TACH PROBLEM IN 2ND GEN.

bldn10

bldn10

New member
I traded out my Pi system for a set of analog gauges and got an old used Mallory tach in exchange. I wanted an in-dash and it would be cool if it worked because it is 3+" and has a shift light and rev limiter. Try to find that at Autometer. However, I am now convinced that the thing is reading way high. It shows an idle of 2500+ and I am way over redline far earlier on the track than usual. I would like to think my newly rebuilt engine is just a revving SOB but I am going slow. I think I'm actually shifting early but I'm afraid to ruin a new engine if it is right. Is it possible that some tachs won't work w/ our ignition? It is set on 4 cyl.
 
The stock setup is tach connected somehow to the trailing ignitor, which has individual spark for each rotor. I'm not sure of the details of the tach connection, so I might be blowing smoke. I do know that the leading ignitor is a wasted spark arrangement, so it gets blipped for both rotors. If you have the tach connected to the leading spark you might be getting twice as many pulses as you expect.
 
Marty, I deciphered the stock wiring diagram and connected the green signal wire from the tach to the yellow/blue signal wire to the stock tach, which is from the trailing coil.

Andy, I haven't tried it but will. I have generally heard that rotaries are set on 4 cyl.

Do you guys know off hand what our injector volume is supposed to be? I sent 6 out and they were all between 41 and 44.
 
Measured injector volume depends on the test. Our injectors are rated at ~440cc/min by Mazda. The test they used it called a d-cell test. This test holds the injector open versus other tests that pulse the injector. To see how the injector flows versus it's factory rating have the d-cell test done at 37psi. Tests that pulse the injector are hard to compare to each other unless you know the duty cycle used for each test. Even then tests using different duty cycles are hard to compare to each other.
 
Guys need some help here.....Loaded the car to head for Nelson this weekend and noticed that my Tach has become very lazy and starts jumping like crazy....Any ideas? :unsure:

Thanks!
 
Check the ground to the trailing coil or replace it with a spare if you have one. Fouled trailing plug will also show this as well.
 
Thanks Steve but I have it wired to the leading coil will check the ground though....Also there is a wire on each coil for the techinicain to connect a hand held tach which is where I have mine wired into could this have something to do with it. Needless to say the stock tach is still functioning. Where do I disconnect the original tach and hook up the aftermarket set-up? Or is this how you guys have yours set-up?
 
I gave up on the old Mallory tach but the problem all along may have been how I had it hooked up. I spoke w/ a tach tech [couldn't pass that one up] at Autometer and he said to wire it on 4 cyl. and connect it to the leading coil. I believe that the OEM tach takes its signal from the trailing coil and that is the wire I had the Mallory connected to. I too plan to connect my new one to the female connector on the leading coil.

BTW Autometer is offering for the first time a "Mini-Monster" tach. It is a 3 3/8" unit that can be installed in-dash (or pedestal) and it has an external shift light like the Monsters. It should be available in the next week or so for around $200.
 
FWIW I had the car on a dyno last week and was able to find the right setting on the Mallory tach. There is a detent between 4 and 6 cyl. and it matched the dyno at 3000 and 5000 so I think that's it. On 6 cyl. when it read 3000 the dyno said 4300. I have it wired to the leading coil but I tried it on the trailing and got the same reading.
 
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