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Ask Hank F. Your Own Question
Hank F.
Hank F., RV tech - emphasis on gensets
Category: RV
Satisfied Customers: 15270
Experience:  RV tech since the beginning of the millenium.
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Our onan gen runs very rough

Customer Question

our onan gen runs very rough
Submitted: 1 year ago.
Category: RV
Expert:  Robert replied 1 year ago.

Hi my name is Robert..

What type of Onan do you have? IS it a LP, gas or diesel unit? Have you done any maintenance on the generator?


Customer: replied 1 year ago.
gas, just oil
Customer: replied 1 year ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Customer: replied 1 year ago.
how does this work are you going to call?
Expert:  Robert replied 1 year ago.


What I would do if you have not done so is replace the fuel filter, inspect the air filter and change the oil to start. IF the generator is still running rough I would get a can of fuel and put the supply line in the can and see if it runs better, sometimes the lines from the generator to the tank can weather and develop cracks, once this happens the generator can suck air in the lines and cause it to run poorly. This will eliminate the lines and tank as a possible problem.

There is an extra charge for the phone service.... Robert

Customer: replied 1 year ago.
sounds like its blowing compression like a steam engine
Expert:  Robert replied 1 year ago.


What I am going to do is put your question back up, there is an Onan tech that comes around and he might have some more ideas on this for you. I do not know when he will respond as most of us have day jobs but I will send him a message about your issue. My goal is for you to be satisfied and have the best possible answer. Robert

Customer: replied 1 year ago.
will he email me?
Expert:  Robert replied 1 year ago.

Yes he should respond like I have if he feels he can assist you, it might be helpful if you can get the full model of the generator.....


Expert:  Hank F. replied 1 year ago.

Hello, my name is ***** ***** Robert has asked me to take oer this for him.

Could I please get the full model number of the genset, exactly as it appears on the data label?

When you say it runs rough, could you elaborate on that? Is it missing? Is it surging? Is it smoking?

How long has it been since the genset was run regularly?

The more information you can provide, the easier this will be. Remember, we can not see or hear what is going on, so we only know as much (or as little) as you tell us.


Customer: replied 1 year ago.
model # ***** 6.5 nhe-fa/11579p volts 120 amps 54.2 it is a onan 6500 It has set for about 6 months and it works but it sounds like its missing or spitting it is not smoking. its quite a loud noise. The gen ran fine when we used it about 6 months ago it is our only source of power in the winter. thank you hope you can help !
Expert:  Hank F. replied 1 year ago.

Sorry for the delay, but I actually work 2 day jobs, and help on Just Answer in the evenings.

Yes, I can help. I deal with this all the time - it is actually one of my day jobs.

I need you to do 2 things here.

1. Remove the ar filter. Start the unit. Once it starts, spray in some carb cleaner (do not use starting fluid). Does the engine smooth out and run better with the carb cleaner?

2. Do a cylinder balance test.

To do a cylinder balance test, start the engine.

Using a pair of spark plug pliers, or other insulated tool (so you don’t get shocked), remove the spark plug wire on the front cylinder. This should make the engine change tone, and possibly run rough.

If it does, that means both cylinders are firing.

If it makes no change in the way the engine runs, it means the rear cylinder is firing, but not the front cylinder.

If it shuts the engine down, it most likely means the rear cylinder was not firing.

Now reconnect the front wire, and pull the rear. The effect should be just the opposite.

Are both cylinders firing or not?

If they are not, we need to find out why.

Customer: replied 1 year ago.
ok pulled the front wire it dies , back wire no change it did not die so whats next??
Expert:  Hank F. replied 1 year ago.

Now we have to figure out why the rear cylinder is not firing.

First check the plugs. Check the condition of them, and clean/regap/replace as necessary. Make sure you are getting fire to both of them.

If the rear plug is not getting fire, switch the plug wires on the coil, and recheck. Both plugs fire at the same time, so it does not really matter which wire goes where.

If you now have no spark on the front cylinder, the plug wire is bad.

If you do get spark to both cylinders, check the compression. Make sure you have good compression in both cylinders.

Let me know what you find,.

Customer: replied 1 year ago.
how do you check compression do we need a mechanic to do that?
Expert:  Hank F. replied 1 year ago.

To check compression, the basic steps are to remove the spark plug, screw in a compression guage, hold the throttle to the wide open position, and crank the engine.

You need to follow the directions of your guage.

If you do not have a compression tester, you will need to buy one, or take it to shop.