Holley Carb Gures, Secondary adjustment!!
. Just thinking, will cracking the secondary's cause a over rich condition due to the lack of power valve and mixture screws?
No, she will go lean, but not so much that you cannot compensate with the low speed circuits.Every combo is different and the cam is the biggest player. And the relationship between the cam and initial timing is pretty powerful too.
I can't say how the exact mechanics work, but in my experience,this is what I have found;
As the cam gets bigger, the idle vacuum gets lower. I believe the engine doesn't get enough air, so I crank up the idle speed screw until it idles at a reasonable speed. But when I do this the mixture screws quit working, and the transfers start flowing. This messes the A/F up and the engine runs lumpy and the exhaust burns my eyes big time. The fuel molecules are just too far apart and the fire goes out before it all gets burned, in the allotted time, or it continues to burn in the pipes.
-So ok, I will bump in some timing, and start the fire earlier. Then I can back up the speed screw to maintain the idle speed. Then if the cam is small enough, maybe the mixture screws come back on line. So seeing as that works, maybe I crank some more timing into it, and back out the screw some more. If I back up too far, eventually the transfers dry up, and I get a hesitation at tip-in;so it idles great, but maybe it won't take throttle.
-If the cam is too big, and the secondaries are fully closed you can never get a good balance between idle speed, timing, and clean exhaust.
-This is why I need to sync up my T-ports(detailed elsewhere). Once the relationship between the transfer slot and the idle discharge ports is established, it is just timing and secondary air that I have left to play with. So then I crank the timing in until the rpm peaks, cuz the engine is telling me she likes it right? She is making more torque, so the idle speed goes up.Then I use secondary air to establish the idle speed,INSTEAD of the curb-idle screw. By this point, she should be idling nicely..
-Next,I crank the idle speed up to about 2000, and reset the mixture screws for highest rpm. Then I return the idle speed to exactly the former setting.
-Next I check the throttle tip in, without the accelerator pump. At this point I might have too much idle timing. And here is how I can tell; If the engines quality of idle goes bad or it stumbles on very slow tip-in, the Tport sync needs work. Usually they want a bit more fuel, which I give it by cranking in the curb-idle screw a quarter turn at a time. If the idle speed gets outta hand, I have two choices;back off the timing, or close the secondaries, whichever produces the best results.Or, if I have a vacuum advance unit,with plenty of stroke, I will try to sacrifice a bit of idle timing, and bring in the Vcan as fast and hard as the engine will accept it. (FWIW, my 367 likes over 50* at 2000rpm when cruising down the hiway.This allows me to run lean there and pick up some mpgs.)
-Then I move to power timing and rate of advance.
-Then I go back and make changes as may be necessary, to the slow speed circuits. Things like accelerator pump,and fuel level.
-Then it's on to jetting. And a revisit to midrange PT timing.
-The one thing I am confident to say is; do not force the engine to accept any preconceived amount of advance, especially power-timing. If you are a streeter, a couple or even a few degrees less than optimum,is easier on the engine, than running on the edge to extract those last couple of horses, that you almost never use. What I mean to say, is that the top of first(A/T), with 3.55s, is 60 plus mph, and speeding here in Manitoba. Having just 2 degrees too-much of detonation-producing timing, that engine will have been hammering for 3 to 5 seconds. The math says each of the bearings,and each of the skirts,will have been pounded for, in the neighborhood of, 500plus times! Better it would be, to be, 2 to 4 degrees short.
-So that's how I do it. I don't do anything until it idles nice.
The above assumes that the engine temp is stable, that the valves are correctly lashed,the cam timing is within a couple of degrees of where it is supposed to be, and the compression is even and "good".
The biggest engine I have tuned is a 360 with a 250*cam.
