Hi Neal I have a 2000cc VW with the same configuration in a motor glider, top set of plugs on a slick magneto and the bottom set (automotive plugs) fires off of the Bosch electronic ignition and it idles smooth as silk with both set to 28 degrees. I seem to recall that the slick magneto is set to 28 degrees as well. There is no mechanical advance in the slick magneto, just an impulse coupling. The impulse coupling is what makes the clicking noise when you wind the engine over by hand. The impulse coupling retards the timing when the revs are below about 500 rpm (cranking speed). Now it is real important to have the timing for both at 28 degrees because if one fires before the other then the first one to fire is the engine timing as the fuel burn in the cylinder has occurred, the second plug's energy is wasted.
Now to your back firing. I would say that the timing for the electronic ignition is more advanced than you think because at idle, 800 rpm, both ignition systems should be at 28 degrees. Also the difference between running on any one ignition and both is only about 100 rpm when at 2000 rpm There should not be any "lighting a rocket in the cylinders" performance improvements. I set the timing on the magneto as per http://www.pa18.de/Technik/Slick.pdf and then set the electronic with a timing light when running at idle and then check at 2000 rpm (though the rpm makes no difference). Other things that can affect the magneto, is the rotor timing inside the slick magneto. I have found one where the rotor was retarded by one tooth. The engine ran perfect up to 2400 rpm and then missed under load. This is not picked up during normal run ups as they are done at 2000 rpm. I picked it up when the I forgot to turn the electronic ignition on after a shutdown during flight. Now during test flights of aircraft I test each ignition system when at full throttle and under load in a climb to ensure they work correctly. Hope this helps Barry Kruyssen k...@bigpond.com Australia -----Original Message----- I have an 1835cc VW with a Rev-Flow carburetor. The engine fires the top set of plugs on a slick magneto and the bottom set (automotive plugs) fires off of the electronic ignition from Great Planes. The electronic ignition is preset to 28 degrees before top dead center as was recommended in the setup instructions. That's of course why it's not started using the electronic ignition. The engine is started using the mag with it's mechanical advance and once it's running the electronic is switched on using a switch on the instrument panel. When it's switched on, it's like lighting a rocket in the cylinders as the engine really picks up power, I'm sure due to the hotter spark of the electronic ignition. My problem is in getting the engine to idle correctly. With the electronic ignition turned on, the engine backfires some during slow taxi (at this point there shouldn't be any other kind, right?) and I believe it's due to the preset timing of the electronic ignition firing too soon for the slow speed of the engine. If I taxi out using only the mag it seems to idle slowly at a lower RPM but this does seem to load up the automotive plugs so that when the electronic ignition is finally turned on it takes a few seconds to clear the plugs. I've had it stall several times at idle but I think that I just need to preset the throttle slide open a little more, as it might be set so it's too far closed and cuts off the air flow when the throttle is all the way closed. This is with the mixture set at full rich by the way. I need some thoughts on how to get this to fire evenly with the electronic ignition on at idle. Does the timing need to be decreased some? I believe I can conquer the idle problem just by readjusting the slide in the carb a little but I'm puzzled on the backfiring at idle. Once on the runway when I roll in the throttle, it really accelerates quickly and is smooth running.
