I have a ih 756 gas tractor--replaced spark plugs and spark plug wires. It started but ran very rough. What could be the problem???

I t is behind teardrop shaped cover below r ight hand brake drum.

remove the kill wire to coil and try

pull the valve body and replace o rings

drain the water out of the carby bowl
clean out the main jet

to be honest the enigne ahas a control that the left side must fire and run before the right starts to fire it sounds to me like you have a fuel issue possibly the carb. is gummed up and dirty.

white smoke is more than likely a water issue. Oil usually has a bluish tinge to it but is some cases, it could be white. Drain the gas a fill with fresh to try and eliminate the possibility of water in the gas. Watch the oil level to see if it changes after using the mower. it may take a few times but if it is burning oil, the spark plug will look "wet" and oil soaked, with possible black soot type build up.

I would start by switching plugs. Then does the wet warm one get hot and does the hot one now stay only warm. If the same plug stays wet, replace it.
If the same cylinder is not firing, check the wiring to the plug. Check the coil wiring and coil for spark to that cylinder.

You should be able to pull the whole distributor out by taking one bolt out that holds a Y bracket on that holds the distributor down. Make sure you mark the distributor and the engine before you pull it out. Also mark the rotor. Once you get it out, you will be able to determine if the shaft is broken. Hold the gear and try to spin the rotor. If the shaft is broken, you will probably need a new distributor. Seems strange that the shaft would bust, but I guess anything is possible.

carb is probably gummed up from the old gasoline. try squirting some gas into the carb where the air goes in to start it. probably won't keep running but sometimes it can shake something loose like a stuck FlOAT VALVE.

It is time to rebuild your carburetor paying special attention to the float and seat.
Gasket set is about $30 delivered and gallon can of gunk carb cleaner with a parts basket is $21 at kragen auto parts. takes about two to three hours. Or you can but a new carb for about $150.