520-HC - Sputtering at higher RPM

[TC10284 113]

Condenser swap didn't help a thing, as I suspected. It still wants to get poppy/backfire some at half throttle or above. 

 

Might try a coil next... 

[Handy Don 13,581]

The cause of backfires is unburned fuel getting into the hot exhaust pipe or muffler. Two sources, right?

 

Too much fuel in going in so it cannot fully burn. This is usually has other symptoms like rough running, stumbles on acceleration, weak power at high speed, and black smoke.

 

Or, there is no spark at all for one or more cycles leaving a full charge of unburned fuel. These backfires are usually loud and can damage the muffler. These are, in my experience, usually down to a faulty connection/short or failing points. I'd guess an ignition trigger module having intermittent fails could do it also.

[TC10284 113]

46 minutes ago, Handy Don said:

Too much fuel in going in so it cannot fully burn. This is usually has other symptoms like rough running, stumbles on acceleration, weak power at high speed, and black smoke.

 

Or, there is no spark at all for one or more cycles leaving a full charge of unburned fuel. These backfires are usually loud and can damage the muffler. These are, in my experience, usually down to a faulty connection/short or failing points. I'd guess an ignition trigger module having intermittent fails could do it also.

 

Exactly. It runs fairly decent up until about half throttle. Then that's where the trouble starts. 

 

Are you referring to the ICM or the rotor? 

[Handy Don 13,581]

Either. Both. They have to work together and I have no experience diagnosing this system. 

My sense is that since it doesn't seem heat related, something is loose somewhere.

It could even be a carb issue--bad valve seats? incorrect or damaged needle valve? loose throttle plate? missing Welch Plug? clogged internal passage somewhere? (But I recall you swapped carbs and the problem didn't move, right?)

 

Edited June 27, 2022 by Handy Don

[TC10284 113]

2 minutes ago, Handy Don said:

(But I recall you swapped carbs and the problem didn't move, right?)

 

That is correct sir. Same behavior. 

[TC10284 113]

Been a while, but just an update:

I drained all the gas out of it because it looked pretty dark yellow and smelled pretty stale. Put fresh in.

 

I decided to let this tractor idle for a while and warm up. After it warmed up maybe 10-15min while I worked on something else, it would finally allow me to get to WOT, and I could engage the deck without it trying to backfire on me. 

I'm not blaming it on the bad gas, because it still had the same poppy behavior before it "warmed up". But at least I can use it now. I think what I'm going to try next is perhaps bending the carb float down some to see if that helps. Still acts like it's getting too much fuel. As mentioned, before it warms up, if I throttle up - not even quickly - it acts like the engine takes a split second to catch up to the extra fuel and always has black exhaust when it does this. 

 

Oh and the engine does shake unusually (comparing to my other 520's) around maybe 2500-3000 RPM. Have already checked compression on both cylinders - don't have a broken rod. 

 

Other than that, the only other thing I can think of is taking off the flywheel chaff screen and just checking the timing. 

Otherwise, I got nothing else. 

Edited July 20, 2022 by TC10284

[TC10284 113]

I checked the timing according to the P220g manual. Chaff screw lines up just after the first screw hole where it should and flips from 1v, back to battery voltage at the same time. 
Guess I'll try bending the float a little this afternoon. 

Edited July 20, 2022 by TC10284

[ineedanother 1,635]

I would also consider the simplest culprits as well, plugs and wires. New plugs if they have some age on them and wires if they look suspect. I've found bad spark plug wires in the dark (not unlike a blind squirrel) by running it in the dark and looking for spark jumping. An un-fired cycle here or there can be the source of fuel buildup and subsequent popping.

[TC10284 113]

16 minutes ago, ineedanother said:

I would also consider the simplest culprits as well, plugs and wires. New plugs if they have some age on them and wires if they look suspect. I've found bad spark plug wires in the dark (not unlike a blind squirrel) by running it in the dark and looking for spark jumping. An un-fired cycle here or there can be the source of fuel buildup and subsequent popping.

 

Plugs I've tried. I can't recall on the wires. I think so, but not 100% positive. I will check that again.