Onan 4000 Revs up and Down

I could be a too lean condition and require an adjustment of the altitude adjuster. The surging could also be caused by a bad adjustment of the idle speed screw. It's on the carb next to the throttle wire. It should have a small clearance when running at idle (3600, no load) but if the clearance is too large, it will cause hunting because the carb goes below idle at load off. If I remember right, the screw is adjusted to produce 90 volts when the throttle is held against the screw. USE THREAD LOCKER ON THE SCREW WHEN FINISHED ADJUSTING.

JimL

96Bounder30E wrote:

wa8yxm wrote:
I believe Surging is "Too Rich". (Sputtering and misfiring is too lean). But do verify that with a good instruciton manual please.. It has been a long time since I played with carbs.

Surging is a function of running lean.......the carburetor is essentially running out of gas and the engine is starting to die when just enough gas re-enters the bowl of the carburetor bowl to let it continue to run again.....in otherwords.......your carburetor has a blocked passage....usually in the main jet.....the Onan jet can be removed once you remove the bowl......blowing out the jet with compressed air or using a piece of mono-filament fishing line will clean the small passages.......never use a paper clip or other metal wire......

96Bounder30E got it right. Surging is normally caused by a lean condition. If rich it will blow black smoke and sputter.

Richard

wa8yxm wrote:
I believe Surging is "Too Rich". (Sputtering and misfiring is too lean). But do verify that with a good instruciton manual please.. It has been a long time since I played with carbs.

Surging is a function of running lean.......the carburetor is essentially running out of gas and the engine is starting to die when just enough gas re-enters the bowl of the carburetor bowl to let it continue to run again.....in otherwords.......your carburetor has a blocked passage....usually in the main jet.....the Onan jet can be removed once you remove the bowl......blowing out the jet with compressed air or using a piece of mono-filament fishing line will clean the small passages.......never use a paper clip or other metal wire......

I believe the altitude adjustment on the Onan carburetor works similarly to a choke on a lawn tractor, though not as predominantly. It changes the air-to-fuel ratio. I wonder if it has a partial butterfly valve, as I have never seen it taken apart.

When the altitude is set appropriately, our Onan 4000 Micro-Quiet likes to wander RPMs until warmed up and under load. But we have never had it run a while and then stall unless we ran out of fuel.

It is not supposed to run out of fuel until the main chassis fuel tank reads 1/4 full. But our generator runs out at 1/3.

Altitude setting as mentioned above was the fix for our Onan 4000. It was set for about 5000ft or more can't remember.. we're not that high up! Been flawless ever since.

If it is surging the problem may be an adjustment. DO The cleaning as suggested above, you may have to go to "Level 2" (Which is a lot of work) But also check the altitude setting, it may be set too close to sea level,, You may also need to make a fuel mixture adjustment. I believe Surging is "Too Rich". (Sputtering and misfiring is too lean). But do verify that with a good instruciton manual please.. It has been a long time since I played with carbs.

RVDisney wrote:
There are not many hours on it. I want to say about 250 is a good estimate. I do not use it often. I many run it to excercise it more than I use it.

My main concern is making sure that it is the carb before I replace it. Can anything else cause this to happen?

This week I had a new fuel pump put the them unit with 120 hrs. It ran but stopped after 5 mins, then ran a few mins later.

96Bounder30E wrote:
Hook up the fuel supply line to a gas can with a can of Seafoam cleaner in it....run the genny for a half hour and then shut it off and let it sit overnight.....the next morning run and if the "Hunting" or "Lean roll" is gone.....rehook up the fuel supply line.....if not the carb will need to be rebuilt.......

I did this a few times AND I sprayed carb cleaner in the carb, following directions on the carb cleaner can. Worked after a while and has been great since...

I have been doing more reading on here. Some people had to replace the control box for the generator. How do I know if this is going. I remember reading that the box controls the idle speed which is an issue I am having?

There are not many hours on it. I want to say about 250 is a good estimate. I do not use it often. I many run it to excercise it more than I use it.

My main concern is making sure that it is the carb before I replace it. Can anything else cause this to happen?