Ok, some new developments.
To recap, truck has had a vibration above 3k under hard acceleration since the 5 speed swap. Just granny shift and it drives fine. It has been driving me nuts though trying to make it right. Played with driveline angle etc. If Duff ever got back about piecing together a kit I would have put traction bars on it too thinking maybe axle wrap was screwing with the pinion angle. Hot starting was also less than ideal.
Oddly playing with the driveline angle (swapping trans crossmember) allowed me to use a carb spacer again which helped hotstarting.
Fuel pump was only doing 3psi so I swapped the Mr Gasket pump with a in tank pump. It could also only muster 3psi. But you know it is new... so 3psi with the right volume might not be so bad.
Check the volume with the pop bottle trick mentioned in one of my posts in the V8 section. It was way down.
So my in tank pump stranded me twice, I could get it going again by thumping the tank which was less than ideal. First time it was like... well maybe it was just a bad connection because I couldn't get it to act up again. And then 6mo later it did it again. That time I remembered I could reach the connector but it made no difference, so I kept punching the fuel tank until I heard the pump. So I shifted gears and got an red Holley fuel pump for it.
Hot starting was dramatically worse, worse than it ever has been. Top end there was flat spot when the secondaries hit but it went nuts after that. So we have improvement with longstanding issues, that is neat. We are FINALLY getting somewhere and it really improved the top end... and made the bottom end end worse. But we have more symptoms to muddle with.
Pump really wanted to run 7psi so I stuck a Summit Holley knockoff regulator on it a few days ago and knocked it down to just under Edelbrock's spec. Also popped a new set of needles in it while I had it apart. I read that too high of pressure can screw with the viton coating on needles where they seat and cause them to leak. Possible hot start cause?
I did more digging into adjusting floats since I would be messing with them and found this video:
Then we operate.
Float top end was way high, no wonder it hot started so weird. Supposed to be 7/16" right at the edge of the curve of the top edge of the float.
Like this:
Ok, got those adjusted. Now onto the drop. Spec is 15/16", mine were like 1-3/8"
Left is adjusted, right is how they both were when I started.
Needles were not horrible, since they are a special order thing I threw them in my tuning kit just in case I get in a bind down the road.
Cranked it up, ran fine. Got in the highway and let her have her head. Just smooth, no flat spot it just freaking runs like a striped ape. Revs really snappy, generally runs better than it ever has.
In retrospect I had multiple issues screwing with me.
The low fuel pressure, I think my original pump somehow lost pressure capability while I was doing the 5 speed swap which probably started a lot of this. It went from 6psi to 3psi while the truck was down. My new in pump tank was junk straight out of the box which is a nice plot twist when troubleshooting experimental things. I couldn't find a pressure spec but it is rated the same flow as a factory low pressure F-150 pump.
I also wonder if the open element air filter ventilated a flooded engine better too. My dual snorkel came into the picture during the 5 speed swap too. It is a factory housing though...
Anyway, theory time.
Low fuel pressure/flow kept the hot start thing at bay because it struggled to outrun the engine to overfill the bowls before shutdown depending on drivecycle. It was exasperated initially by not being able to run a carb spacer. Flat spot above 3k was the fuel pump running out of capacity to feed the secondaries.
With enough fuel pressure/capacity the fuel supply could outpace the engine demand. If I shut the pump off a bit before I shut the truck off it hotstarted a lot better. The flat spot was the needles opening late and then the Holley had enough surplus capacity to easily recover.
That's my current theory anyway.
And to give credit where credit is due,
@Uncle Gump mentioned checking floats years ago which I swear I did at the time. But I think I had a lot of things in concert screwing with me simultaneously.
1. Low fuel pressure/flow
2. Upper float adjustment
3. Lower float adjustment
4. Carb spacer
5? Air filter venting?!