420 spark plug gap
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Onan engines are notorious for surging, and there are several things that can cause it.
Make sure that the throttle linkage is free and unobstructed. Also that it is installed correctly.
Dirt in the carburetor air bleeds can give this problem. Also a missing welch plug at the front of the carburetor. Check that you can adjust the idle mixture. Should come in at about 1-3/4 turns open.
Air leaks under the carburetor are a common problem. The intake manifold is made in two pieces sealed by RTV. On an old engine, you'll need to reseal the manifold halves. Surging from air leaks is generally more pronounced under light load and reduced when you load the engine (mow grass).
Generally, with a "new to me" 318 or 420, it's wise to R&R the engine, remove all the shroud sheet metal and clean out accumulated dirt. Also, remove the flywheel and clean off the alternator coils and (on P-series) clean and inspect the ignition components. While you are at it, you can seal any oil leaks (crankshaft seals, oil filter mount) and make sure the crankcase-fuel pump hose is in good condition.
Hank
Make sure that the throttle linkage is free and unobstructed. Also that it is installed correctly.
Dirt in the carburetor air bleeds can give this problem. Also a missing welch plug at the front of the carburetor. Check that you can adjust the idle mixture. Should come in at about 1-3/4 turns open.
Air leaks under the carburetor are a common problem. The intake manifold is made in two pieces sealed by RTV. On an old engine, you'll need to reseal the manifold halves. Surging from air leaks is generally more pronounced under light load and reduced when you load the engine (mow grass).
Generally, with a "new to me" 318 or 420, it's wise to R&R the engine, remove all the shroud sheet metal and clean out accumulated dirt. Also, remove the flywheel and clean off the alternator coils and (on P-series) clean and inspect the ignition components. While you are at it, you can seal any oil leaks (crankshaft seals, oil filter mount) and make sure the crankcase-fuel pump hose is in good condition.
Hank
Hank,
This is where we got today. We got it running but it surges pretty bad at 1/2 to 3/4 throttle. So we pressure washed all the tractor except the motor because it was incredibly dirty. We were using a small hanging bottle as the gas tank to run the tractor, while letting it dry off it filled the carb and cylinders with gas. I had the float and needle out last night and it seemed to be working correctly. So needless to say I bit the bullet and pulled the carb and intake manifold off, and ordered a rebuild kit for it. What stumps me is where did the fuel leak through to fill the cylinders? I also noticed some fuel leaking out of the throttle shaft. And lastly I noticed that the intake manifold is riveted together, I just redid my 318 manifold it is held together with little machine screws. How should I go about resealing the manifold? Drill out the rivets and replace with small bolts?
This is where we got today. We got it running but it surges pretty bad at 1/2 to 3/4 throttle. So we pressure washed all the tractor except the motor because it was incredibly dirty. We were using a small hanging bottle as the gas tank to run the tractor, while letting it dry off it filled the carb and cylinders with gas. I had the float and needle out last night and it seemed to be working correctly. So needless to say I bit the bullet and pulled the carb and intake manifold off, and ordered a rebuild kit for it. What stumps me is where did the fuel leak through to fill the cylinders? I also noticed some fuel leaking out of the throttle shaft. And lastly I noticed that the intake manifold is riveted together, I just redid my 318 manifold it is held together with little machine screws. How should I go about resealing the manifold? Drill out the rivets and replace with small bolts?
Brent, yes there is a thread on how to reseal a onan manifold with screws and fuel resistant sealer. You are correct! The only way fuel can flood the carb and crankcase is a float needle not seating due to dirt or bad needle or seat. Could also have a bad float creating the problem.look, to be sure someone did not install a electric fuel pump with to much pressure as it will cause the float needle to allow fuel to pass by.You should have a pulse fuel pump for the factory spec. Do a good cleaning of the carb. and soak it in carb. cleaner overnite. Blow out all of the air and fuel passages with a compressor.Also make sure float height is set to proper specs. and make sure welch plug is in place and sealed properly.
Joe
Joe
Joined
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5,084 Posts
the idle jet clogged is a common issue on mine. I take off the carb top and unscrew the idle jet, then take a fine wire to poke out the crud.
I found that the new fuel keeps moisture in it and the bowl corrodes after time. Since then I add some 2 storke oil to the gas. 2 oz per tank and the issues has not returned in the past 2 years. I only use it for snow removal and it sits for a long time.
Yesterday after priming the fuel pump and polish the points it started right up. This is my usual fall ritual. Dont know why i have to polish the points every year, but it runs all winter after.
wonder is a boat primer bulb thing would be better, but its only once a year I have to prime it.
I found that the new fuel keeps moisture in it and the bowl corrodes after time. Since then I add some 2 storke oil to the gas. 2 oz per tank and the issues has not returned in the past 2 years. I only use it for snow removal and it sits for a long time.
Yesterday after priming the fuel pump and polish the points it started right up. This is my usual fall ritual. Dont know why i have to polish the points every year, but it runs all winter after.
wonder is a boat primer bulb thing would be better, but its only once a year I have to prime it.
Joined
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5,084 Posts
tell my wife to do that
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