1971 IH cub cadet 109 Hydrostic garden tractor back firing

My 1971 Cub 109, 10hp Kohler keeps back firing out the exhaust. Sometimes when it hasn't been messed with for a couple days, it will start up and run smooth for a while as long as you don't try to throttle all the way up or engage the blades...at that point it misses a little and starts back firing out the exhaust. I have emptied the gasoline and added new with sea foam, changed fuel filter, replaced fuel line, checked the points (amateur)-sanded them a little and watched them spark, new spark plug, have not checked the spark at the plug. I have heard different guesses... Does any of this make sense to anyone?... What am I missing or haven't gotten to yet? I don't want to keep replacing things such as coil, condensor, etc unless I need to... Anyone have this problem and possible solution? I'm still not sure if it is fuel related, fire related or maybe an unseated valve... the title should say Hydrostatic, btw

When did the issue start, was it out of the blue or did this start happening after you changed/fixed something else that was wrong? It seems as if your issues happening more commonly when you put a load on the motor, is it every time you turn the blades on or try and throttle up? The next time you run it engage the blades and when it starts to back fire shut it back down and pull the plug and see what that looks like. What the plug looks like should be able to tell you weather its a fuel issue or not and should help to narrow down your trouble shooting. As far as it being a valve that's unseated I have never had that issue first hand so cant say what that issue is like but I do know what an engine runs like with a bent valve and what your explaining doesn't seem like that's the issue.

Possibly the point gap is incorrect which is effecting the timing.

If your points are sparking enough to be seen in day light then the capacitor ( condensor to some ) is duff and a bad can will cause these symptomes. replace it.
Cans go off over time regardless of weather it has been uses or not so consider fitting a points eliminator ( hall effect trigger ).

Thanks for the advice... I mowed for about 20 minutes some time early last week... Going along fine and all the sudden it felt rough and choked out a backfire and then within the next five minutes it was backfiring terribly... first thing I did was put a new spark plug in it and that didn't do anything... next I changed the fuel filter and it was running like a top again but only idling at about have throttle... then when I went back to where I left off mowing, started throttling more and tried to engage the blades, it was backfiring again. I ordered a new condensor yesterday and it will be in today... I will try to put that on after work and see what happens...

Thank you all again... I will put the new condensor/capacitor on tonight and be back on here tomorrow to let you know how that went...

UPDATE: I put on the condensor/capacitor last night and so far so good. I could not give it a true test as far as I am concerned due to a wheel being off and at the shop. However, I did start it...throttled it up full...engaged the blades with no hesitation or backfiring... I did this two different times last night and then once again this am. I will get the wheel back this evening and take it out to mow and then will know for sure... Thanks again...

*****H*E*L*P***** Ok, so I put it all back together with new condensor. No more back firing... mowed for about 15 minutes or so and it just shut down on me. NO BACKFIRING THOUGH... Then I started it back up but it wasn't easy. I took off again but within a few minutes it just powered down like i dropped the throttle and shut off again. Still NO BACKFIRING (so I believe the condensor fixed that problem). This time I couldn't get it to start again but it tried to a few times and after that it just turns over like it has no fire... I laid the spark plug and wire on the motor cover and it had good spark... And again, now it just turns over and doesn't make any sounds of trying to start up... I have gas running out of the tank hose and through the filter... all new hose to the carburetor...