Assuming the trans is NOT auto downshifting during this event;
My guess is that the plugs are overheated and are pre-igniting the coast-down mixture. As soon as they cool off, or you add gas with the gas pedal or, or take away air with the brake pedal, the pre-ignition stops,and everything returns to normal. This is my guess.
If I'm right, you have three possible solutions;
1) richen up the cruise mixture
2) futz with the coast-down mixture
3) run cooler plugs
I vote for a little of each.
Retarding the timing will probably not have any effect.
That 235/249 cam wants some bypass air. The proper way to set the idle-timing is;
Begin here >
Take the carb off. drain it. Flip it over and drill one very small hole in each primary throttle blade; on the frontmost half, about half-way between the shaft and the edge and I center mine between the transfer slot and the idle-discharge port. Start with 3/32 holes.but be prepared to go up to and no more than 5/32. Chamfer the holes on both sides, just enough to get rid of burrs. Clean off the chips. Flip it back right side up.
Next; reset your curb-idle screw to achieve a transfer-slot exposure on the manifold side to appear slightly taller than wide, just enough to see it.
Next; reset your mixture screws to the mid-point of their working range; With Holley or Holley-type set it to 3/4 turn. All others are 2.25 turns. If you have a metering-rod type of carb, you may have to drop them down a hair; we'll see.
Next; make sure the WET fuel-level is correct and stays correct. Make sure none of the air-bleeds are dirty.
Next; make sure your vacuum advance is plumbed to the SPARK-port
Next; DO NOT TOUCH THE IDLE-SPEED SCREW.
To set the idle speed you will be using timing. After you start it up, and it is warmed up, it will probably be idling too fast, so RETARD the timing to no less than 12*.
But if the idlespeed is too slow, ADVANCE the timing to no more than 18*.
Your target for that cam is about 750rpm in Neutral, fully warmed up, with the timing at around 14*.
If you cannot get it there;
You may need to increase the size of the drilled Idle-air bypass holes. but do not do it yet.
If you can get the Idle speed close at 14*;
First try the mixture screws.
If it wants more fuel; increase the transfer-slot exposure by turning the speed screw 1/2 turn faster. Then put the mixture screws back.
If it still seems to want more fuel; Increase the idle-timing to 16*
STOP here>
The idle speed does not have to be 750. I have a 230/237/110 cam in my 11/1 360 and I can get her to idle just fine down to 550rpm in first gear (manual trans) and she has enough power to idle around the parking lot like that, on just 5* of idle-timing.
Your transfer slot exposure only has to be enough to prevent a tip-in sag or outright stall.
Your idle speed has to be low enough so that it doesn't
bang/clang going into gear.
You achieve these by controlling the idle-power. The three players are; throttle-opening, idle-timing, and cylinder pressure.
Don't let people bully you into trying to run more idle-timing than necessary. I can make your beast idle just fine on 5*. I'm just saying.
So now, if you don't have a tip in sag, AND the trans does not clang when engaging a gear, and the rpm drop during engagement is 100 rpm or less; you are ready for another roadtrip, but let me say it again; from this idle-speed setting, do not move that screw! If you do, and you cannot get it working, or you get lost, go back to "begin here>"
After you have found an idle timing that satisfys the engine, you will have to revisit the power-timing, to make sure it is not too much.
The very first time that your engine cares about it's Power-Timing is at stall. And with a distributor, and a stall lower than 2400rpm, it is almost impossible to give it enough timing. We can talk about that another time.
Your cruise timing will likely want to be more than 52*@2400rpm.
I for one will be waiting to see your results.
I run Champion copper cores; they last forever
Happy HotRodding
