Hello Kjell,
Different instrument and devices act differently to the noise from the ignition system. Your shift light may just be more tolerant of noise, be better shielded or have better filtering inside to reject the noise.
You may need another signal converter if the shift light has lower impedance than the tach...it'll steal most of the signal and the tach won't get a good pulse. Separating the wires may help. Shielding the wires can help.
You could also cut down the noise the ignition makes by putting a RACING suppression core wire on the mag. No street car stuff, but something 50 ohms/ft. or less. Most of these types of wires are hard to use with a Vertex cap...the holes are too small. Taylor makes a Thundervolt 50 wire that will work...even their big 10.4mm wire has an inner core that's 7mm so you can strip the outer jacket off and get it in there. As long as the wire is 50 ohms/ft or less, there won't be any damage to the mag, rotor or cap...you'll be fine.
20 crankshaft degrees refers to the built in centrifugal advance in the mag. If you set the STATIC timing (with a buzz box for example) to 18, then it'll be at 38 when the RPM is high (add 20 to 18 for the total advance).
If you have trouble starting the motor (bucking against the starter), then the advance can really help save your starter and ring gear. Most race cars on methanol are "locked out" meaning they have no internal advance mechanism. Street cars, Fords or motors on gasoline are generally found with anywhere from 8 to 28 degrees of advance built in when running a Vertex. Usually, the advance is "all in" by 2500 RPM or so.
Oh, another thing that can cause havoc with a tach or shift light...running the plug gaps too big (more than .018-.020) or having a carbon center contact in the cap that's missing or stuck up in the hole and not touching the rotor. In both cases, the load on the mag is increased, the voltage skyrockets (in order to get to ground) and the resulting RFI makes everything mad.
At FIE, we're currently working on a fiber optic solution for a tach signal since light isn't affected by EMI or RFI. It will only work with EXTERNAL coil mags unfortunately, but it will pickup the ignition signal and transmit "light blinkies" to where ever you want on the car to a receiver. The receiver is mounted right at the device needing the signal which is even powered with nice filtered juice from the receiver. It's in testing right now and looks really good.
The MSD signal converter boxes work ok, but the signal gets polluted upstream or downstream and what you have by the time it gets to the tach can be very different than what you want your tach or shiftlight to see. Plus, it's in a plastic box and that doesn't help things either.
Good luck with your tach! Let us know how it goes...
Spud
