1987 318 Tuning with Summit Carb

AJ/FormS said
Ok hang on, I musthave got my wires crossed. In though you had installed a Carter type carb.
With a Holley type carb, and the 6900 cam, the Mixture screw adjustment should be ~3/4 turn out.
But I'll say it again; MAKE SURE the choke is OFF and is NOT interfering with the curb-idle screw.

The Transfer slot is your main low-throttle-opening fuel circuit. At idle, with the secondaries closed tight You have to have a slot exposure AT BLEAST square, as said before, AND you have to add a lil from the mixture screws just enough to smooth the idle.
Theoretically, you can idle the engine on one or the other circuit. But, the carb will not function correctly either way; it must have enough transfer fuel to not have a tip-in hesitation, as you have found out.

Otherwise all that I said is the same.

No, it's neither a Holley OR Carter style. He gave you the part number. Did you even bother to look it up? It's a Summit carburetor modeled after the old Ford Autolite four barrel. Do you know anything about those? If not, you're sending him down an unnecessary rabbit hole trying to show everybody how smart you are(n't). That's why I didn't respond. I don't know a thing about how those carburetors tune.

To the OP, I will say this. Holley (and everyone else) says the transition slots being square is a STARTING POINT, so keep that in mind. You may need "less" of a square, or you may need "more" of a square. I wouldn't box myself in trying to hold it to square. Also, IMO, you should play around more with the timing. You said it would idle up more when you pulled in 18 initial. So just for curiosity's sake, try pulling in timing until it stops idling up. You'll know when. It will start running "rough" when it gets too much. At that point, back off on the timing until that extra roughness clears up. This is really easy to feel for if you'll have your hand on the valve cover when you are pulling in more timing. You'll feel it, trust me.

Also, I see you said you closed the secondaries? HOW did you do that, exactly? I know. Seems like a dumb question, but here is how "I" 100% verify they are closed all the way. I adjust the screw until they close AND stick closed. Then, I carefully turn the screw a LITTLE at the time until the secondaries JUST BARELY don't stick closed anymore. From your description, it almost sounds like they might be a little open still. Might be worth checking into. That's a pretty mild cam and IMO should have more like 18 or maybe a little more hg vacuum at idle and it doesn't. That tells me maybe it needs a little more initial timing.

You might also play around with full manifold vacuum on the vacuum advance, but don't do that now. Work over what you have again and see. Just some ideas. Good luck and keep us posted.