Can a tachometer effect your ignition?

[Zedyone_kenobi]

I ask this as I am chasing down a very predictable miss and back fire out of my exhaust around 4000 rpm. I have been playing with mixture ratios and timing and the more I do this, the more I think they are acting just fine. Everything in the fuel and ignition portion of my car is less than a year old.

Now I have seen my tach on some days just go nuts and peg at 7000 rpm when I am slowly idling in traffic, but it always returns, and there are other times when I floor it and the tack needle just swings insanely fast upwards 5x the speed my engine can actually rev. Most of the time my tach does behave normally, but it does act pretty funny above 4500 rpm often. So, I always thought my engine was not running well and my tach was showing it. However, I am starting to think that maybe my tach is causing my coil to miss fire and that is sending a lot of unburnt fuel into the exhaust causing the popping.

Has anybody had any experience with this. I may try to locate the wire going from the coil to the tach, and disconnect it and then see what happens. This has been a fascinating problem to track down. And electrical problem would explain SO much with the behavior of this missfire.

[DonaldTrumpshair]

Mine does the exact same thing, if you find anything out please let me know. I'm wondering of its tied into my fusible links, because when my links wiggle and shuts the car off really quickly or what, but I would be interested if you find anything out

[zKars]

Please tell us what distributor/coil system you're using (240 points, 280 reluctor, 280zx E12-80/92 or aftermarket) and if you have the stock 240 tach.

If you have a stock 71 tach then the power to the coil runs through the tach, making it pretty tricky to just have the car run with the tach disconnected. So, as a first and easy test, disconnect EVERYTHING from the + side of the coil, and connect a temp jumper wire from that post to the + side of your battery thus eliminating the tach and regular power supply to the coil. Do this JUST before you start the car, and go for a drive. Note that turning your ignition off will NOT cause the car to quit, you'll have to remove that jumper to get it to stop.

Let us know what happens.

[Zedyone_kenobi]

That is awesome advice, and my sincere apologies for not including details...

I run a Pertronix Flamethrower coil

Advance Distributors rebuilt Dizzy that was recurved to close to european specs

Vacuum advance plugged up.

Pertronix electronic ignition installed.

Taylor 8mm plug wires

New NGK plugs

I am running with a balast resistor with a jumper wire across it.

I was looking for a wiring diagram of the 71 to find the tach to coil relationship. I have a 73 FSM, but I know they may be a bit different than the 71. Just for reference, I have a spare tach in my parts bin... but have not checked it out.

Your suggestion is fantastic zkars. I will give it a try when it stops raining here. May be a couple of days. As we are getting a down pour lately.

Edited December 20, 2011 by Zedyone_kenobi

[Zedyone_kenobi]

Got home from work a bit early, and decided to try out the jumper, to no avail. Maybe it was better, but if so not by much. However, I did do some more digging. I decided to rebaseline everything.

I pulled off my air cleaner and rebalanced the carbs. They were of course unchanged and dead nuts on. Then I started with the MR 2 turns from full lean

I proceded to go half a turn toward the lean side and the idle picked up dramatically.. AHA!!! this is good. Then I went another half and it dropped a bit, so about 1.5 from full lean is my sweet spot.

Next, on to timing. I had it set for 12 degrees BTDC at idle per my dizzies rebuilders request w/o the vacuum advance. However, I am reaching max advance at about 2800 rpm, which is not bad per say, but the most I am getting is 32.5 deg BTDC. I should be closer to 40. I maxed out my dizzies advance and got it to 15 BTDC. I took it out for a drive and I was able to rev almost to redline before the popping out of the exhaust happened. I think I just need to dial in a few more degrees of total advance, maybe hook up the VA again.

Seems timing is my most likely culprit.

[Daily Driven Rev]

Good job