Rough spot between 30-40mph

[nyamwegi]

I posted a few times earlier about having problems with a quadrajet and getting water out of the tank on my '75 scottsdale 350. I just bought and installed a new 600cfm edelbrock carb (manual choke) and it has been running a whole lot better than the old carburetor. I do have a few questions, however:

1. I've cycled through over a tank of gas after I thought I had all of the water and bad stuff out, but yesterday I got bogged down after I hit a patch of something that had been "hiding" in the tank that was real rusty (obviously didn't make the motor run well), so I drained a few gallons out of the line till it was solid clear and put on a new fuel filter. Has anyone used STP gas treatment? It claims to help evaporate water. I'm a little nervous about my new carb but I feel confident if I keep a filter on there it should keep debris or anything that would really mess the carb up from getting in there. I probably drove five gallons worth since draining the bad stuff last night and it's been running really well. I guess if I keep having more crap turn up in the lines I'll have to get a new tank. But I'd prefer it if gas treatment would help. Anyone have experience with it?

2. As far as running the truck runs very smooth and seems to shift into all (4?) gears. Pretty sure it has the TH350. The only vibration I get is consistently between 30-40 miles an hour. If it were the carb it would be in an RPM range, not an MPH range, correct? Could one of my planetary gears be causing vibration? It seems like once you break 40 the vibration stops.

Thanks

[Donut]

I don't know how effective the gas treatments are with todays fuels, (alcohol and other stuff)  back when, I used to run "dry gas" during the winter.
Your vibration,?  Vibration, if it's rpm, usually engine. Mph, usually driveline.  Check the simple things first, I'm thinking I'd start with u-joints.  (after i check the fluid level in the tranny   

Glad the new carb worked out for you.

[78 Chevyrado]

Unless you want to remove the tank to clean it out, just keep filters handy and keep doing what you've been doing.  It may take a while.  I kept the tools and filters with me so I could swap filters anywhere when i started driving my 78 after sitting for 10-15 years.    The water removing stuff won't hurt the carb, the main thing is make sure you have a GOOD filter inline and not one of those screen types.

[Blue 82]

I run a Racor
like the shorty on the left
When Florida switched to E10 I had to drain it weekly

[VileZambonie]

The TH350 is a 3 speed. If you have rust and crap getting into your fuel system do yourself a favor and replace the tank.