Spark plug keeps popping out

Possibly restricted exhaust or valve worn or piston rings worn take out spark plug hold gently but firmly against the plug hole turn over engine Holding the plug obviously not with spark plug cover on !! It should at least push the spark plug away from the hole with a pop pop pop sound if so hopefully that rules out valves if it's not burning oil then it probably not rings could be timing off or valve seals most engine with leak will run full revs under no load ps also check plug gap :-)

Make sure battery has a full charge and battery connections all good. How long since a tune-up?
Have a helper crank it while you check for spark at the spark plugs. If spark everywhere, use a gage and check proper fuel pressure. Also check for fuel injector pulse.

You may get some popping with a leaking exhaust but not common. Sounds like you're in need of a tune up. Spark plugs and new wires to start. The flames are due to unburnt fuel from mostly a miss fire. Try a simple tune up first, also make sure you have a new air filter to help the motor breathe easier and save money at the pumps. Be sure to check and replace any old/cracked vacuum lines as this will cause poor performance too. Hope this helps start you in the right direction.

POPING IS A IGNITION PROBLEM (MAY BE VALVE)? BUT IT WAS RUNNING. CORRECT..NOW FIX THE FUEL PROBLEM AND DO A MAJOR TUNE UP.. CAP/ROTOR/WIRES/AND PLUGS. GET IT RUNNIG FIRST DO NOT COMPLICATE IT

make sure the gap in the spark plug is correct and clean.make sure carb is clean and has proper suction aka loose carb hoses bolts exc.ohhh and if you take the carb off definitely check the reeds while your at it

The FC3S is notorious for this and fueling issues. Check all ground points, battery cables and connections. Check the draw from the starter. If the voltage drops below 10V there is not enough power for the ignition, causing weak spark. It really needs to spin over 200rpm cranking. Pop starting does not necessarily mean the compression is good. You need to make sure it cranks fast enough to get a proper compression test with rotary tester, New NGK plugs, new NGK plug wires.

I don't know the plug number. Off the shelf chain saw plug from small engine repair shop will replace existing plug.

First of all, are you getting spark?
Take out plug. Put spark wire on plug. Hole plug by spark wire. Touch metal threads of spark plug to metal part of engine block. Metal to metal. Pull cord. Look for spark between metals. If there is no spark, then spark coil is bad.

Look at exhaust port. Is there anything that would stop the exhaust from escaping?

Can you get to fuel line running into carburetor? See if gas makes it all the way to the carburetor.

Can you get to the air filter? Is it dirty? Can you spray WD-40 directly into carburetor. A saw that has good exhaust, spark and compression will fire on WD-40.

If your engine can fire on WD-40, but will not stay running, then carburetor is suspect. Maybe gum out will work. Try to keep the engine going on WD-40 and see if that opens the carburetor.

If you can keep engine going on WD-40, that takes both hands ... use your third hand to move choke and throttle levers and see if the engine can start on its own.

Upvote if this information helps. Thanks for the feedback.

First, make sure the spark plug is in tight and not cross threaded (take it and put back in). If the plug is loose, pressure comes out of the cylinder and pushed the wire off.

Second, push the spark plug boot on and massage the air out of the boot.

Hope this helps.

the plug insulators must be leaking compression from the cylinders, have seen this and that is what i have found.

It cranks really good sputters a little and makes some loud pops but it will not turn over? FYI:Cranking and turning over are the same thing. When was the last time it ran. The gas could be bad. Did you get new plug wires as well. Try testing the distributor, remove a spark plug, plug it back into the wire, and touch the threads to ground and crank the engine. It should spark. Keep your fingers away, that's high voltage. Remove all spark plugs, spray some starting fluid in each cylinder, then crank the engine with the plugs out. This will clean any debris from the cylinders. Engines need backpressure to run properly, a major exhaust leak hence the popping, could cause it to not run. If you can get it started try to keep it going, full throttle, keep in the starting fluid, it could just need to get warm. I used to have one of them and they don't like it when they're cold.