18Hp Briggs & Stratton issues

Well I have a 1993 Craftsman Lawn tractor, with straight pipes coming out of the motor and exiting out the back. I put a new battery in the tractor, complete new wiring , and got the starter rebuilt, now it turns over great. but when i went to start it up i heard a loud Pop noise coming from the exhaust. So i checked if the rods were thrown, not i was getting spark and then while cranking the key i checked the exhaust, it was shooting smoke out of each pipe. And when i turn it over using the key, it sounds like it trys to start, but nothing, i thought it could be somthing with the vavles, considering i have already rebuilt the rest of the motor. So it any body knows whats the problemm please let me know thanks.

Only way to tell if its the valves is pull the heads - could be a bad magneto - also might want to check the flywheel key and make sure it hasnt spun the flywheel off timing.

well right now im not able to work on it (vaction) But what if i checked the valve covers to see if a spring might be bad, low back pressure, striaght pipes

If you were getting spark but it's backfiring that's a good indication of a bad flywheel key, as dangeroustoys said. Pulling the breather covers will allow you to check tappet clearances, but that's about it. I'd do a compression & leak-down test to identify any possible compression loss. Are you sure there's no issue with carb or fuel pump?

Well i rebuilt the fuel pump about 3 months ago, what exactly is the fly wheel key, like the key way where the crankshaft matchs with the fly wheel?

its your valves..... either you broke a valve off or keeper came out... mine was making a loud pop/backfire sound and blowing puffs of smoke occasionaly but never starting.

Flywheel key is a small piece of metal that keeps the flywheel and crank timed together - if the crank end ( underneath encounters something- the flywheel key is designed to shear to prevent damage to the motor.

I decided to start by checking the fly wheel key first, for less issues hopefully. got the 1 1/4" nut of the crankshaft, and relized the key way was sheared, i was like thank God! Because i replaced the head gaskets this past summer. so i didnt what to do that. But i went to napa, got a new key way, put it in, and put the nut on, hand tighted. and it did the same thing, sheared it. And then i realized what had happened, the key just keeps it on the right settings, and the nut aucaly hold it in there, so i put the impact on it and tighted it up, and it works. Thanks for every ones input.

yep, gotta get the nut tight. the key is used for indexing the flywheel for timing, but is also used for shear pin. if the motor stops quick (like a push mower hitting a water meter) the key will break instead of twisting the crank.
Dont over torque the nut though. I have seen stripped threads on the crank from using a big impact. the motor is pretty much junk after that, unless you happen to have a spare crank laying around

Yeah, the flywheel is a 'press fit' onto the crank - why the crank and flywheel have a matching taper.

Ive gotten motors with snapped flywheel keys , one i bot at a tractor junkyard, a OHV 14.5 briggs - they 'supposedly' opened it to check it out , well it didnt run at all- kept snapping flywheel keys - they said it was because of a hydro lock ( oil in cylender) but that was BS, knew it had something to do with the valves - i had it sitting upright the whole time- they ended up fixing it for free.