Sears riding mower with B&S 28n707 (15.5 HP OHV) engine won't start

Question

The engine was running fine and almost out of gas so I added some very old gas from my motor home to finish the job. After running several more minutes I shut down for the day. The next day it cranked up no problem and was running at about half throttle when I engaged the blade and it immediately died. Since then all it will do is backfire while cranking and never fires in the forward direction. While cranking it spins nicely but every few seconds it will backfire and stop rotation momentarily and then continue to crank. Is it possible for this engine to jump time and if so should I look at the flywheel or the valves? The valves appear to be opening and closing OK but I haven't tried to figure out their timing. Thanks for any available help.

Rod

Answer 1

The key words here are 'old gas'. At this stage do not jump into disassembling the engine without maybe dumping the old gas out. The carburetor should also be disassembled and blown out. This will clean out the sediment bowls and any water that is remaining. Be sure to have the gas tank drain completely also. What happened was there was a quantity of water in the gas that as long as it was agitated was dilute enough for the engine to still run the first day. Once the water settled and all the engine had to run on was water that was the end. Hope this helps.   P. S. do not try starting fluid without cleaning up the gas situation. Starting fluid is VERY hard on the engine. It is made primarily for starting diesel engines in a very controlled amount...

Answer 2

It is possible to shear the key between the flywheel and the crankshaft.  This will effect the ignition timing.  It doesn't usually happen unless the engine is stopped suddenly like the blades hitting something.  It is nearly impossible for the valve timing to change.  As suggested, clean up the fuel system first and check the flywheel last.

Answer 3

If it was running OK try cleaning this spark plug first. After this spray in some carb cleaner in the intake to see if will clean the carb. then try a small amount of starting fluid in the intake. If starts and runs OK on the starting fluid then it must be the gas. So try fresh gas before you tear the motor apart.