Mallory Hyfire 6a Installed, CDI works great...tach on the other hand |
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Mallory Hyfire 6a Installed, CDI works great...tach on the other hand |
Aaron Cox |
Aug 26 2005, 03:01 PM
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#1
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Professional Lawn Dart Group: Retired Admin Posts: 24,541 Joined: 1-February 03 From: OC Member No.: 219 Region Association: Southern California |
very smooth through the RPM range.... all with a blue coil. I have an ignitor coil to go in also......
But...as we all know...tach no workie with CDI.... and...as i read here.... 914world.com writeup on how to mod a tach.... finished soldering. now for reassembly... AA |
lapuwali |
Aug 26 2005, 03:41 PM
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#2
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Not another one! Group: Benefactors Posts: 4,526 Joined: 1-March 04 From: San Mateo, CA Member No.: 1,743 |
That really is an Andy class picture... (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)
Specifically, what's going on here is the old tachs depend on the fact that the pulses coming off the coil are spikey from noise (up to 30v or so), and there's a resistor at the signal in to limit these spikes. A better way would've been to use a Zener shunting the spikes to ground, but a resistor will do. The mod is to bypass this resistor, so the clean 12v square wave from the CDI boxes will trigger the tach. You'd have to do the same if you used the tach signal from an aftermarket ECU. The later tachs don't have this problem, perhaps because they went to a shunt diode instead of just using a resistor, which will work with either the 30v spiky signal OR the 12v square wave signal. No doubt they were responding to complaints from people trying to use aftermarket CDI boxes. |
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