My 520H has gone hunting - needs some help

I've had my share of 520H surging problems, but now I have a new one. Its hunting. This is my 1988 520H with the Onan P220. It starts up just fine. I begin mowing and all is well is for about 15 minutes, then it starts hunting or surging (if you wish). It only happens under load (PTO engaged with a 48" SD deck) and after the tractor warms up completely. The rpms start to drop, then the governor opens the throttle until the rpms are back up, throttles back as it should, then the rpms drop again and the cycle repeats on about a 30 second interval. Its almost like the the governor is way too slow in reacting.

If I disengage the PTO, the big Onan runs just fine and I can get it back to the stable without issue. I've repeated this about 4 times now.

What I've done so far.

1. Carb was cleaned about 3 weeks ago. (My usual spring chore)
2. I changed the oil and filter, blew out the fins, and cleaned the air filter
3. Installed a new electric fuel pump.
4. Checked the plugs, they look just fine and are properly gapped.

So I thought, I must be the new electric fuel pump. So I disconnected it and put the gas lines back on the stock pulse pump. Ran the tractor and had the same problem. So its not the pump.

I've had carb cleaning fix an idle surge many times and I plan to clean this carb again. I would put in a kit if the price wasn't so darn high.

Any other ideas?

Thanks,

Kurt

I guess other things can cause this but I've never had one that the surging wasn't caused by fuel starvation. When the surging starts give it just a little choke and see if it stops, if it does you'll know.

I'll give that try Geno. I'm thinking I may have broken some junk loose when I put that pump in. That could cause fuel starvation if it lodged in a jet. Not sure how if would get past a fairly new filter, but I'll change the fuel filter and clean the jets and see what happens.

BTW, I checked the electric fuel pump before I disconnected it. Plenty of flow, but the pressure seemed low, but my gauge was acting up. Time for a new one.

Thanks,

Kurt

I got busy this morning and replaced the fuel filter and did a top down carb cleaning. The tractor started right up and I mowed with it for about 30 minuted before the hunting returned. I tried adding some choke and that seemed to help. The best spot was right before the engine started to stumble from too much choke. I ran it that way for a few minutes and then backed the choke off. The engine was running just fine. 10 more minutes of mowing and the hunting returned. This time adding the choke did not help.

I also cracked the fuel cap in case the vent was plugged and that did not help.

I quit mowing and returned to the stable. I broke out my propane torch and played the unlit torch around the base of the carb and the intake manifold. No change in the engine.

About the only thing left to try now is some Sea Foam or MMO. If that fails, I'll pull the carb and put it in a can of carb cleaner for a few days.

Ugh!

Thanks for the help.

Sea Foam is a good next step.

Geno wrote: Sea Foam is a good next step.

Worth a try. Stay tuned, but it might be awhile. I'm traveling for the next week.

Here is something we overlook many times. The strainer in the gas tank, will (don't ask me how I know) cause grief. If there are large chunks of whatever in the tank everything will be fine until you have run for awhile and those chunks get drawn to the screen, then you will get reduced flow until you shutdown and the junk falls off the screen. Anyway if you haven't yet drained the tank pull/replace the petcock blow out the tank and see what happens. :cents1

Thanks GT, I didn't know a strainer was there, I will definitely check it out.

Both of my Fords have this kind of strainer. It is amazing how much crap gets on the screens.

Kurt

Update,

The manifold and carb gaskets are on order as is the gas tank valve. So I'm in wait mode.

Since my sinuses cancelled my trip the show, I had some time to pull the carb and clean it as best I could. Now it is sitting in a can of carb cleaner until the parts get here. I wish these new cans of carb cleaner were a good as the old one. Maybe a few extra days will help.

Once I get it back together, I'll run some Seafoam through it and hope I got it.

Thanks for help,

Sorry I had to miss the show.

Kurt

Update:

I soaked the carb in a can of carb cleaner for two days. It came out a clean as a new one. I blew out all the passages with compressed air and then carb cleaner. I also replaced the fuel and vacuum lines as well as the fuel filter. I also replaced the valve/screen on the bottom of the gas tank.

I was hopeful as I started it up. After 5 minutes of mowing, the hunting was back. UGH!

So I replaced the ignition switch, the connector looked good and started the 520H and I saw some improvement. I would say more than half the problems are gone. So I put some Sea Foam in the gas tank and started mowing. The seem to be reduced quite bit.

My plan for now is to mow for a few weeks with Sea Foam. I will also be checking all the electrical connection. No infamous 9 pin connector on the 1988 520H so that is not it. But there are other connection.

Thanks for all the help. I let everyone know how this works out.

PS this Onan has almost 1200 hours on it. Any major problems and a repower will follow.