overheated ignition coil ?

[poolboy]

Here's another photo from Nelson, of the early non-adjustable bypass valve.

The temperature compensator is that yellow thing just showing at the left edge of the photo.

That round brass object that you see where the adjustment screw would have been on the "adjustable" versions is a tamper-proof seal. If you pry it off you can "tamper with a small adjustment screw.
There are 2 sympyoms of a problem with the Throttle Bypass Valve that I am aware of.
The first symptom would be failure to return to your preset idle speed in the normal fashion.
This is caused by one or two things. The diaphram surrounding the actual brass valve has a leak or has become too brittle to flex. Or the adjustable spring tension on the valve is inadequate to completely close the valve when you decelerate or stop.
The second symptom is backfiring in the exhaust when you decelerate. That happens because the Throttle Bypass Valve is not opening. That is usually due to too much adjustable spring tension on the brass valve.
A good starting point would be to turn the adjustment screw Clockwise all the way until it stops. Then make 7 Counterclockwise turns.
Test drive, check for backfire on decelerations and proper return to idle. Adjust as needed. Make small adjustments (1/2 turns) from this point on, inbetween test drives.
Keep in mind that if the diaphram (the diaphram around the brass valve) is torn or brittle there will be nothing else to do except replace it (Moss # 365-745) or eliminate it's function by blocking the actual bypass port that the valve regulates and take your chances with the backfire issue.

 

[DNK]

There were several different bypass valves, and not all of them had adjustments. But I don't think any of them could be described as "rect." ... perhaps you are thinking of the temperature compensator ?

My bad