4 cylinder Spark plug Gap and Timing

[30 Ply 30U]

info transfer from old board.

Spark plug Gap and Timing
I saw this on a 31 Chevrolet site (at bottom), and thought it was really interesting, especially since the 29U Plymouth also originally was equipped with the AC Spark Plug #G12.  Has anyone tried this with the AC-C86 Plugs in an early Plymouth (28-31)? (These are the modern plugs AC lists for 29 Plymouths, although AC is phasing these plugs out and replacing them with the C87 plug, which is a slightly hotter plug.)  Note that the 1934 Plymouth parts books lists the spark plug gap as .025?.  However, the May 29 Plymouth manual states on one page that the gap should be .027, and elsewhere in the book states that the gap should be .027-.030, but recommends the gap be the same for all 4 plugs.

Also, the 29 Plymouth timing is suppose to be set to .050? BTDC.  This equates to approx 12.5 degrees BTDC (This is not listed in a book anywhere, but can be calculated using trigonometry.)  I have always wondered why I have to advance the spark further than that to get the motor to run well.  In another article I read, it stated that modern fuels burn slower, and thus the engine should be advanced further on old cars (running on modern gas.)

" 31 Chevy Specifications:  Spark Plug:  AC type G-12, Size 18mm, Gap .024"
The gap of .024" actually is the wrong gap for the 194 engine, especially when using the modern AC-C86 plugs.  At .024" gap the car will "pop" out the tail pipe at idle, however when the modern plugs are gapped to .040" the car idles smoothly with no popping, and the overall power is increased dramatically.  The same thing happens when the gap on the original AC-G12 plugs is increased.  When the spark plug gap is increased from .024" to .040", the timing should also be increased from the factory 12 degrees to about 18 degrees.  There is no 18 degree timing mark on the flywheel, so the 18 degrees is approximately when the 12 degree mark is just ready to disappear out of the timing hole on the advanced side.  Setting the timing here will also help to increase the power.  If the engine "pings" at this setting, then the distributor should be retarded slightly until the "pinging" goes away."