My Husqvarna runs poorly when blade is engaged after the engine runs for about 20 minutes. It appears that the electrical is sporadically dying as if the safety switch for the blade is killing the motor briefly then allowing a spark with a loud bang when the spark is restored. It runs fine without the blade engaged in all throttle positions. I do not think it is the seat switch but the reverse/blade safety switch. But I cannot find the @#$% thing. There also seems to be a vacuum throttle advance that kicks in when the blade is engaged but I do not think it is a fuel/throttle problem.

This killing the spark sporadically cannot be good for the engine.

That sounds thermal. Either the ignition module or the coil.

Hi,
I agree with the ignition module/coil "guess" based on information supplied. I can look up many brands and parts when provided numbers.
Peace,
Clarke

Why would the coil only cut out when the blade is engaged? I would think a bad coil would be bad all the time once it got hot. This mower runs fine hot without the blade engaged. Only misses when the blade is engaged. Only 26 hours on the mower.

I think it is posssible that you have a failing safety switch. It thinks you are engaging the blade without being in the seat. Some safety switches are 2 wire and others 3. For about $9 you can replace the switch but not knowing what tractor you have I can't suggest how to test your current switch.

Hi,
The engine works harder when the blades are engaged, so it makes more heat. A marginal fuel pump could work fine and fail under the same circumstances.
Peace,
Clarke