What type of oil does it use? John Deere RX75

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John Deere D110 Lawn Mower Tractor Workshop Service Repair Manual

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Spark plug is NGK BMR7A gap is .025 Fuel is any gasoline mixed 40 to 1 with 2 cycle oil.

The wrong oil won't stop an engine from starting but the right oil will help it to last longer. Buy a respectable brand and there will be an application catalogue making recommendations for which of that brand's products are best in which engine.

It is little use changing parts and hoping. You need to be checking for a good quality spark and for the presence of fuel getting into the cylinder.
An engine needs compression and the right amount of fuel delivered into the cylinder and ignited at the right time by a spark of sufficient quality. An engine that doesn't start has at least one of those requirements missing. The challenge is to discover which...

need more information to assist you--year make model engine of vehicle

check fuel filter , it may be clogged with debris

some models have a fuel pump some are grivitey

could be bad gas, but it sounds like a choke problem. did it sit over the winter with less than a half tank of gas in it. try putting some rubbing alcohol in the fuel or draining the tank and disconnecting the gas line so you can drain the gas out of the carb. also check the gap of the spark plug and make sure that is correct. when was the last time there was fresh oil put in. also check the air filter, it could be full of dirt.

I have a 115 john deere tractor that runs but the engine will not run smooth I've put a new fuel cap on it a new pump and clean out the carb.

Most common is a dirty air filter, followed by a plugged vent hole in the filler cap on the gas tank. Next would be fuel issues either in the supply line or in the carburetor.
Hope this helps, --W/D--