using diodes to take feed off coils

hello.

i have a 2 stroke 2 cylinder motorcycle that fires twice per revolution and has 2 coils. fed from 2 sets of points.so reach coil fires once per rev and is independant from the other. no wasted spark. 12 volt system.

i am trying to wire up an electronic tachometer normally used for a 4 cylinder car with one coil. 

if i feed it from one of my coils it runs at exactly half speed.

i need to somehow give it a feed from both my coils but without connecting them both as the bike then wont start. the feed comes from the pulsing positive connections of the coils. ie the one the points open and close and send a live pulse  down every revolution.

if i attach a wire from each of the pulsing terminals and attach them to the tacho feed wire, the bike wont start. i need a kind of one way electrical valve are they called diodes? that will allow the pulse to be sent one way up towards the tacho but no the other way past the valve and back to the other coil. sort of backfeeding it.

i have been told diodes will work. i bought some 5w 12v rated diodes here

https://www.maplin.co.uk/5w-zener-diodes-19091

but i can connect it either way and the pulse still gets through. they are obviously the wrong rating or spec.

can anyone tell me what rating i need? i hope iv'e given you enough information. i need to be able to wire up the two coils to the tacho  so that they can both send a pulse from  but not be connected to each other. wiring shaped like an upturned v with a blocking diode or valve on one of the lengths. or even each of the lengths

 the bike is a yamaha rd350A 1974 but this doesnt really help solve my problem. the only other way is to make a circuit that will double up my pulses and send twice as many pulses out as it is receiving. im not very clued up with electronics as you can maybe tell and i dont know if this is possible.

cheers

al

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live each day as if it was just another day.

At those voltages, the zenor diode is conducting in both directions.

Get ordinary diodes like IN4007 (1000V) preferably - though IN4004 (400V) should suffice.

Logic-OR connect the points to the tacho. IE Kathode/Line end of diode to wards the pints/wire, the other Anode ends joined together to the tacho.

That should work - as I recall the RD points did not overlap. (I used car coils on mine, and down-tuned the oil pump. The splugs lasted well over 10,000 miles!)


They can also be wasted-spark fired - ie, both splugs together.

yeah i used to use car coils on mine too!!! strapped to the down tubes on the frame. sparks about an inch long!!! last summer my oil pump lost the pin that the pulley rests on and got no oil delivery while i was on a runt o york. nipped up twice before i realised. not cheap to rebuild crank and new piston and rings so i now run premix at 33-1. motul 710.

anyway thanks for help. you lost me a bit with the logic-or but i think i get it in the way i have to have the diode pointing the right way.

if i connect both points wires together to try and achieve a wasted spark the bike wont start. cos that was how i hoped the tacho could be fed and give me a true reading. maybe i need to insert diodes before the coils to get a wasted spark?

im on denso iridium and am on 2000 miles. and counting. but the washer has worn flat and leaked so i borrowed ones form some ngk plugs.

all the best

al

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live each day as if it was just another day.

Actually I might have the diode thingy wrong....

By wiring the points together, you won't get spark because at no time is there a break from ground - ie, either one or no points are open - at least one is closed to ground, hence charging the coil(s) but never sparking.

But the same will apply through the diodes.
Therefore you need (say) a series cap to produce a pulse - but that has to be short enough for max RPM, and if the tacho is an "impulse" type...


Can you get into the tacho? It may be much easier (and reliable) to change its timebase. There will be a resistor-capacitor combination that sets the sample time. Usually the resistor is variable for calibration. Simply double the resistance value (else double the cap value) to double the sample time & hence the RPM displayed.


Rethinking the points issue, the solution is logic gates - a pulse when one OR the other points is open (or when both are closed) - that is timing independent (as opposed to inserting a series cap).
Hence my "diode-OR" logic (pun intended but inapplicable) - but I reckon that is incorrect. Not that I can think any clearer now....


PS - keep mud of the HT end of the coils!
I used plain NGK splugs as I always do. I have never been one for Iridium etc (except for phones) preferring instead CDI etc.
I avoid KLG & Bosch splugs. The Bosch would last 300 miles and NEVER recover after a clean up! KLGs broke threads, and were "not good".   
Denso are good. But in my "surveys" and experience, NGK. Else maybe NGK. Or NGK.

i can get inside my tach and take some pics if you would be so kind as to assist me.

ive ordered the diodes, only pennies but you reckon they wont work? surely if they cant back feed to the other coil then i will get two pulses? i suppose there is one way to find out. isnt there something simple i could insert between the one coil and the tacho to double up the pulse rate? or is it not that straight forward? i dont really understand when you talk technically. i need a dictionary or a tad more explanation if that is possible?

got densos in my car. done god knows how many miles in 4 years. when i was working out of town used to do 134mph everyday and cruise at 120 all on the way home on a 50 mile drive. got them in my zx6r. done 12k very fast miles in the last 2.5 years. and i mean very fast. up to 175 and usually 140-160  everywhere on twisty roads. no chicken strip on front.

finest tip on the market when i bought them, supposed to wear out quicker than the ngk ix but i havent found that yet.  most of it is confidence. i havent been let down. yet....

 had plenty of fouled (rich petrol) ngk b8hs but cant wet foul the densos. even ran without main jets trying to understand the throttle range the mains influence has and had to go to the densos. 

will try and post something tomorrow. i know the tacho has a switch for 4,6and 8 cylinders. it just slows down if i go to 6 or 8. 

thanks for help so far.

al

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live each day as if it was just another day.

If it has the 4,6,8 switch, then you should be in luck. You just do what the switch does, but the other way.
It probably switches in or shorts different resistors.

In simple terms, the #8 resistor value will be half that of the #4 resistor.
You want to halve the #4 value.

Try to pic the trackside as well as the resistors "connected" to the switch showing their color bands/codes.
Otherwise you could write/describe the color bands.
With that I can decipher the circuit etc.

If you convert the tacho, is there a setting you will NEVER use - ie, 4, 6, or 8?   Why? In case you want to use the tacho elsewhere (on a 3 or 4 cyl bike, or 4, 6, 8 cyl car etc - modify the switch position that you won't use.


Doubling pulses is possible, but this is easier.