For a SBM with a 276?110 cam or smaller,
here is your starting point;
Rev the engine in Neutral to 2400. Leave the VA operating.
Then add timing . When the rpm goes up, close the throttle back to 2400, and add more timing.
Repeat until more timing does not increase the rpm, then back it up 3 degrees. Write down your timing for 2400.
What you have done is started the fire early enough, that peak pressure is being delivered to the crank, at the right time for 2400 rpm. This will give you the smallest throttle setting to cruise at 2400.
Next, fix the throttle at 2400 rpm. Your engine will be running partly on the transfers, partly on the mixture screws, and maaaaybe a lil on the PMJs but I doubt it. So screw in the mixture screws until the engine slows down. Then back them out 1/4 turn. You now have the right amount of fuel to cruise at 2400 regardless of which circuit is operating. Now kick the throttle off and let the engine return to idle.
Then turn the mixture screws in all the way, counting the turns as you go, then put them back where you found them. If the screws are in about the center of their operating range, then you are good to go.
But if not in the center, then the engine may be wanting more or less fuel from another circuit, to satisfy the screws.
To help determine which, start her up, and check the idle-timing. write it down.
Then if it is not in the range of 12 to 16, set it to 14 for now. Next, set the mixture screws to in the middle of their range. Set the rpm to 700/750 in Neutral. This will set the transfer-slot exposure under the primary throttles pretty close to correct.
Make sure you are running a PCV to the front of the carb, to discharge just below the primary throttles. All this assumes that the Float level is correct and stays correct at all times. And if you have a 4-corner idle carb, a cam smaller than 276/110 does not need them so shut them off.
Finally determine if the carb is running rich or lean by first, jamming a rag into the secondaries. If the idle changes, then make sure the secondaries are closing all the way, but not sticking. and remove the rag of course. Reset the idlespeed to 700/750 again. Now, take that rag, bunch it up in your fist a bit, then slowly begin to cover the primarys. If the idle speed goes up,then she's already getting to much air. If the idle speed eventually goes down but never rises first, then she is already short of air.
What to do;
Firstly, at 2400rpm, the engine does NOT care about the size of the Primary MJs. In fact, it will run 2400 in Neutral just fine, even if you take them right out. And I don't think they make MJs small enough to run out of fuel at 2400 in Neutral. So forget about tuning your cruise fuel with MJs.
At 2400 in Neutral, your engine can only be AFR tuned by fine tuning the fuel deliveries from the Idle-circuit against the transfers, and let the MJ fuel be what it will be.
So then, to adjust the Afr keeping the idle screws fixed, the only way to do it, is to alter the fuel coming from the transfers, by using the speedscrew. And if the idlespeed goes too high or too low, then the only way to change it is with Idle-Timing.
So do what you gotta do.
If the engine develops a tip-in hesitation, the transfers are too lean.
If the exhaust starts to burn your eyes, the engine wants some bypass-air, which you need to supply on the primary side of the carb.
Now, lets go back to cruising at 2400.
From wherever your Idle-Timing ends up at, you need to get to whatever your engine wanted earlier.
And you still have to satisfy the power-Timing.
Lets say the idleTiming comes in at 16*. and
the power timing needs to be 34*. and
the required cruise timing was 56*
How are you gonna marry all that?
Well first, you gotta install the right advance cam, which is 34 less 16=18*, so you build or buy one. Then you gotta get the rate of advance set, as quickly as possible, that does not put the engine into detonation.
Suppose you are able to install some springs that get you 34* at 3200rpm and the advancing begins at 1000 rpm. That would be a rate of advance of 18 degrees over 2200rpm, right? Which is 18/2200 x100= .818* per 100 rpm. So then, at 2400 the mechanical will be
(2400 less 1000/100) x.818= 11.5*, which you then add to the 18 initial=29.5 at 2400... but you need 56 so you need 26.5 from the Vcan. Your Vcan does not make that; in fact, the most I have heard of one being modified to get is 24*. I modded mine to get 22*. Say yours goes to 22* as well; now you are still short 4.5*.
Your options are to leave it at that, or to increase the Idle-timing by whatever you need, or to install a faster rate of advance. But honestly .818 is pretty fast. Furthermore, in my experiments, that last 3* is almost not worth chasing. And adding 4.5 degrees of idle timing will likely drive the idle-rpm up higher than you want.
BTW; 67s in the front for a hot 360, is guaranteed to be too small at sealevel. 70s with a 12.5PV is about right with a 230*@ .050 cam, depending on your elevation.. 68s at the smallest, If you try and cover the transition with pumpshot, your city fuel-mileage will suffer.
