This isn't a bike, but the generator in my RV. It runs, finally, and produces 120vac as it is supposed to, but the engine surges. I have had other single cylinder equipment do this as well, but never really worried about it on those (usually old lawn mowers). Any thought as to causes? Carb trouble is my first thought, this thing sits unused for great expanses of time. I did remove the carb , disassemble and clean it to the best of my ability with spray carb cleaner, and it looked pretty good. This was actually what got it running at all. If no other ideas surface, is it usually worth it to just replace, or are these small carbs rebuild-able with any success?

It isn't an Onan 5500 is it?  I had issues with mine, rebuilt/cleaned it 5 times.  No worky.  Did a bunch of research (no, not on how to calibrate it), spoke with an Onan tech who stood to gain nothing, and finally broke down and paid $230 for a new carb.  Purs like a kitten now.  And I must add, a well calibrated kitten at that

Buy some Techron and run a couple of tanks through it. It'll clear up.

Buy some Techron and run a couple of tanks through it. It'll clear up.

I'm sure it would.  If its plumbed like my RV generator (Onan), one tank would would be 35 gallons until I switched to the second one.  I usually run mine once a month for about half an hour or so and it does pretty good all year.  If I forget during the winter months it starts the surge thing for a while.

Make sure you use the blue stabil (if you use it) in your fuel now for alcohol.

My Poppa worked on mowers for decades, besides s little bad gas most times you can get that surge controlled by closing down the high speed needle screw just a scoche, and opening the low speed one a tweek if needed to maintain smooth idle, the throttle governer sounds like its cycling, and when the engine revs, it  throttles the butterfly to drop the high speed down. There also may be a couple holes the spring that attaches to that governer arm can be moved to on the arm, and also at the carb, moving that sometimes helps.

What MOB said. I would guess governor. Does it surge under load? Plug something that uses almost as much power as the generators is rated. If it stops surging then sure sign of a governor and its really not a big problem. Some just have a plate positioned next to the cooling fan and adjustment by spring such that when the engine over revs, the fan moves more air and pushes on the plate which reduces throttle. Without load the engine over revs, the plate pulls back the throttle then engine slows down, less air pressure on the plate which lets the throttle increase, which speeds up the engine again and starts the cycle all over again. Under load it doesn't do it as much because the load naturally keeps the engine from over revving. Some adjustment of the spring on the governor plate can reduce or eliminate the surging.

It's not a 5500, it's an Onan 4000. The governor is not driven from cooling fins like a lawnmower, it is driven internally from the crank shaft and is working. It is essentially what is causing the surge, otherwise I suspect it would simply stall. That being said, it could need adjustment. Important piece of info I neglected to include, while spraying carb cleaner down the throat while running, it smoothed out perfectly. The surge is unchanged by load, I can have the air conditioner on inside the rig or no load at all, the surge goes on. At this point I'm going with the carb, I will try to remove without having to remove the generator from the RV. That is not the worst project I've ever done, but seems like a lot for 2 screws that hold the carb in.

I'm no MOB or SISF(IOW, I only wish I had their skills), but I have had many carbs apart of many types.  The Onan carb is the one weak link on this gen set.  When I get a 20 year Onan tech tell me a new carb is the answer, I ignored him, tried yet again to clean the carb.  I used the full RV fuel tank (yes, 35 gallons) to cut my grass, even burned some in the dirt bike.  Finished emptying the tank (Stabil for Ethanol added), added fresh fuel, ran fuel pump until it was clear and no signs of contamination.  Fresh fuel added, super (I thought) clean carb, same issue.  Gave in and bought new carb.  Installed, runs perfect.  I will use a magnifying glass to see if I can spot the blockage so I can have a backup carb, but that will be a winter project. 
As to adjustment?  There is an altitude adjustment screw with one of those stupid plastic caps to limit how far.  Removed, cleaned needle and seat, reinstalled, and still, no love.  There is also a fuel shutoff solenoid on the bottom of the carb, bottom line?  I gave up, for now.  I have a 24 hour race coming up and I needed everything to work smoothly so I can focus on the race...  And then there is elk season

If it is running, and the governor is causing the pulse, and runs smooth when you spray carb cleaner into the intake, I'm sticking with the "too much fuel thru high speed needle screw" answer, because carb cleaner is not as combustable as gasoline, it reduces the fuel quantity to the combustion process.

In webbing, I stumbled upon some stuff, interesting reading, there is a manual available on this sites link midway down
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/25457462/gotomsg/27104585.cfm
http://www.smokstak.com/forum/showthread.php?t=85628

This guy covers surging because of mis adjusted choke linkages..

http://www.justanswer.com/rv-motorhome/705au-onan-4000-microquiet-runs-great-just-shuts-down.html

Another manual here
http://www.rvforum.net/SMF_forum/index.php?topic=50331.0

More stuff here, a bit dated, but still pertinant

http://www.irv2.com/forums/f65/onan-generator-maintaining-and-running-a-4000-watt-137495.html

Usually surging on small engines is caused by a lean mixture. This is all- too common these days as the EPA wants everything to run clean (if not quite right).

It is easy to check too- just slowly engage the choke and the engine will run cleanly before it starts to load up and stumble. If that is the result of engaging the choke, you have a lean condition.

As an aside, I listen to a LOT of small engines run this way in the neighborhood while others mow the lawn, etc. Very annoying to anyone who appreciates something that runs correctly. 

Brian

This isn't a bike, but the generator in my RV. It runs, finally, and produces 120vac as it is supposed to, but the engine surges. I have had other single cylinder equipment do this as well, but never really worried about it on those (usually old lawn mowers). Any thought as to causes? Carb trouble is my first thought, this thing sits unused for great expanses of time. I did remove the carb , disassemble and clean it to the best of my ability with spray carb cleaner, and it looked pretty good. This was actually what got it running at all. If no other ideas surface, is it usually worth it to just replace, or are these small carbs rebuild-able with any success?

Yep, that sure sounds like it is running lean and the carb. cleaner enriched the mixture (hey, flammable is flammable, right? It does not have to be gasoline).

Brian

It's not a 5500, it's an Onan 4000. The governor is not driven from cooling fins like a lawnmower, it is driven internally from the crank shaft and is working. It is essentially what is causing the surge, otherwise I suspect it would simply stall. That being said, it could need adjustment. Important piece of info I neglected to include, while spraying carb cleaner down the throat while running, it smoothed out perfectly. The surge is unchanged by load, I can have the air conditioner on inside the rig or no load at all, the surge goes on. At this point I'm going with the carb, I will try to remove without having to remove the generator from the RV. That is not the worst project I've ever done, but seems like a lot for 2 screws that hold the carb in.

Yep, all (well most)  indications are pointing to the carb. I will start with the easy stuff, checking for a vacuum leak. Posts here have indicated both lean and rich condition, lean seems to make sense to me as when the governor engages it activates the choke which "saves" it to keep it running. I also believe I am adding flammable fluid and enriching the mixture when spraying carb cleaner into the carb throat. I'm at home hooked to a chemo pump for the next fer days, so further diagnostics will have to wait until Sunday at the earliest, depending on how I'm feeling.

Thanks to all who offered suggestions and help. I finally got some time to revisit this issue and quickly determined it was not a vacuum leak. I removed the carb, without having to remove the genset from the RV, though it was a little challenging. Disassembled it and soaked it in Berrymans for a few hours, rinsed and blew it dry and put it all back together, runs like a champ!

I'm at home hooked to a chemo pump for the next fer days, so further diagnostics will have to wait until Sunday at the earliest, depending on how I'm feeling.

I didn't catch that until you posted an update today. Hope all is going according to plan & you're getting better.
Glad you got the genset problem ironed out.

Thanks. So far all is good and going to the plan and schedule the Dr's have laid out. Still ride on non-treatment weekends, did about 200 easy miles on Sunday with my wife and some light trail hiking in a state park.