engine struggles when secondaries open

[elmariachi]

First, you should be sure your base and total timing is correct for the engine. What feels like sputtering or cutting out at high rpms could be knocking from too much timing. You aren't going to hear it over the pipes naturally, but it feels like the engine is missing. Past that, you should step way up to 84 jets (the larger the number the more fuel you are feeding it) just to see if that induces a radical change. Putzing around with incremental jet changes (75, 76, 77) is the long way to sorting out if its a fuel rich/lean issue. Just give her the gas and see what she does. FYI you wrote he went from a 78 to a 74. That is a DECREASE in jet size.

The above comment about power valves is worth looking into. If a 4-barrel with mechanical secondaries has a power valve in the primary AND secondary blocks, then the jets are meant to be the same value on all four corners (what is called "square.") If there is no power valve in the secondary block and there is a plug, then its intended that you run a considerably larger jet in the secondaries. Otherwise, the power valve in front only affects how the car responds on take-off to full throttle condition. Once you are in afterburner mode and rolling fast, the front power valve has already done all its going to do and you are relying on front and rear jets to feed gas to the carb.

Innovate Motorsports makes a system that uses an oxygen sensor to analyze air/fuel ratios. Install the O2 sensor, hook it all up to a laptop and go for a ride. Its nice to be able to see what the fuel condition is when "things" happens.