Lean out at WOT. Fine at 3/4 throttle

[BigBlockMopar]

Opened up the 425, didn't find any burrs or anything. Still looked like almost new. Cleaned everything up inside.

I remembered I had once altered the top of the carb to try and lean out the mixture slighty at idle by machining down the springseat slighty.
This worked nicely at the time I had the mixture going rich at idle.
Decided to swap the cap with another unmodified one just to see if the rich-idle issue was still present.
What I found was the engine pulled much better, even with less then full tanks, but it didn't change the lean-out at further throttle openings.

So my next plan is to install the 8mm main high-pressure line.

[C3H8]

I must admit to a little frustration here. Lean conditions have a specific process to figuring them out. I made some suggestions earlier and from what I can tell it appears you haven't checked the basics. You keep going back to the mixer and ignoring the process. I'll repeat the process. The system is made up of four main areas. Fuel supply (tank and hoses), Lockoff/filter, Vapourizer and mixer. Diagnosing a lean condition requires following a process.
1. Is liquid coming out of the tank and making it to the lockoff.
2. Is liquid making it through the lockoff and to the vapourizer.
3. Is the vapourizer getting adequate fuel and supplying enough fuel to the mixer.
4 Is the mixer big enough.

I believe you checked the first two sort of but you still haven't identified if the vapourizer is getting adequate fuel at WOT. The only way to verify this is with a two pressure gauges. One on the supply line just before the vapourizer and one on the primary port. At WOT the pressure should remain constant on both gauges The primary pressure should remain constant at approximately 3/4 psi to 1 psi. It should not bounce around. If the pressure drops off or varies greatly then you check the inlet pressure to ensure it is constant. If the inlet pressure is steady and the primary pressure drops the vapourizer is not big enough and a second vapourizer is required. If both pressures drop then the fuel supply lines, lockoff or service valve is the problem. If both these pressures remain constant only then should you turn your attention to the mixer.

As for the mixer you should make sure (in this case) that you have removed the restrictor plate from the mixer located inside the vapour inlet. The mixer was originally designed for NG. When used on propane the plate is normally left in to compensate for the BTU difference of NG and propane. In your case, since were talking a higher CFM capability, it would be best to remove the restrictor plate to maximize fuel volume. Concentrating on the mixer will not solve your issue if the vapourizer is being starved or the vapourizer is not supplying adequate fuel. You won't know unless you do the pressure checks.

[BigBlockMopar]

I fully agree pressure-checks will pinpoint to the problem-area quickly and I can understand seeing someone 'work around' this can be hard to look at ... but, getting proper gauges installed, and viewable from the cabin, with an asortment of T's, lines and fittings that can't be bought easily on every corner overhere will become a project by itself.

Installing an 8mm main-line is something I've wanted to do earlier but never got around to it.

[storm]

Let's be frank (not Frank) here. I had actually given up on this. This is an issue that has been going on for quite a while. I can understand, due to the current pandemic, if this was a recent thing that getting parts would be difficult (I order a battery for my wagon a week ago and it probably wont be here until this coming Friday) but its not a recent thing and there has been plenty of time to sort parts out. If its an issue of funds to pay for things then so be it but the length of time this has been going on for indicates that getting parts should not be a problem at all.

To be honest it appears your fuel system was built to a budget that didn't suit the intended purpose. You have an engine that creates more power than the combination of fuel system parts are able to supply. There may be more problems but until the fuel system is built to suit the purpose you require it to fulfill you will just keep chasing your tail.

With regards to the gauges you don't need them to be viewable from the drivers seat. There are a few different ways you can find out what the pressures are. Electronic pressure sensors do not require "T" fittings but you will require some way to log the data (Innovate Motorsports may be an option). You could also set up mechanical gauges that are viewable outside the car, e.g. from the guard, and put the car on a dyno and watch what is going on. The point of this is there are usually always options and I think you need to think outside the square so you can get the information you need so you can fix this issue.