318 Rough Idle Require Choke

[Tjaltz]

Hello All:
I have a 1989 318 (serial #600XXX) with an Onan P218G engine and about 400 hours on it. I was mowing the lawn, machine was running fine, and it stalled. When I went to restart it, it had to choked, and then it would only run if it is 3/4 choked, and had very little power. It has fresh gas, I checked the plugs, they are okay. It has spark to both cylinders, if I pull either plug wire while it is running, it runs much rougher and stalls. I sprayed the carb and linkage, and noticed that sometimes while spraying the right side and base of the carb, it seems to smooth the idles a little better. When I spray more than a little of the cleaner into the carb itself, it will stall out. I have 150 lbs of compression in each cylinder. The tractor also has a John Deere AM109263 hard start kit installed.

I have thoroughly cleaned the carb (while on tractor) and all linkage, and ran carb cleaner (in the tank) through it. After the carb cleaning it seemed to run okay for about an hour. I ran it at idle, mid-throttle and full throttle for almost an hour. I then drove it around for a while, and the engine started to surge (hunt up and down) and would only run smooth with about 3/4 choke. below are two videos I took of it. The first is the tractor with the choke wide open and the engine surging up and down.

YouTube - ‪John Deere 318 one‬‏


The second video shows the engine hunting, and then smoothing out when the choke is applied.

YouTube - ‪John Deer 318 two‬‏

I'm thinking that because the machine seems to runs okay cold (about an hour), and the surging doesn't happen until it is very hot, I have a leak or crack on the carb or intake that opens up enough to cause the problem only when hot. With the hood and both sides off it seems like it takes even longer for the heat to affect it.

Any help would be appreciated.

 

[rayikeo2]

It may not be heat related at all and sounds just like how my mower ran when the vent in the gas tank cap got plugged. If the vent gets plugged the mower will run until the fuel punp sucks out enouth gas to create a vacuum in the tank. Usually around 30 minutes depending on how much gas is in the tank. At this point the fuel pump can not overcome the vacuum to feed the carburator. By chokeing the mower you are forcing the pump to work harder because of the fuel/air mixture. If this is the problem the stress will rupture the fuel pump diaphram. Fuel pumps are expensive, $130 or more. Try a new cap or Test when it happens the next time, immediately try to unscrew the gas cap, if it is very hard to remove listen for an air release when the seal opens. Gas caps are less than $10, fuel pumps are over 10 times as much. It doesn hurt to have an extra cap anyway. If the cap is original you need to change it anyway.

 

[RandyT]

smoothing out when you spray cleaner around the outside of the carb and intake means you have an air leak in that area.

 

[jinman]

You also could have varnish gummed up jets in your carb and restricted fuel flow causing a lean mixture. You have to increase intake vacuum by adjusting the choke butterfly to get enough fuel to the engine. The choke enriches the fuel mixture just enough to make the engine run properly. When you spray cleaner into the area, it goes into the engine and burns in addition to the fuel, so that is doing the same thing as choking the carb does.