305 V6 Vacuum Issue

[67GMC]

I have a 1967 GMC C20 305 V6 with the NP435 4 speed that has a vacuum / backfire / stumbling issue. If you lightly press the accelerator, it wont stumble or backfire but if you try to take off normally it will. Some background information.

I bought the truck 3 weeks ago, brought it home on a trailer. Originally had the Stromberg WW carb on it and it had the same backfire / stumble issue.

Replaced the points & ignition coil with a Pertronix 1162A kit and Flamethrower Coil.
Replaced the Stromberg with a Holley 4412-3, 50 Powervalve with 74 Jets, idle adjustment set to max vacuum. Manual choke. Definitely still running rich after adjustment. Maybe too the jets are too large?
Installed new spark plug wires, cleaned cap & rotor. Also cleaned and re-gaped the AC Delco R44XLS plugs to .040
Ran a compression test last night. #1 125, #2 115, #3 115, #4 125, #5 125, #6 120.
Vacuum is 17 to 18"Hg but not steady, I have attached a YouTube link showing the fluctuations. I move the throttle and it's not backfiring but if I do it any faster, it will.
Listening to the exhaust, it appears to be misfiring as well - see the 2nd video link. Not sure how to embed them in the post.
Timing was set to about 2.5* with the distributor disconnect. Vacuum advance does function properly as it advances well over 10* when attached.

Any thoughts as to why its stumbling and backfiring thru the carb? I can adjust the distributor with enough advance that it doesn't backfire but it runs too rough due to pre-ignition.

I have searched for similar issues on the board here but don't really come up with such a small vacuum variation. The vacuum should be steady, correct?

I am thinking possibly an intake leak? The carb has a good seal, new gaskets on top and underneath the Mr Gasket adapter. I am thinking about doing the WD-40 intake leak test as well.

There is a tick on the driver's side but I don't know what effect it might have. See the 3rd video. Sounds like a lifter tick. Maybe I need to adjust valve lash?

My next step was to install a fresh set of plugs and possibly a new cap and rotor but I'll have to order them as no one stocks them for the 305 V6.


https://www.youtube.com/watch?v=-vzMDL5TCqY

https://www.youtube.com/watch?v=yJ9lnCy0eCs

https://www.youtube.com/watch?v=LODZtsji5VI