I've had this old thermocouple gauge for very long time & just got a chance to use it.
Inserted into a length of SS tubing with the junction end sticking out a little and inserting it into each exhaust tube by the same amount I got readings of about 225 Deg F
& it didn't seem to vary much with small changes in idle mixture screws, so I just left carb settings where my engine guy left them. Insertion into the pipes did vary temperatures observed, so a little hard to be precise with this.
it started right up after sitting for several months while body stuff getting done, about to put it all together soon, for first drive.

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Post a pic of that gauge. Does it convert mv to temp or do you have a scale to convert manually. I ask because I have a millivolt meter for testing thermocouples .It's an old analog meter.

EDIT: LMAO, just spied it in the pic and enlarged to see it. I'll bet that sucker is accurate.

I am told that you get the best EGT readings if the thermocouple probe is about 18" from the exhaust port. In your case, it's considerably farther. And in addition, the exhaust for each cylinder travels different distances to get to the end of the heat exchanger pipe, generally losing more heat to the pipe the farther it travels.

In short, your setup might not be that accurate. It may, however, be accurate enough. I don't know enough to say.

--DD

Gauge reads in Deg F directly. i really don't have any long term use for it if you want it.
Max tmp reading is 225 F., so what I was seeing was very slightly off scale.