Generator Questions

Mark Lewis · 01-12-2014, 07:50 PM

I have a Onan Marquis 5500, model # 5.5BGMFA26105H.
Do not know the hours on it and it runs good .Unit is at least 13yrs old.
I recently had to replace the fuel pump.
The only issue I have is it seems to get lugged down when I turn the AC on, has a 13500 roof air unit on trailer.
Is there anything that needs to be done to the generator part as far as routine maintenance goes? All I have is a 5 gal hot water heater and microwave besides the ac.

Thanks for the help
Mark Lewis

Rich Biebel · 01-13-2014, 04:30 PM

Mark......Those older Onans run well and should easily run your AC and other items at the same time. The rated output on a 5500 is about 45 amps at 120 volts. An ac unit probably draws no more than 15 amps and a microwave maybe 10, Hot water heater I would guess another 10 tops so you are well within the generators capability.

The older Onans were flathead engines. 1800rpm. They tend to carbon up heavily inside the cylinder heads and intake ports. The rear intake port often is much worse with that carbon buildup. The engine supplies the power to spin the rotor of the generator. The magnetic field created as load increases is the load the engine sees. So if the engine is weak it cannot "carry the load" of the magnetic field in the generator. There is basically nothing in the generator end to service other than the brushes. They are either good or bad and if they are bad you will not have any AC output and the unit will likely not stay running due to loss of AC output.

The first thing to do is make sure the std tune-up items are good and if the machine has a lot of hours on it......a heads off carbon cleaning might be what it needs. Valve lash is also a big factor that causes them to run weak. Exhaust valves get tight, not enough lash and once the engine is hot they run poorly from improper valve seating. lash is .005 Intake and .013 exhaust cold.

I remove the heads......remove the manifolds.....clean the carbon in the heads and ports and the backs of the valves. Adjust the valve lash and reassemble it.......Carb has to be good.....Governor arm and all adjustments set carefully under load for best operation. I use a resistive load bank on all the units I service and repair. Everyone gets tested and adjusted with no load and right up to full rated load. Carbs are very sensitive on mixture and governor arm adjustments.....

01-12-2014, 07:50 PM	#1
Mark Lewis Member Join Date: Apr 2006 Posts: 346 Likes: 63 Liked 207 Times in 44 Posts	Re: Generator Questions and Answers, by R&J Control Inc I have a Onan Marquis 5500, model # 5.5BGMFA26105H. Do not know the hours on it and it runs good .Unit is at least 13yrs old. I recently had to replace the fuel pump. The only issue I have is it seems to get lugged down when I turn the AC on, has a 13500 roof air unit on trailer. Is there anything that needs to be done to the generator part as far as routine maintenance goes? All I have is a 5 gal hot water heater and microwave besides the ac. Thanks for the help Mark Lewis

01-13-2014, 04:30 PM	#2
Rich Biebel VIP Member Join Date: Jun 2007 Location: Northern New Jersey suburbs Posts: 2,314 Likes: 25 Liked 544 Times in 213 Posts	Re: Generator Questions and Answers, by R&J Control Inc Mark......Those older Onans run well and should easily run your AC and other items at the same time. The rated output on a 5500 is about 45 amps at 120 volts. An ac unit probably draws no more than 15 amps and a microwave maybe 10, Hot water heater I would guess another 10 tops so you are well within the generators capability. The older Onans were flathead engines. 1800rpm. They tend to carbon up heavily inside the cylinder heads and intake ports. The rear intake port often is much worse with that carbon buildup. The engine supplies the power to spin the rotor of the generator. The magnetic field created as load increases is the load the engine sees. So if the engine is weak it cannot "carry the load" of the magnetic field in the generator. There is basically nothing in the generator end to service other than the brushes. They are either good or bad and if they are bad you will not have any AC output and the unit will likely not stay running due to loss of AC output. The first thing to do is make sure the std tune-up items are good and if the machine has a lot of hours on it......a heads off carbon cleaning might be what it needs. Valve lash is also a big factor that causes them to run weak. Exhaust valves get tight, not enough lash and once the engine is hot they run poorly from improper valve seating. lash is .005 Intake and .013 exhaust cold. I remove the heads......remove the manifolds.....clean the carbon in the heads and ports and the backs of the valves. Adjust the valve lash and reassemble it.......Carb has to be good.....Governor arm and all adjustments set carefully under load for best operation. I use a resistive load bank on all the units I service and repair. Everyone gets tested and adjusted with no load and right up to full rated load. Carbs are very sensitive on mixture and governor arm adjustments..... __________________ Rich Biebel S/C 1479 Stock 147R Last edited by Rich Biebel; 01-13-2014 at 06:32 PM.