1970 Johnson 25hp.. A coworker who had let it sit for a few years. When I start it it runs rough and backfires/sputters and then dies. I took apart motor and re-sealed the crankcase halves, new points and condensers, carb re-built, and new hoses for both fuel and crankcase breather hoses. I pulled off flywheel, one of the ground wires for the coil had rubbed against the flywheel. I moved it and coated wire with liquid electrical tape to cover wire. Could I have a bad coil? Could the wrong plugs (J8C) cause this?
(A FYI Note)
Whenever re-sealing the crankcase halves, keep in mind that the block is cast in one piece, then split. This means that the two halves are a perfect match of which neither will fit to another engine's half. As such, DO NOT ever (hard force) sandpaper either of the two halves but rather carefully clean them with lacquer thinner or acetone, and a sharp bladed putty knife... a very light sanding with #320 or #400 sandpaper just to clean away residue is okay.
Normally, when sealing a metal to metal surface, the sealant to use would be "Gel-Seal" which seals in the absence of air. As one tightens the bolts, the sealant hardens ans seals thoroughly (used on the later model engines thoroughly). This would be sealant.... "Gel Seal" or "Loctite #518". (End of FYI Note)
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The J8C plug runs so hot that it will eventually cause a aluminum piston to start melting. That engine requires either Champion J4C or J6C plugs gaped at .030. I'd recommend the J6C plugs due to the engine age.
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(Point Setting Of Magneto Models)
(J. Reeves)
Note that there are other ways to set points such as using a ohm meter or timing light... however, the following will result in a setting so close to being exact that one could not tell the difference.
Make sure that your feeler gauge is absolutely clean so as NOT to transfer oil/grease to the points.
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 embossed 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.
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Clean and rebuild the carburetor if you haven't done so already.
Be sure to carefully clean the brass high speed jet, located horizontally in the bottom center of the float chamber, way in back of the drain screw. Solvent and air pressure just doesn't do that job properly... and fuel must flow through that jet before it can flow to anywhere else.
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(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.
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