Magneto....Weak Spark??

Sluggish/underpowered SC CASE still driving me crazy. 115-120 compression on all cylinders. I THINK timing and fuel are good, but still no power.

Can a magneto get weak, causing severe underpower??

Glenn F.

Coils can go bad causing a weak spark, pull a plug and watch when you crank it over. Is the spark bluewhite, or redish orange, if the latter it could be bad coil, wires, plugs or points out of adjustment.

You MUST NOT be using Carbon-Core Plugwires with a Magneto..
BUT, I have had excellent results with Autozone Low-Resistance "Spiral-Wound" Super Stock wires..
They deliver a very Hot Spark (if the mag is sending it).

Your compression is plenty good..

Ron.

"Low power", NO, without a misfire, but "misfire", yes.

Is it misfiring, or just not getting enough gas/air to run well?

I'm not that familiar with magnetos, but I would think they could get weak.

But, I would think it would react like any other ignition system that had weak spark, typically hard/no start, or misfiring under load. Try checking the spark at idle and at full throttle. It should have a strong enough spark to jump a 1/4 inch or more.

Do magnetos have a mechanical advance? If so, is it free and working? Have you tried advancing the timing to see if it made a difference?

Other things to check,(I may be repeating some things, sorry my memory isn't the best)

Is the governor reacting quickly and opening the throttle fully? You should be able to set the throttle to high speed. Be sure engine is up to rated RPM (2200-2400). Then manually push the governor arm to idle and feel the spring resistance increase as the RPM drops. Then release the arm, it should snap wide open, then return to the regulated RPM and level off. If the governor doesn't open the throttle fully, or is slow to open, something is binding or out of adjustment. If it hunts or surges, the fuel mixture is too lean.

Have you checked each cylinder to see if all are firing? Pull one plug wire at a time, both at idle and at full speed. See if each makes a somewhat equal difference in engine speed and sound.

Have you tried disconnecting the air cleaner, just for a test, to see if there is a restriction.

Is the choke fully open?

Have you adjusted the idle and main jet? With the engine up to temperature and at idle speed, lean the idle mix until speed drops, turn rich to good idle, then slightly richer. A good test is shut off the engine, wait 5 minutes or so, it should restart without choke or opening the throttle. Set the main with the engine at full speed. Turn the main jet adjustment in until engine begins to stumble, then back it out until it runs smooth. Then back it out another 1/4 turn. Push the governor to idle and release it. The engine should respond without hesitation. If it hesitates, back out on the adjustment another 1/4 turn. Quick throttle response and maybe a puff of black smoke is what you want. Warning signs would be if turning the adjustments make little or no difference.

Any possibility of exhaust restriction? Can you take the pipe loose from the manifold, just for a test?

This is a long shot, but if all other reasons are eliminated, check the valve timing. Remove the valve cover. Rotate the crankshaft (this will be easier with the plugs removed) in the direction it normally runs. Watch the #1 or #4 cylinder rocker arms. When the exhaust is just closing, the intake will begin to open. This is called the "overlap" position. Stop turning when each valve is equally just cracked open. You'll need to rock the crank back and forth several times to get it right, just remember to make your final turn in the direction the crank normally runs. Then look at the timing marks. The engine should be very close to top dead center.

All I can think of for now, let us know...

If it is not misfiring the mag isn't the problem,
Is the air cleaner free of obstructions? Is the timing right?

If that carburetor has a main jet adjusting screw try opening it. This is off a B&S engine. Hal

Cannot agree, just last month had a fresh rebuild on a wisconsin including carb. It would start but not get above an idel. Mag had blue spark. Tryed retiming , checked carb, rechecked valve lash. Sent mag to standard mag in chicago, Mag coil had small cracks. Got mag back reinstalled and engine started and ran excellent at all rpm. Years ago my CC Case puller ran good just wondered if hotter mag would help. Rebuilt very hot mag gave more power, and more blue ribbons.

That does make sense.

A mag with an age cracked coil could work at idle. But as speed increases, so does the voltage. The higher voltage would be looking for a place to go, and through the crack to ground would kill the spark available to the plugs.

Used to see that on old outboard motors, cracked coils were no good.