Sounds like a sticking valve to me. Marvel mystery oil in the gas will help if the problem isn't too bad. I have pulled the valves and used a wire wheel to clean the stems and reinstalled and had no more trouble like you describe. Old fuel turns to lacquer and doesn't burn worth a flip. That was my malady. Good luck

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??

Yes, this is the time of year when the leetle buggs are looking for a tiny hole, to lay their egg in! This to prolong their time on earth so their offspring can have the problems you are having! So, they see the tiny breather hole in yer gas cap, and thats where they lay their egg! Then they cover the egg and hole with a bit of poo from their anus, and then they go away, and in the alotted time, the hatchling bores its way out into the sunlight, and flies away. And, the hole in yer gas cap is the perfect place for an egg to be secreted away! And, the preceding is just what gives the problem you are enjoying! I have seen old engines laying in a junk yard, and every hole has mud daubbed in and around it! Hence the name:"Mud Daubers!"

Sax , long time no hear Bro , Good to see you and Rday are still active participants ! I have been away from the forum for a little while (formerly aka Ewalk). Photo , great forums for debates and brainstorming with very capable people. Just a note of info since the op has not given any further info , I don't see indicated in his original post of Ethanol based Fuel . I have no doubt that this may be suspected just not really expressed . I have further briefed myself within this issue , apparently in the US E10 is apparently pretty well status quo with E15/20 & E80 added recently. Unfortunately most current Small Engine Manufacturers only approve E10 usage , along with proper fuel management and treatment . As previously stated I am fortunate here in the Great White North , Ethanol is still a lamb duck fuel , slowly gaining usage . Non Ethanol is still the Norm in smaller populated areas. Having worked for years on small engine units and various recreational equipment (snowmobiles & motorcycles)as a Hobby , I have a reasonable working knowledge. I welcome your and Sax,s perspectives , I originally sided with Rday and more recently with Sax , since I have ran numerous chainsaws with 70:1 even old Circa 60,s Pioneers also a 4HP 1989 Yamaha outboard rated for 100:1 with Amsoil Sabre Syn Oil . I have numerous American Friends who routinely drop of their E-fueled units for seasonal servicing with only carb issues from the dreaded E. To this date have not witnessed any E-fuel related engine failures . However I agree with you that Toro , Briggs and even Mercury Outboard today preach that E-10 use only with proper preventative measures (fuel treatment) further they will not warranty any E15 + fueled units which fail. Only Briggs approves Mtbe Usage also . Anyways , great talking with you Gents , perhaps the Op and Rdaystrom will drop back in and give us further perspective with this E-Fuel usage issue within Small Engines !