I don't know if the core of what I have been trying to say is clear, or not. But Bill did hit on a different way of saying it. Once a long time ago, there was no such thing as an anti-afterfire fuel solenoid on any brand of outdoor power. Back in those days, everybody's outdoor power engines might pop if you shut them down without a short cool down idle period. Some engines and applications did not pop as much as others, due to different configurations of the exhaust muffler or length of exhaust pipe. Then suddenly, back in the late 1980s and early 1990s, the anti-afterfire solenoids began to appear. So, the only way to solve your problem is to have a true professional, well experienced, small engine technician/mechanic evaluate your old tractor and tell you whether or not the tractor ever has an anti-afterfire solenoid on it. If you can remember a time when it did not pop when you shut it down, there may be hope that there is something faulty with an existing solenoid. But if the tractor never was equipped with the anti-afterfire solenoid valve system in the first place, then you are going to have to spend some money to have it retrofitted to behave like a modern tractor. It's kind of like if you want a 1935 Chevrolet with air conditioning on it, you are going to have to pay someone to put it on there because they did not have air conditioned cars back then.

If it was me I would do complete fuel system clean up. Said this before with that many hours and years on it your fuel system is dirty and about to fail. I would Start at the tank. remove it, clean it out either by swabbing or pressure washing. rinse with gas or alky. Next I would replace all the fuel hoses, fuel filter, install shut off valve, replace the hoses after the fuel pump and the fuel pump vaccum hose. If you don't want to clean the carb. bowl and spray the main jet with carb. cleaner add some fuel system cleaner when you get it filled back up with fresh gas and clean system up to the carb. Also while I was in there I would also inspect the elect harness, switches, looking for loose hanging wires, chafed wires, rusty connections. plastic tie wrap any loose wiring that could get chafed or rubbed against moving parts. If you want it to last you need to do this very few years 6 is pressing it, but not bad depending on conditions you work your equipment in. I would also make sure I check the air filter often older vanguard are fussy with the small air filter between the cylinders. Now cylinders reminds me of cooling fins. have you checked them lately? Might want to revisit the owners manual on pre-ventive maintenance make sure you got all the bases covered. do that and that tractor may last life time???? another thing if you going by the 50 hour oil changes you better be mowing in dust free invoriment?? if not then 50 hours between oil changes is way too long hence extreme conditions in the owner maintenance schedule??

It could be as simple as a little water in the bottom of the fuel bowl on the carb. A bit of extra vibration sends it up where it affects the running. If you do find water/ debris- run the line complete and install a new filter. You also may want to confirm that something even more simple such as a fuel cap that isn't venting isn't starving your fuel supply. Try it with the cap loosened to test. A third possibility would be a safety switch/ connection that is intermittent.

you might try some SeaFoam in your fuel mixture for a tank or two if you don't want to tear the carb down. i don't want to touch the carb, i'll try the local auto parts store or mower dealerships for SEA FOAM. if that is not available is there another product or 2 you'd suggest? Did you replace the filter also? (with the correct color, Red) RED? no it was a generic gas filter that looked exactly the same as the one i took off. this was from a mower/tractor dealer in town. you mean i need to use a B&S filter? i have used generic filters for years without any trouble. tom