67' S Tach with Dancing RPM w MDS Box and Ignitions ???

[joshkras]

Greetings:

I have a 67S that I have owned for the past 9 years. I have always had a problem with a dancing, somewhat "bouncy" inacurate tach. I was told it was do to the MSD ignition box that is installed and that it could be fixed, but I never really went for a fix.

The engine has always run a bit cold and would take several miles to smooth out, but the tach would remain the same. IT also apears to register higher than the actual RPMs at idle.

6 years ago I did a total rebuild to 2.2S P/C (rally heads), etc. and still had the exact same problem with the tach - no change. The engine also too would run cold as previous, before the rebuild, but after warmup, it would be smooth.

After the last tuneup service and clutch replacement (from a 914 GT clutch to a standard 911 clutch) the car feels different and vibrates heavy at low RPMs at idle. It also feels slowe than it used to be. I have gotten used to it but it is kinda annoying. Could this be do to a diffent, heavier clutch?

My mechanic says it is do to crappy gas in California and that I should really upgrade to a non-points ignition. He named a specific brand of ignition, which I cannot recall. I searched on the web but could never find one.

I do recall his 67'S was fitted with this ignition and it ran a lot better. Any thoughts or advise would be greatly appreciated.

Thanks,
Josh

[Kaleid]

Josh,

I have a Bosch ignition box/coil and have the same issue, so I don't know if it's solely attributable to the MSD box...

[kenikh]

The bouncing tach is from the MSD box's signal being incompatible with the tach. There is an adapter you can buy that will fix this. The cold motor issue is odd, though. Do you mean the oil temp? Have you checked the oil thermostat to make sure it can close? If so, perhaps your timing is backed off too far? If your 'S' motor is running a 'T' distributor and wasn't recurved, this might be possible, but really isn't the likely culprit.

As far as the last tuneup, your mechanic is feeding you a line of CRAP. Isn't the bad California gas the same stuff you were using before you went in for the tune up? Did the gas in your tank change overnight? I don't think so. Sounds to me like your car might not be firing on all cylinders or perhjaps some plug wires got crossed. That will make a car feel sluggish and vibrate at low RPM. I'd go back to him, call bull**** (nicely) and have him take care of it.

Is the 914-6 GT clutch significantly lighter than the 911 clutch? This might be the culprit on sluggishness, but doubtful on the vibration (barring a defective part of ham fisted install). My '69S clutch and flywheel don't make my car feel sluggish.

As far as using a Crane or Pertornix non points ignition, they are nice, but I don't use one and have none of the problems you are having. I had my car in California last month, running "crappy" California gas with no issues.

Bottom line: your mechanic did something wrong during the tune up. Get him to fix it.

[joshkras]

Good points on the gas! Thanks for the advise. I will have him check it out or maybe even bring it to someone else to check out first.

Is the adapter just a plugin in or is it more complicated? What is the life of an MDS box? Maybe it's time to change to a newer unit.

[kenikh]

The MSD units are very nice indeed. They last, too, unlike the Perma-Tune units you see on older cars. Regarding the tach, it is an inline filter you can buy directly from MSD. A number of CD boxes had the issue, so it is a pretty ubiquitous part. FWIW, my car has the same problem (Perma-Tune) and I have never gotten around to fixing it (or replacing the crappy Perma-Tune).

[joshkras]

Forgot to address the cold running statement. By cold, I mean that upon start up and a little bit afterwards, you need to nurse it and give it revs at a traffic light or it will stall. It has always behaved like this. To me it is strange that after the total rebuild, I would think this characteristic would have changed. However, after a tune up, it does usually stall less and requires less nursing to get warmed up.

After a couple miles on the highway it warms up. Temp guage never really moves or rises that much. Not sure how accurate it is. I

[kenikh]

Forgot to address the cold running statement. By cold, I mean that upon start up and a little bit afterwards, you need to nurse it and give it revs at a traffic light or it will stall. It has always behaved like this. To me it is strange that after the total rebuild, I would think this characteristic would have changed. However, after a tune up, it does usually stall less and requires less nursing to get warmed up.

After a couple miles on the highway it warms up. Temp guage never really moves or rises that much. Not sure how accurate it is. I

This is normal, especially on carbs.

[Zeke]

Good points on the gas! Thanks for the advise. I will have him check it out or maybe even bring it to someone else to check out first.

Is the adapter just a plugin in or is it more complicated? What is the life of an MDS box? Maybe it's time to change to a newer unit.

MSD will test the box if you want to send it to them. I'm sure you could rig another ignition up for a couple of weeks while it's gone. This might give you some answers as well. Agree on the mechanic, although the gas is not good anymore. I wonder what your timing is set at?

Distributors are suspect in these situations. Yours may be old and tired. Well, we know it's old You could also send your dizzy off for curve testing and refurbishment. There is a place in SoCal that does this for a good price. If no one can supply the name off-hand, I will get it for you out of my records.

Testing all the components is the only way to go. For instance, after I got my S running after purchasing it dead, I changed the plug wires. BIG difference! That's a $100+ proposition, so testing is in order. If your "mechanic" can't do this, go elsewhere.

[joshkras]

I will have to check the receipt, but I think I got new wires, cap, and points at the time of the tune up, but testing is probably the best approach.

I have used the same mechanic since I purchased the car and he has never BS'sd me and has done 10 or so tuneups and the engine rebuild.

When I got the car back this time it just felt different. So, there has to be something wrong. It's not a big deal though. Last week I took a high speed run from Tiburon to Aptos and performance was fine - just annoying big vibrations at lower certain lower RPMs.

[304065]

If you are still using Kettering ignition, yes, it would be a good idea to upgrade to some sort of CDI. The Factory did in 1969.

If your tach is bouncing with Kettering ignition, it suggests voltage fluctuation, possible defective voltage regulator. Check all the ground cables first for oxidation or fraying, then check the battery voltage with a voltmeter on the batt terminals with the engine on and then with it off. Report those voltages here for further guidance.

I have used both MSD and Bosch, both work fine. If you don't have a Bosch CDI sitting around the MSD 6AL is a good starting point but your tachometer will have to be modified to take the MSD's output signal.

The Kettering ignition sends a high-voltage pulse to the tachometer to get it to trigger. Old VDO tachs are based on a monostable multivibrator or "one-shot" design, the trigger pulse results in the tach needle bumping up. It's like a volleyball, the more you hit it, the higher the average level. Tachometers post 1971 are a different design, still a one shot but a high impedance type designed to be triggered off the very low current flowing in the CDI trigger circuit. Some report that they are compatible with the MSD's 12V 50% duty cycle square wave but YMMV. Anyway, the ultimate solution is to either get a modern tach and have it refaced to look like a 67, or have the 67 modified to work with the modern ignition. I'm currently doing both. . . .but I have fun with this kind of stuff. . .