AM
consolenut consolenut is offline
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Default Re: Onan 7.5 JB Fuel Efficiency

Hi

I have a JC and it consumes about 1.4 gallons of propane per hour. That is with a 20-22 amp load per leg. The JB is only 2 cylinders and depending on load it might be close or perhaps a little less. I have run my machine at about the same and a little less load and fuel usage seems to be about the same. Flow meters on propane pumps are in gallons. Hence I get my numbers that way based on run time.
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Old 04-13-2018, 10:32:12 AM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 JB Fuel Efficiency

According to the Onan table, page 3 in T-015, the 15JC consumes 110 CU. FT/HR at full load. For propane Onan says 1 gal.=36.5 Cu. Ft and 1-LB = 8.5 Cu. Ft.

For the 705JB it is 63 CU. FT./HR.
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Old 04-13-2018, 12:20:38 PM
Vanman Vanman is offline
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Default Re: Onan 7.5 JB Fuel Efficiency

Realize that the fuel mixture which develops the greatest power (highest frequency under a given load) will be richer than the mixture which consumes the least fuel for a given load.

Only a fuel flow meter can determine the most efficient fuel mixture. But securing reliable transient response will limit how lean the mixture can be set, especially on simple carburetors. Too lean and the engine will stumble upon application of load.

I set the mixture just rich enough such that the set can reliably and smoothly take full load in one step. This will be leaner than maximum power, yet richer than maximum economy.
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Old 04-13-2018, 01:37:04 PM
len k len k is offline
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Default Re: Onan 7.5 JB Fuel Efficiency

On gasoline heard JB consumes 0.6 GPH @no-load, 1GPH full load (but I suspect the 1 is really 1.2 )

For propane by ratioing my NHM specs I'ld say add 23% to above numbers
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