[gtofreek]

Well, today I learned something very interesting that I have never encountered before. I have been working on Pontiac's since 1979 and cars since 1976. I have seen a lot of weird things.

A month or so ago I finished a 461 cid stroker engine with 389 heads and tri-power. The same engine I posted a dyno sheet for. As some of you may have read, we had an issue with some noisy lifters. Well, we were able to get a set of slow bleed lifters after a few weeks of waiting. I went out to the customers house last night and we installed the new lifters and took it for a drive. I should back up. The first time I went out to figure out why the lifters were noisy in the first place, we readjusted them and took it for a ride. Much to our surprise, when we stepped on the gas, the car hardly accelerated. we looked at each other with that, WTF look. It sounded funny also, kind of a gurgling sound from the carbs. To make a long story short, we found a bunch of dirt in the carbs, piled up behind the main jets. So we cleaned them good, did a couple of mods to increase the air flow and put some better needle and seats and new gaskets in them.

Now, back to yesterday. We get it fired up and it sounds real throaty and responsive. It's gotta run good now. Keep in mind that this thing makes over 500 ft. lbs. from 2700-3800 rpm. It only made about 386 HP @ 4500 because of the small heads but it's a torque monster from hell. 516 ft. lbs. from 3100-3300 rpm. We drive it and when we step on it, it still doesn't go. Here comes that WTF look again, only now it's got twice the emotion to it. You know, like WTF!. So I tell the guy, you need to bring it into the shop tomorrow so I can figure out what the heck is going on. He already tried another coil and distributor, and a hot wire right from the battery with no change.

So, today I get to looking at it and test the voltage to the coil. 11.2 volts with the engine running. Good. It has a Pertronix billet distributor in it. I figure next I'll try an MSD Blaster II coil. When I am disconnecting the old coil, I notice a tach wire hooked to the negative terminal. So I think to myself, I should disconnect that and see how it runs. Just then, my business partner, Jeff Koerner walks up and says, "hey is that a tach wire? Because years ago I was tuning a circle track car and the driver said it would not pull at all when he stood on the gas. We disconnected the tach and the car pulled like crazy."
Well, we drove it without the tach hooked up and it ran strong. Set you back in the seat and accelerated like it should. It has a 2 speed tranny with 3.73 gears and 27" tall tires. Hooked the tach back up and it's a turd again that the stock 2 bbl 326 would have outpulled! The engine ran good when just idling or putting around but when you floored it, nothing but strong turdness! I might as well hopped out and pushed, it probably would have accelerated faster!

I never in a million years would have guessed the tach would cause an engine to run like a turd, but it must be interfering with the electronics in the distributor. We are going to try installing a diode between the coil and the tach and hopefully that will allow him to run this tach[it's a reproduction made to fit in the dash and look stock].

If anyone else is running a tach with electronic ignition and wonder why your engine doesn't run like you expect it should, you might just want to check this.

[lust4speed]

Yep, that's strange. I wonder if it is that specific tach that is causing the problem. I'd be interested to hear what the results were with a different manufacturer's tach.

[Radman]

MSD module has a seprate wire to feed the electric tach. Also the tach instructions say that the warrenty is void if hooked to the coil with a high voltage ignition module.
It is time to get the books from both the ignition system and the tach.

[Sun Tuned]

It sure will, but not that common of a problem.

I've only run into it once before.

Worked up a really nice point dist for one of the members on this board...
No worky right he calls back, serious head scratching going on, I'm thinking condenser took a dump, but I extensively checked all that before sending it back to him.

He was about to send it back when he was changing out parts that weren't making any difference when he decides to drive it and see change if any and Voila! Now it works fine! Smooth idle. Which was the main complaint it was very rough.

He said he got to the end of the driveway and noticed he'd forgot to plug in the hood tach. So out he gets and the second he hooked that up it started shaking all over the place again. Unplugged it and everything was golden.

Undoubtedly something had grounded/shorted/malfunctioned in that and caused MUCH head scratching to ensue. I would have never thought a tach would have caused that much trouble either, but who knows.

Gotta check everything these days.

[Half-Inch Stud]

Have had many Tach problems with my HEI, and the TACHs are always the problem.

Really never noticed the HEI suffering. Now i got a fun thing to do : disconnect the dead tach.

[67cruiser]

If i may say so, id be concerned why you don,t have a full 12 volts to the distrib.??? What size wire you running?? 10 gauge??

[lust4speed]

On a different tangent, I installed the MSD replacement module with rev limiter into a friends HEI distributor. While the unit checked out fine on my distributor machine, his dash tach wouldn't work when we got things installed. This unit uses a gray tach wire coming out of the distributor. I wired in a spare aftermarket tach and it worked fine off the grey wire. Called MSD and the tech said to not use the grey wire and plug the tach wire into the tach terminal. We did that, and his dash tach came alive and no more problems. Tech's explanation was that the cap terminal supplied a stronger signal than their tach wire.

[gtofreek]

Originally Posted by 67cruiser

If i may say so, id be concerned why you don,t have a full 12 volts to the distrib.??? What size wire you running?? 10 gauge??

It has 12.7 when not under a load. 11.2 under load is fine. This engine was dynoed with 7 volts. Pertronix said that is fine. I have seen engines run on 3 volts. Doesn't take much to fire a coil.

HIS; the tach works just fine, it just causes the engine to run sluggish at full throttle. Run fine otherwise.

Lust4speed; it has Pertronix ignition, not MSD. There is no tach hook up with it.

[george kujanski]

The '60's OEM tachs are connected to the coil neg terminal and are actually powered by the coil connection. Since the coil neg terminal is what actually triggers the ignition pulse, a malfunctioning device such as the tach, bad capacitor(condenser), etc. will interfere with the rise time of the primary voltage and therefore affect the secondary, (high voltage) output.

Normally, the tach input has a 330 ohm resistor in series with the coil connection and that works out to about 38 mA of current stealing from the coil to power the tach; the coil probably peaks at 5 A or so so the tach current is insignificant if operating properly.

Quality aftermarket tachs (not the direct replacement kind) have a transistor input buffer circuit to minimize the loading on the coil.

I'd say something is not right with the tach even tho it seems to work. It should not disturb the ignition. Any idea what version of Pertronix is in that distributor?

George

[gtofreek]

Originally Posted by george kujanski

The '60's OEM tachs are connected to the coil neg terminal and are actually powered by the coil connection. Since the coil neg terminal is what actually triggers the ignition pulse, a malfunctioning device such as the tach, bad capacitor(condenser), etc. will interfere with the rise time of the primary voltage and therefore affect the secondary, (high voltage) output.

Normally, the tach input has a 330 ohm resistor in series with the coil connection and that works out to about 38 mA of current stealing from the coil to power the tach; the coil probably peaks at 5 A or so so the tach current is insignificant if operating properly.

Quality aftermarket tachs (not the direct replacement kind) have a transistor input buffer circuit to minimize the loading on the coil.

George

So, you don't think a diode will help it? What your saying is the tach is drawing too much, therefore a diode probably won't help?

[HWYSTR455]

Hey Paul, out of curiousity, what brand/model tach is in question here?

.

[gtofreek]

Don't know. It's one the customer bought to install in the dash and look like a factory tach. He bought it from some guy that makes them.

[george kujanski]

Without knowing the design of the tach, it's hard to say, but even the simplest design OEM tachs of the day allowing the ignition to work normally, so I'd say something in the one you have is defective, or the design is poor. it shouldn't act like you describe.

george

[F ROCK]

i had an issue with a points eliminator kit and a sun tach.
eventually burned up the tach and some wiring.
i'm back to points, 2nd tach works fine now.

[lust4speed]

Originally Posted by F ROCK

i had an issue with a points eliminator kit and a sun tach.
eventually burned up the tach and some wiring.
i'm back to points, 2nd tach works fine now.

Which kit?

[A.W.Dille]

I also had a problem with a Sun tach last year, shut down my GTO going into AutoFair. Took me about 15 minutes to figure out what was wrong. took it out and threw it in the trash. I have an HEI distributor. Haven't gotten another tach back in it yet.

[F ROCK]

Mick, mine was Accel. engine still ran but had a miss. the tach still worked for a while, but was on the steering column and i happened to notice it was getting hot. burned up completly when i was pulling it out of the garage to work on it.

went back to points; got a py hood tach now. no problems.
i still like points better. (but i've had my share of bad sets too.)