Way to Determin if throttle is open too far into transfer circuit at idle

[jboyer]

Cliff,

Please let me know your thoughts on how Im determining IAB and also if there is a ratio of what size to keep between the Idle tubes and restrictors?

I'm experimenting with the IAB (Bypass air at idle), im trying to get the engine to idle without opening the throttle blades too far into the transfer slots. What I have been doing is putting a vacuum gauge at the ported vacuum line for the the distrubuter. I adjust the idle speed screw and note the vacuum reading on the guage. What I'm thinking is that if I see any vacuum from this port at all then the primary plates are open too far and I start add more IAB. I started with 6" and worked up to .100 IAB to get it down to less than 1" but enough vacume to see on the needle move from its resting spot. I cannot kill the motor with the Idle adjustment screws, but it does lean it out quite a bit when they are in all the way in with the holes drilled to .90. 2 turns out seems to keep me between stoich and slightly rich.

What I would like to know is if you think that no vacuum to this port means you are low enough not to pull too much from the transfer circuit or if It may still need to be lower.

Im using a 79 qjet with Daul main Air bleeds off of a 429 Cadillac
51P rods and 73Jets.
Idle tubes at .036 and restrictors at .049.

I'm running this on a 517cid Cad with a bigger cam, ported heads, 9.1 Comp, headers, edl perf intake

Cam Specs
LSA 112
Duration Int/Exh - 296/306
Duration @.050 228/235
Valve Lift .519

They recommend that idle set arround 950rpm but said i can work it down to about 600 even 400 if the carb were setup correctly. I like the sound it makes at 600 better than 950. 600 sounds nice and lopey, 950 sounds like, hey that idiot has his idle set too high.

Love the book, thanks
Jason

[Cliff Ruggles]

Jason, sounds to me like you are very close with the settings.  If you have control of the A/F at idle and can lean it out with the mixture screws, and then back them out for a good idle in and out of gear, that's good. 

Usually if the ported vacuum slot is activated, the throttle plates are too high and you may also see some "nozzle drip" as well.

A quick test to see if it wants or likes more bypass air is to remove a small manifold vacuum hose, then re-set the mixture screws.  If the engine speed increases, and you can lower the idle speed screw even further, AND still have full control of the idle fuel from rich to lean, more bypass air is going to make it even better....Cliff

[jboyer]

Thank You Cliff.

Im using an O2 sensor in the exhuast after the headers, and a 02 guage. Sometimes the mixture bounces back and forth between Stoich and Lean, I suspect this may be due to the lump in the idle. So Im thinking I may try increasing the Idle tubes and Restrictions to compensate for a low signal? The bleeds are sized so that the total of both is about .140. Also is there a specefic ratio I should keep in mind between the Down Restcrictor and the Idle Tubes?

[Cliff Ruggles]

Depends on the size of the upper and lower idle airbleeds.   The upper idle airbleeds, especially the airhorn located version have a significant impact on fuel delivery at idle.  The idle tubes and idle down channel restrictions must be based on airbleed size(s).....Cliff