Is your glow plug OK. The magnum 52 runs best on an O.S. type F plug. It should fire with any of them though as long as they light up OK.

Inital needle valve settings are 2 1/2 turn out after closing the high speed, Same for the idle. These will get the engine running so you can tune it.

Take both valves out. and then remove the line from the muffler and blow into it with fuel in the tank. You should get a good flow of fuel out the ends of the spray bar with not to much lung power. Also a quick check to make sure the lines are not reversed.

The magnum 52 four stroke doesn't have much compression. It is quite different than the O.S 52 fours stroke. You can feel the engine come up on compression, but it is no where as much as you feel with the O.S.

Has the engine been taken down?? could the valve timing be off? Pull the glow plug and using a small wood dowel find top dead center. Mark the front bearing housing and the prop thrust with a single line. Use a sharpie fine point and it will clean right off after. Remove the cover on the cam. There is a dot on the cam face and it should be facing straight down or straight up, depending if were at the top of the compression stroke or top of the exhaust stroke. Don't crank the engine with the cam cover off as you may skip a tooth without the end bearing support.

Fuel, the Magnum four strokes need 15% nitro. They just don't run right with less.

Once you get the engine running, and warmed up, adjust the needle valve for maximum RPM, you should get around 10,000. The ramp on the highspeed valve is stubby and one click makes a big difference. After each click wait 15 seconds or so befor changing again. Once you have the engine maxed out, then go rich to drop the high end by 500 rpm or so. Better more drop than less. Now idle down as slow as you can and keep the engine running. Screw in the idle for maximum speed at this throttle setting. A long thin screwdriver, 8" blade is best, for this. Now go back and readjust the top end again. back to the slowes idle and this time when you have it running as slow as you can without stalling, punch the throttle. If it stutters and fumbles, you are running to lean on the idle, back it out 1/8 Turn. If it slowly comes up to speed, you are running to rich and in this case 1/16 turn closed on the idle. This takes some hunting to get to the sweet point. After every couple adjustments on the low end, re-do the top end. What you should end up with is an idle around 2000, and top end at 9500, and when you punch it from idle, it barks to life and doesn't stall. If the engine is older, I would suggest gettting a replacement set of Orings for both the needle valves and the carb to manifold. The fit of these rings is very important to having an engine that runs well. I have two magnums now, the 52 and a 70. The 52 has been troublesom in the idle, even after 1 gal or so of fuel through it. The 70 is a sweetheart, right out of the box. I've two O.S. 52s, actually one as I lost my plane with one of them, and they are rock solid and quite eaiser to adjust.