Idle Rpm High in Park, Low in Drive
Go to posts 36 and 37, and
Tune for the most power in gear. That is the least drop in rpm when placed in gear and or the highest vacuum in gear as long as the rpm kept the same. Can't use vacuum as a comparison between two different rpms. The engine naturally makes more horsepower as rpm increases.
now, I gotta tell you, that the biggest cam I have tuned is the Mopar 292/292/108 , in an Eddie-headed,
[email protected] Scr, in my personal engine. If your cam is bigger, the following might not apply.
The First thing to do is to put the vacuum gauge back into the toolbox. And Immediately followed by the Timing light.
The Second thing is to lay the groundwork:
>Make sure the valves are closing, sealing, and that the cam is timed reasonably close to correct. That is to say that split-overlap occurs within ABOUT 4 to 6 degrees of TDC-Overlap cycle.
>make sure that no air is getting into the intake from anywhere else than thru the primaries, past the throttle blades.
>Make sure you have a working PCV system, plumbed to between and below the primary blades. You MUST have a working PCV; this will be your Idle-Air Bypass. If you have a brake booster, PROVE that your engine is NOT sucking air thru the charge line. I like that line NOT to be plumbed to any one specific intake runner. If your carb has a port on the Secondary side or a port in the Plenum; use it. If there is not one there, put one there.
>Disconnect the V-can and plug the port on the carb.
>make sure the fuel is fresh, and that the WET fuel level is stable and Not low. Fresh fuel is somewhat critical. If the Light-weight VOCs have evaporated you will have trouble.
The Third thing is to prep the carb;
>Take the carb off and Set the primary exposure per posts 36/37; to a little taller than wide; visibly taller..
>Close the secondaries up tight but not sticking.
>Set your mixtures screws full rich; on a Holley this is about 1turn out. On anything else, set them to 2.5 turns out.
> If you have a big air-mover coolingfan, install some kind of non-combustible baffle on the carb's vent-tube, so the fan cannot affect your float level; the level must remain stable.
Fourth thing;
>Start the engine, and warm it up, For an alloy-headed streeter, try to get the engine heat up to in the range of 185 to 205, AND stable. Do not attempt to tune your idle until the temp is stable., whatever it is.
>After the engine is warmed up, and stabilized, and NOT before;
If the rpm is too slow DO NOT change the speed screw. Instead, advance the timing.
If the rpm is too fast; DO NOT change the speed screw. Instead retard the timing.
In either case, do not install a timing light, just rotate the distributor to get whatever Idle speed your engine likes, but as slow as it will comfortably tick over, or no less than 700. Let it warm up.
Take a walk behind your car. If the exhaust burns your eyes, you got some work to do, and;
Now you can begin the idle tuning.
As for that 292 Chrysler cam in my engine (it's long gone now); I got it to idle at 550 in First gear with a manual trans, pulling itself on a flat level, hard parking lot; but climbing over a dime was sometimes a lil iffy,lol. I measured that cam at
[email protected]. The starter gear was 2.66 x 3.55=9.44
IMO; This is about the biggest cam you should ever consider installing in a streeter, and most definitely not with 3.55s, which I found out the hard way.
To get to that point; the Neutral idle was 750@ 14*advance, and with additional Idle Air bypass amounting to one hole drilled in each primary throttle valve at 7/64ths inch.
Your bypass will have to be determined by trial and error, because
we have NO IDEA as to the size of the cam.