Adjusting Vintage Tach

Down to the last couple of issues on my punch list and need some help.
My dwell/tach/ign tech meter tells me my tach is off. The tach instructions say to make sure it is the 4 cyl position, but there are no marks on the rear to indicate how to position. There is a black plastic button w/ a slot, but when I try use that the tach either goes off or directly back to the RPM I had before ( I think it is in 6 cyl mode )- I am doing this as the engine is running.
The other posible adjustment says " cal " next to it and has a very small slot screwdriver slot. I assume this is to calibrate the tach, but when I try to turn the screw it seems very tight and I am afraid to break it.
Anyone done this? It should be a simple adjustment, but like w/ most things I am working on it never is.
Thanks for any help, Dale

Anyone using the original ( I believe ) CMC tach's that say " Vintage " on the bottom?
Thanks for any help on the above message.
Dale

The CAL slot you mention is for the "calibrating potentiometer" (a variable resistor). The threads are normally covered with a paint like substance to prevent the adjuster from vibrating and moving. It will be difficult to turn until you break this sealer loose. Curious - do you have a CD or transistorized ignition installed - that will mess up the calibration and cause the needle to jump (had problem in old '72 914). If the CAL-POT doesn't provide enough adjustment you may have to add a resistor.

I sold off my CMC gauges before using them - but they did come with instruction sheets which I believe I included with the gauges so perhaps someone has a set of the instructions available.

Wolfgang,
I bought them off Ebay- it my have been you- I don't remember.
I just have the standard ignition- nothing special.
I do have the instructions as well, but all it says is to set the tach to either 4cyl or 6cyl. If you look on the back of the tach there is nothing printed back there to indicate which or how to adjust. Per my message above it seems to be working as a 6 cyl because I am comparing it to my Dwell/Tach meter and the RPM's are not correct for the 4cyl, but matches the 6cyl reading.
To things on the rear to adjust- the one that says " cal " and a flat plastic button w/ a slot in it w/ no markings. I have turned that button and it seems to either turn the tach off or back to the same 6 cyl RPM that it was before - no change ( doing this while it is running ).
If I am able to move the " cal " adjustment will this move me from 6cyl to 4 cyl or is that just a fine adjustment after having it in the right mode?
Thanks for you help, Dale

The CAL-POT is only good for fine adjusting of a few hundred RPMs. The switch probably changes the resistance to a different rating. Sounds like that circuit is bad. Somewhere (Toyota 4x4 Forum?) I read to try a 1000 ohm resistor (or a variable one) on the feed line (not sure if that was to get you from 8 cyl to 4 or if it was 6 to 4). Think my old gauges (sold here off SOC Classifieds) went to Crestview, FL - not sure if he got to hook up stage yet or not.