Husqvarna with KT725 suddenly started surging/pulsing/cycling

Hi,

I'd appreciate any suggestions you might have for the following problem.

About 15 minutes into the first cut of this spring, my mom's Husqvarna lawn tractor with KT725 engine suddenly started "cycling" like somebody was pushing the throttle back and forth about once per second (see video below).

What I've done so far: drained gas, run dry, filled with fresh gas, changed fuel filter, and verified that fuel line from tank to fuel filter is clean.

Some clues:
1) The mower had been operating fine for about 15 minutes before this behaviour started very abruptly.
2) This was the first use of the spring. It had been stored for about 4 months in an unheated building with a small amount of unstabilized gas in it. The gas probably was ethanol-free, but I'm not 100% certain of that. (Yes, I know, bad to leave gas in, normally this does not happen.) I added a few litres of fresh gas. It started almost immediately and ran fine for about 15 minutes before the problem started.
3) The problem is most noticeable at idle or low power. At high power it is not noticeable. At idle it can even cause the engine to die.
4) I thought I saw some puffs of black smoke but generally the exhaust is pretty clear.
4) I checked the air filter. I'll replace it, but it isn't too bad. I briefly ran the engine with the air filter off and this did not solve the problem. I did notice some water droplets in the air filter (see photo below). I don't know if that is normal.
5) Spark plugs: The driver's right side had minor carbon deposits, but the driver's left had serious deposits (see photo below). I initially thought that they were oily, but after a few minutes it dried out and just looked dirty.
6) The fuel filter had a little gunk in the folds of the filter paper. It has been replaced. Fuel flows freely into the filter.
7) The mower has around 250 hours on it. The oil and filter are replaced every 50 to 60 hours.

What do you think I should try next? Should I put carb cleaner in through the air filter? Remove and clean the carb? I've never done either of those before.

Thanks for any help you might offer.


Air filter with water droplets:

Spark plugs:

Classic sound of running out of gas. I'll guess fuel pump. If a vacuum pump, replace its lines as well.

I’d be looking at a head gasket problem or the float needle not seating properly. Reason I say this is that as the engine runs at high speed you state that the engine runs good at high speed, but not idle. Plus an occasional puff of black smooth indicates a rich mixture, adding carbon to the plug on that cylinder. Head may be expanding when heating up on the cylinder with the carbon plug, allowing a small amount of oil being sucked in. Finally, it may be a combination of these to problems.

Yardman999 said:

Classic sound of running out of gas. I'll guess fuel pump. If a vacuum pump, replace its lines as well.

Click to expand...

Thanks for this suggestion. I'll investigate this.

I'm not knowledgeable about fuel pumps. Would it make sense that the fuel pump would work at high throttle (when the problem seems to be absent) but not at low throttle/idle?

Also, would it then just be coincidence that the one of the spark plugs is badly fouled but the other is reasonably clean?

Rivets said:

I’d be looking at a head gasket problem or the float needle not seating properly. Reason I say this is that as the engine runs at high speed you state that the engine runs good at high speed, but not idle. Plus an occasional puff of black smooth indicates a rich mixture, adding carbon to the plug on that cylinder. Head may be expanding when heating up on the cylinder with the carbon plug, allowing a small amount of oil being sucked in. Finally, it may be a combination of these to problems.

Click to expand...

Thanks very much for these suggestions.

Would the head gasket problem appear suddenly, like this problem did? One second there are no issues, the next it is doing this cycling/surging? Now the problem is there even when the engine is cold.

Yes a head gasket can blow at any time.