91formulaSS

I already posted this in the "Tech General" section but no one seems to know why this is happening. (Mods: if this isn't kosher feel free to delete) I figure maybe some carb guys who've done this swap might know.

https://www.thirdgen.org/forums/tech...-new-post.html

can anyone explain this one? first off I swapped in a holley 4150 for my TBI, used a Mallory 4309 FPR with stock in-tank pump. when I set the pressure to anything over 4 psi the pressure gauge needle fluctuates wildly, the more pressure the more swing in the needle. If I try and set it to 6 psi the swing is from 3-9 psi and it's very rapid. The pump starts making a ticking noise that coincides with the swings in the needle and the lines actually start to move with it too. Once I lower the pressure to 4 psi or below the needle doesn't move at all. This happens regardless if the engine is running or not. Do I have a bad FPR? could my pump be going bad? any and all comments welcome.

Supervisor42

I don't think it's a "bad" regulator as much as that it is for the wrong "range".
The vacuum port on it indicates that it is a regulator like for TPI that is going to work in the 35-50 psi range.
The diaphram in the regulators that work in the higher ranges are smaller than the ones that work in the lower ranges. I used a JEGS part #793-30-900. Quick Fuel makes it and it is very steady from 4-10 psi as I adjusted it. (I know, they're expensive)
Here's a pic:

Notice how much bigger the diaphram is. It's working range is 0-15 psi.

EDIT: I was wrong about the pressure range. It is expecting a Mallory F/P to feed it though. The volume of the stock TBI pump may be too low for it to be stable.
Also mine is being fed with a TPI pump which is different also.

91formulaSS

so I ran a 1" rubber hose from the return side of the FPR to a gas can and all pulsation ceased. I guess my next step is to look for kinks or obstructions in the retun line. Do you think I could use compressed air to blow it out? maybe there is something in there causing an obstruction. Last thing I want to do is have to drop the tank to run a larger line.

Supervisor42

Yeah, you could run compressed air thru the line. There shouldn't be anything in there because everything that went thru it came from the fuel filter (filtered gas). Just be sure to remove your gas cap so you don't overpressure the tank. Which leads to the answer to your next question.
There is always an "obstruction" in the return circuit... the pressure in the tank that goes "Psssss" when you open the cap.
You can look for kinks or places where the return line may have been smashed. I think you would see a "can't get the pressure low enough" problem if it was obstructed though.

Apeiron

The 4309 is 3-12 PSI.

Supervisor42

Which is why I added this minutes after posting: