Trouble With M

I have an M that has been restored (by my grandfather). It has always run well, but recently, I've been having trouble with it. It had been running rough and then was starting hard, so I replaced the spark plugs, which really seemed to help a lot! It was running great, but now, suddenly it seems that whenever it is "under load" it starts to spit and sputter and loose power and starts to die unless I press the clutch and then usually it will return to running. There are times when it will even backfire with a "gurgling" sound, especially at full, or nearly full throttle. It's so bad that it can't even seem to pull itself up a small hill or push snow. I can feel the engine start to shutter when I am using the hydraulics.

replace points and condenser first and possibly the coil,but the carb can do the same thing.I would do the tune up first

Their is a screen in the fuel bowl above the in let that could be dirty.