Compression gauges are dirt cheap at Wal*Mart, K-Mart, many stores... buy one. You need to make sure that the head gasket is okay and is not allowing water to enter the cylinders.
Backfires? Is the flywheel key sheared or missing? In either case, the engine would be out of time. Make sure the key is a tight fit in the crankshaft slot and the outer straight edge is vertical with the engine, NOT aligned with the crankshaft taper.
The aluminum seat that the coils sit upon has a slight taper at the edge. The metal vertical portion of the coil must align with the top inside edge of that taper to set the coil the proper distance frorm the flywheel magnets.
With the s/plugs removed, the spark must jump a 1/4" gap on that model with a strong blue lightning like flame... a real SNAP! Does it? Note that the gap is important... checking spark by using the spark plugs is a waste of time.
Spark plugs should be Champion J6C plugs gapped at .030.
Set the point gap as follows:
(Point Setting Of Magneto Models)
(J. Reeves)
Set the points as follows. Have the flywheel key aligned with the fiber rubbing portion of the ignition points. Adjust the gap so that a .020 gauge will pass thru but a .022 will not. Should there be any question of the points being dirty (touching the contact with your finger would cause them to be dirty), clean them with a small brush and acetone or lacquer thinner.
NOTE 1: Should the operating cam have a small portion on it with the word "SET" imprinted, align this portion with the fiber rubbing portion instead of the flywheel key.
NOTE 2: Should the cam have the word TOP imbossed on the top of it, that is a cam that could be installed upside down and this is simply telling you which side is up. It is not a position where one would set the points.
Now, if the engine is in time, has compression, spark, and the proper fuel mixture, but still will not start and run.... that leaves the carburetor. Clean and rebuld it with a complete carburetor kit. Make sure to manually clean the brass high speed jet that is located in the bottom center portion of the float chamber.
(Carburetor Float Setting)
(J. Reeves)
With the carburetor body held upside down, the float being viewed from the side, adjust the float so that the free end of the float (the end opposite the hinge pin) is ever so slightly higher (just ever so slightly off level) than the other end. And when viewed from the end, make sure it is not cocked.
(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.
Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
