Initial timing should end up around 10-12 degrees for most builds. This will insure easy restarts when it's fully warmed up and heat soaked. For a well thought out engine build you should be making PLENTY of vacuum at idle without having to add any more timing.
If the engine isn't pretty happy around those numbers then the engine compression ratio is too low for the cam choice.
When you get a moment wonder over to my thread in the Lobby on camshafts:
https://cliffshighperformance.com/simplemachinesforum/index.php/topic,3650.0.htmlEnough on camshafts. The Big Block Chevy emission years Q-jets had TONS of idle bypass air because those engines had pathetically low compression ratios and crappy camshafts. I've heard they were even retarded some to help get them thru emissions but have never actually verified that.
What I do know is that a big block Chevy 454 in any configuration is a "sleeping giant", and all that you need to do is tighten up the squish and put a much better camshaft in it and they will absolutely ROCK your World.
Small camshafts on tight LSA's aren't going to make the grade in that arena. They don't have enough seat timing for the big displacement and close the intake valve way too early. Just look at the dyno sheets above and you can see what happens when you put a long duration cam in a big engine with much wider LSA and later intake closing.
The wide LSA really spreads out the power curve and gives them a fighting chance to make the big power they should be making.
I've had scores of engines brought in here to tune, some from great distances, and not one single time have I been the first bit impressed with any of them when the builder used a relatively small cam in them on a 110 or tighter LSA.
Even with all that said you have to tune for best results. If the engine wants/needs more timing and fuel at idle speed then you have to give it to them. Tight LSA also causes "reversion" (my terminology for the shootty/quirky idle we often see from them and the fact we have to add more fuel/timing to make them happy at low RPM's).
So bottom line the carb has to be set up to provide adequate fuel at idle speed to keep it completely on the idle system and good sensitivity/control with the mixture screws. Adding idle fuel will also help off idle, transition and put more fuel to the engine in the "normal" driving range via the transfer slots.
From what I've seen here 48B rods are too fat unless you are using the APT system in the baseplate and can get them up out of the jets some.
I use a different jet/rod combination for the Big Block carbs with the huge MAB's and have had perfect success with them. I also use our orange power piston spring as it is a clone of the same spring used in the 350/350hp and 454 and 427 high output engines.
The big block carb also requires the high flow N/S assembly and I'd use our HP accl pump in it........Cliff
