max. R.P.M.'s for zr4 megneto to fire

[Guest tony12345]

I need the R.P.M.'s of a zr4 2 spark magneto too spark. also the craking speed of the engine. the engine is a 1914 stutz bearcat

[Guest cben09]

Is it 2 spark or 2 spark DUAL ??? ,,this mag will turn at crank speed,,sparks at 180 deg,,,not sure of rpm,,Hope this helps a [small] bit,,Ben

[Ivan Saxton]

I presume you mean the R4 model which fires two spark plugs of the same cylinder simultaneously. This is done by the two ends of the high voltage secondary winding feeding two separate distributer caps. You can assess what sort of spark you get through two spark gaps when you turn the armature slowly by hand in the correct rotation. If the magnets are strong, the points are clean, flat, and correctly adjusted, there is no high voltage leakage at the slip ring, (which has two conductive brass segments for the two opposite carbon brush pickups), if the capacitor is fit and healthy, and if the insulation of the windings is healthy and has not become thermo-plastic.

Normally you do not do repairs or alterations unless it is necessary. However, the varnish insulating old windings is frequently perished so that it will melt , then set and cool during a lunch stop on a tour so the magneto will no longer rotate , and if you succeed in turning the engine over, something has to break in the magneto or drive. Capacitor incompetance was always insidious and frequent in older cars, and you need a capacitor tester for the several different tests required. A modern capacitor of the same capacitance as the original is usually fairly simple to use, and is much more reliable. As compression ratio increases, so also does the difficulty for the high voltage spark to ionise and discharge through the mixture in the spark gap of the plug(s). Ifeverything is in good order, it should be easy enough to start by hand with the armstrong starter. It is most usual to time the mageto for maximum strength of the magnetic field at the point the points open and the field is collapsed at full retard for easiest starting, beause the magnetic field collapses more rapidly at higher engine and magneto speeds. This is some practical compensation that that the points do not separate exactly when the poles of the armature core break the magnetic field.

(I hope the above is not just as clear as mud).

One further trap for the otherwise experienced is if the breaker ramps are unevenly worn so that the magneto sparks 1&4 substantially earlier or later than #'s 2 & 3. It can be very puzzling if the engine seems too far advanced yet also too far retarded. Please pardon any late night typographical errors in this late night post.

[Layden B]

I doubt there ever was an absolute definition of what the cranking speed should be. Same for the magneto as to minimum speed to spark the plugs.

In those days things were more likely done by effect. If your mag needs to be spun faster than than the starter will do, the magnets perhaps need to be recharged. As the spark is sent to 2 plugs in series in a Bosch 2 spark magneto as has been mentioned, be sure to have the switch in the 1 position when starting. This sends all the mag output to one plug and thus one plug gap. This effectively halves the voltage requirement and lowers the shaft speed required to spark. You do realize that with a 2 spark magneto the gap should be .017". The mag should also be wired so the 1 switch setting fires the intake side plugs.

[Guest cben09]

Layden,,What can you tell us of a ,,Z-K 6 Bosch,, it has a very nice breaker assy for the battery side,,Cheers,,Ben

[Guest tony12345]

Thanks for the help guys It is a 2 spark. from your replys I think that the engine is turning over to slow. I dont know why yet. I had the mag rebuilt a week ago. . I don't have a tachometer that work's on mag system I guess I will try to find one. Thanks again tony

[Guest cben09]

Are you sure it doesn't have a coil for starting on BATTERY,,??? If it has 2 sets of points,,its a dual[battery start],,,Ben

[Guest tony12345]

No it has only one set of points and no coil for starting on battery.

[Layden B]

Not familiar with the ZK, I don't believe they were a model that was offered here. Probably came over the pond on a car.

Stutz went to Bosch at the same time they added starters, straight 2 sparks, not dual. I realize there are Stutz cars out there with 2 spark dual mags but not original equipment.

[Layden B]

Ben, I hope you are not refering to K series magnetos. They used magnetic spark plugs.

[Guest cben09]

Hi,,This one is an alluminum base,,,looks like a Z-R-6,,real nice points box on bottom of breaker assy,,I have a couple of those K Lo tension mags and one that looks similar but issn't,,Puts out a spark every 90 degrees,,marked H-D,,its an early one Ben

[Ivan Saxton]

I am afraid I cannot see how one set of plugs could be made inoperative on a 2-spark Zr4, except perhaps by means of a earth switch for the high voltage transfer to the distributer cap externally. You cannot prevent two sparks from the secondary winding. This is not earthed, so it must spark from both ends.

I know the book says the plug gap has to be 18thou, but this is nonsence if the coil is properly rewound with high quality enamelled wire. The plug gaps on both sets of my 1918 L-head Mercer are 25 thou. I have to go to melbourne again tomorrow, but I shall run a test on Wednesday for you to see what length of spark it throws.

[Guest cben09]

BE CAREFUL,,,,,,on making the spark longer,,,thats when the higher voltage can puncture the insulation,,turn slower and watch for fatter spark,,,Cheers,,Ben

[Layden B]

On a ZR 2 spark mag setup the switch in #1 position grounds the #1 wiring connection on the magneto. This is the internal connection from one end of the armature to the distributor rotor. Thus the full voltage is applied to other end of the armature and the wire going externally to the face of the distributor cap. That is why this wire from the mag to the switch is a high tension insulated wire rather than a low tension as the ground wire to the points is.