I understand the BB is where the shop manual says to set timing to. Shop manual also states it should be running 5 degrees advanced when idle is below 600RPM. So does that equate the BB to being 5 BTDC? Mine does not have the UIC stamping. It has the triangle for valve timing, 2 vertical lines, the BB, then another vertical line. I'm thinking the vertical lines are 5 degree increments. The reason I'm asking is my truck is stock but for the rear end has been swapped out from 4.56 to 3.73 gearing and when I set timing to the BB it idles and runs smooth as silk. But is gutless above 25MPH with the lower rear end gearing and I can't even take a small hill. I'm currently running by my timing light 3 degrees advanced of the BB which gives me better highway power but the idle starts to suffer. What I want is the smooth as silk idle below 600 per the manual but I need a steeper advance curve when I come off idle for highway driving. So I'm looking at the Pertronix billet distributor with the recurve springs kit, advance limiter, and vacuum advance. But I would like to establish baseline timing off of the BB so I know what my actual timing curve and all in advance is which from memory is around 24 to 28 degrees. I can't know where I'm at if I can't establish if the BB is 0 or 5 degrees advanced.
There’s a lot of unknowns here. Valve adjustment, compression, vac leaks, carburetion, etc.
But taking your timing concerns at face value:
1) Make sure your dwell is within spec (or at least verify/adjust your points gap).
2) With the engine warmed up, and ignition timed to more/less the dot….hook vac gauge to vac advance port. Set idle SPEED screw to the point you JUST start to see the vac gauge lift off zero. Adjust idle MIX screw to max vacuum and then another 1/8-1/4 turn richer.
3) Whether or not it’s ideal….you now have at least a baseline to start playing with the timing. Advance the dizzy a bit at a time until you find a sweet spot where RPM almost maxes out, but things are nice and smooth and happy. Snug up the clamp. Shut off and try to restart. If it bucks against the starter, loosen clamp and back timing off a smidge until it starts easy again. Go for a drive and see how it feels.
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There are lots of ways to skin this cat. That is the way that works best for my well-worn engine. I am not convinced that a numerical approach to timing works well on a Stovebolt of less-than-pristine condition. It’s more of a feel game, IMO.
I can say with a lot of confidence that my 235 runs LOUSY timed right at the dot. It wants more timing than that.
Edit:
I typed all that before I saw your last reply. Ignore the points gap/dwell thing if you’re running Pertronix. All the rest should still apply. The Pertronix is a variable that’s hard to account for. Sometimes they work, and sometimes they don’t. Some people love them and some people hate them.
I am highly skeptical that you need a different distributor curve. My truck is even longer legged than yours….3.38 rear and same tire size. I can make it run nice at both idle and 70 mph.
I think something is amiss with your ignition or carb. Perhaps dizzy is worn internally and timing not stable at low RPM. Or could be carb….what carb are you running?
