timing my 440 TNT

[CBODY67]

Is your hot base idle to WOT response a slight "soft spot"/flat spot rather than a "stumble"?

Reason I ask is that on my '67 Newport 383 4bbl (with the stock production AFB), when I dressed-down and smoothed the casting ridges on the venturi cluster's outer surface and carefully matched the carb sections upon reassembly, doing the WOT from idle, then produced a softer throttle response than previously "just moving quickly" from rest. As in trying to hit a gap in traffic from a parked position. In more normal throttle movements from hot base idle, no problem.

Note that on this particular AFB, it is FULLY MECHANICAL SECONDARY operation. The secondaries contain no intermediary "air valve" above the secondary throttle plates, so at WOT, it is all four throttle plates open and un-restricted (once the automatic choke is fully open).

Many, many years ago, every time I might try the full WOT from hot base idle, I usually got a "pop" out of the carburetor, although friends with GM cars had no issues doing that. THEN I figured out that as the car did not "like that", I would not try to make it like it, so I discovered the car would move quicker (even laying some rubber) with a solid 1/2 throttle punch instead. Worked fine, just needed to add more throttle to get to the WOT automatic shift points.

WHY can the 1/2 throttle punch work better than full WOT? Simple. The intake manifold vac does not drop to a point below which the vac advance starts to work, so part-throttle keeps more vac advance in the engine. MORE TOTAL advance results! More torque to get the car moving. Then after moving/launched, add more throttle to get to WOT and open the secondaries.

Enjoy!
CBODY67

 

[66/440]

Maybe this is relevant. You need to make sure the timing marks on you balancer have not shifted. The vulcanized rings will sometimes allow the outer portion of the balancer to shift which makes timing difficult. Just check that piston is at TDC when the timing marks show "0". It's tough to really difficult to figure out your timing if your base line is off.

 

[furious70]

I became a believer in manifold vacuum for vacuum advance last year when switching my charger over. Marked improvement.

 

[stain]

try the full WOT from hot base idle, I usually got a "pop" out of the carburetor

This what I am experiencing. It was much worse before my latest adjustments but is still slightly present.
What I find strange is the car idles best with the 2 adjustment screws almost all the way out AND the bypass screw all the way in. This bugs me because now there is no adjustment and just does not seem right. The P.O. had the stock carb redone before I purchased and it seems fine otherwise.

 

[CBODY67]

You might double-check the MasterTech items at www.mymopar.com to verify the "correct" way to adjust the "air bypass" used on the 1968 4bbls, just to make sure of what is going on.

I know that the hole the idle screw point goes into in the throttle bore, to control the idle fuel flow, was decreased from earlier times (decreased in 1969 on Rochester carburetors) so that a "too rich" idle mixture could not be obtained with the idle mixture screw setting. Enough fuel flow to do a correct idle mixture adjustment, but not rich enough to be "too rich" to cause black smoke from the exhaust pipes. Perhaps the air bypass AFBs were that way, too?

Seems like the air bypass system was supposed to be easier for techs to adjust idle speed without taking the air cleaner off? But probably had its roots in emissions compliance, too? Yet it only lasted about one model year. Or was only on CAP cars? It's been a while since I've thought about that system.

Perhaps you might need to enlarge the accel pump shooter holes for a bigger fuel shot?

CBODY67