I have a Craftsman LT100 with a briggs and stratton 16.5 engine the engine runs but it is losing power and back fireing through the carby I have just had the head reconditioned with new valves new head gasket but it is still doing it I have set the valves at the correct settings was told that the cam lobes are OK the coil is OK also compression on the piston is just below 90 could that be the problem

Sounds like the timing could be off here or blow by through head gasket/ valves

Did you hold the compression for a while to make sure a head gasket blow by wasn't happening. Also if compression held.

Check the key on the flywheel key to see if its broken.

Below is for a 16.5 HP B&S engine Model #31L777 Type-0126E1 Code-020214ZA. Not sue exactly what yours is there.

Do you hear any clicking noise in the OHV cover area. If so check push rods.

Who ever put it together this may have happened. The push rods are the same length, but have different style ends. The ends make the difference in length. Because of that the Intake valve would be slightly held open and the exhaust valve never fully opened. This could be a reason why it backfires through the carb. Check the push rods since it was taken apart and new valves installed. If that was done incorrectly then switch push rods.

A lot could be going here get back after

In short if it was recently done take it back and have them look at it since they did they work

Yes it did hold compression will have to check the flywheel,also will check the push rods I put the head back on myself with 20lbs torque,if the push rods are different which one goes where,there are no noises coming out from the OHV , when it is running it seems like it misses then backfires and when I engage the blades it makes it worse.
Model-313707 , Code-000606ZD , Type-0133-EI but the last couple of numbers are missing from that