Just to be complete, you might consider checking the vacuum advance to make sure the diaphragm has not ruptured. Make up a little stub fitting out of a piece of brake line, attach a small rubber hose. Disconnect the vacuum line from the advance and install your testing tube. Suck on the hose or use a mitivac if you have one. If you are easily breathing through, the diaphragm is toast and likely the root cause, or a contributing cause, of the backfiring. If the diaphragm is good, reconnect the vacuum line, start the motor and set the timing.
You set the timing by using the screw on the clamp, not the octane selector bolt. Ideally, the selector should be set to 0 (hash mark on the distributor pad and the 0 mark on the selector plate lined up). Set the motor to anywhere between tdc at the bb to 5 degrees btdc. You should certainly be able to run 87 octane.
The other issue that might be contributing to the problem, a malfunctioning mechanical advance in the distributor. It is not all that uncommon when I am rebuilding a distributor for a 235 to find the advance springs missing, or one broken off, or jammed in the weights, or just rusty, gummed up weights that are just not working.
Last edited by Dragsix; 09/10/2024 8:16 PM.
