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Old 04-23-2021, 10:36 AM   #1
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Typical Class C generator fuel prime time?

I have an 11 year old 31 ft Fleetwood Class C with 115 hours on the generator. Every month I run the generator for 1-2 hours under load and during the past year or so the time to prime the fuel system in order to start has increased to 1-1.5 minutes. After starting it ran fine. This week I installed a new fuel pump and filter and now the prime time, after sitting a few days, is 14 seconds. What is typical fuel prime time for a long class C ? I have a full fuel tank and religiously use fuel treatment. The fuel line back to the tank from the generator seems good as the new fuel pump had a steady flow when disconnected from the carb.
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Old 04-23-2021, 10:56 AM   #2
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Maybe something different but I only prime our 18 year old Onan 4000 if it has set for a long time. A few days, couple of weeks, no need for priming on ours since the bowl already has fuel in it.

When you said "1 to 1.5 minutes", how do you know? Mine will prime as long as I hold the stop button in. There is no indication that the fuel system is pressured up. If I feel there is a need for prime, I just hold the stop button in for a few seconds after I hear the pump running then release and hit the start button.
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Old 04-23-2021, 11:16 AM   #3
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Maybe something different but I only prime our 18 year old Onan 4000 if it has set for a long time. A few days, couple of weeks, no need for priming on ours since the bowl already has fuel in it.

When you said "1 to 1.5 minutes", how do you know? Mine will prime as long as I hold the stop button in. There is no indication that the fuel system is pressured up. If I feel there is a need for prime, I just hold the stop button in for a few seconds after I hear the pump running then release and hit the start button.
Prime the O4000 with the stop button? I've been doing it wrong for some 20 years? No wonder it spun out the dried up copper bearings in the starter! I have always pushed the rocker switch to start. And pushed the same rocker switch the other way to stop. But I never, ever got a start on the first push to start. Always had to push to start at least more than once, at five second intervals. and how many times depended on how long since the last run. Fuel evaporates from the bowl and oil leaks all the way down between runs. More so between extended periods. Once a month almost guarantees a start on second try. Your results may differ
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Old 04-23-2021, 11:28 AM   #4
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The amount of "prime" time for me is like I suspect for others, the longer the generator hasn't ran the longer I need to prime. That equates to no prime time if the unit has ran within the last few days to maybe a minute if it hasn't ran for a month. Keep in mind that the prime (as I understand it for the onan generators) doesn't actually flood the engine or directly inject fuel into it, what it does is simply turns on the fuel pump and once the carb fuel bowl is full, the float inside the carb will stop the flow of anymore fuel.

TandW, check it out, I suspect yours is like everyone else's, just hold the stop button down for about 30 seconds then try starting, if it doesn't start then I hold the stop button down for another 30 seconds. I don't see where any harm would occur (based on my above statement) if one was to prime the generator whether or not priming was actually needed.
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Old 04-23-2021, 12:02 PM   #5
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Only by holding in the start button does the fuel pump become energized. Check it out in your troubleshooting section of your manual.
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Old 04-23-2021, 12:11 PM   #6
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Only by holding in the start button does the fuel pump become energized. Check it out in your troubleshooting section of your manual.
Holding the stop button energizes a booster pump on my carbureted onan 5.5kw, but the owners manual says to only prime if itís been run dry or the bowl drained.
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Old 04-23-2021, 12:25 PM   #7
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We were talking about the O4000 Microlites.
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Old 04-23-2021, 12:40 PM   #8
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We were talking about the O4000 Microlites.

I suspect a lot of the Onan's have a prime (maybe not the lpg models?). I purchased my RV used and in the plethora of manuals I didn't get one for the generator (onan 4000 microlite). In any case, I am certain on mine that holding the stop button down for more than a few seconds engages the fuel pump as I can hear it if the windows are open and it is quiet outside and a little light at the panel also lights up while holding the stop button in. Also, I am sure that the generator will start on the first or second try even when it has been sitting for a month, but only when I have held the stop switch down for ~30 seconds (once or twice), if I fail to prime it then it takes a very long time and many tries to get it started after sitting (there was a time early on that I didn't know about the prime function so I always prime if it has not ran in many weeks). Also, in the beginning I didn't know that all that was needed to turn it off was a simple ~1 second push of the stop switch as I used to hold it in until the generator completely stopped. Try it out, yours could be different but it is easy to test. ~Craig
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Old 04-23-2021, 12:42 PM   #9
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I prime until the generator starts. I try to start every 15 seconds and if it doesn't start I prime another 15 seconds. I'm sure the old pump was weak and will have a final test of the new pump after 30 days when it's time for the next exercise.
just don't know what's normal.
I'll try and find specs for the Onan pump and run a flow test of the old pump to confirm it was weak. FYI I did install an Onan pump not an aftermarket unit.
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Old 04-23-2021, 12:47 PM   #10
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I prime until the generator starts. I try to start every 15 seconds and if it doesn't start I prime another 15 seconds. I'm sure the old pump was weak and will have a final test of the new pump after 30 days when it's time for the next exercise.
just don't know what's normal.
I'll try and find specs for the Onan pump and run a flow test of the old pump to confirm it was weak. FYI I did install an Onan pump not an aftermarket unit.
Once again, from the troubleshooting section, pressing the "Start button" energizes the fuel pump.
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Old 04-23-2021, 01:23 PM   #11
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Once again, from the troubleshooting section, pressing the "Start button" energizes the fuel pump.
I don't think anyone disagrees that pressing the "Start" button energizes the fuel pump, if it didn't then the generator wouldn't run... The question is about the amount of time it takes to prime. I will add back to the OP, when mine hasn't ran for only a couple of weeks I usually only need to prime for 15 seconds then if it doesn't start, I prime another 15 seconds and then try again like you mentioned and that is usually enough. I only need a 30 second prime in the winter and for those times when I may not have ran the engine for 4~6 weeks (which I try not to go that long).. but sometimes I get delayed and I don't exercise the generator for 4~6 weeks and in those cases I start with a 30 second prime and if it doesn't start right away, I prime a second time for around 30 seconds which is about the longest prime time I have ever needed. ~Craig
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Old 04-23-2021, 01:36 PM   #12
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All this prime time leads to worn out starter bearings. Do like the manual says, run it every thirty days at 1/2 load. It is what it is. If you think tapping on the can keeps the foam down, then go for it. In the meantime, I'll be ready for another.
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Old 04-23-2021, 01:44 PM   #13
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I am not sure we are on the same page, priming doesn't rotate anything, not the engine, or the generator, or the bearings, etc. priming just turns on an electric fuel pump (to correct myself, the fuel pump has a motor in it that rotates) and not too mention, the faster starting allows for the generator's electric starter to run a lot less which in turn will provide for a longer starter life. You really should try priming yours and see, if it starts faster for you after not running for an extended time period then I suspect you would like that it does and you don't have anything to lose if it doesn't.
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Old 04-23-2021, 01:47 PM   #14
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Originally Posted by craigav View Post
I suspect a lot of the Onan's have a prime (maybe not the lpg models?). I purchased my RV used and in the plethora of manuals I didn't get one for the generator (onan 4000 microlite). In any case, I am certain on mine that holding the stop button down for more than a few seconds engages the fuel pump as I can hear it if the windows are open and it is quiet outside and a little light at the panel also lights up while holding the stop button in. Also, I am sure that the generator will start on the first or second try even when it has been sitting for a month, but only when I have held the stop switch down for ~30 seconds (once or twice), if I fail to prime it then it takes a very long time and many tries to get it started after sitting (there was a time early on that I didn't know about the prime function so I always prime if it has not ran in many weeks). Also, in the beginning I didn't know that all that was needed to turn it off was a simple ~1 second push of the stop switch as I used to hold it in until the generator completely stopped. Try it out, yours could be different but it is easy to test. ~Craig
In looking in the Q4000 Owners Manual pdf it appears it can only be primed with the switch that in on the genny and not the switch mounted inside the coach.

The Control Switch is labeled Start and Stop/Prime and has the little yellow light. The switch on the dash does not.
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