I am adding a return style fuel pressure regulator (Holley 12-881). The old regulator is returnless. Holley blue pump

Old regulator is before the carburetors. Would it be better to mount the new regulator before or after the carbs? Instructions show diagrams with the regulator before or after the carbs.

Thanks for the help

I put my return regulator before the carb to set the pressure.

I put them after the carb in the end of the fuel log.

Most people these days mount the regulator after the carb. The recommended mounting method changed about 20 years ago but I'm not fully sure why. I think the carb guys decided that it was just better to always be circulating fuel by the carb rather than circulating the fuel between the regulator and the tank and leaving stagnant fuel in the pressure line between the regulator and the carb.

I've seen both routings work just fine but the full circulation model probably works in some situations such as really high heat, where the stagnant fuel model has problems.

THIS ^^^^^^^^^^^

I've always ran a deadhead style regulator on my cars. Probably around 15 years ago I started having issues with fuel as I ran straight pump gas at the time. I ended using a mixture of pump/race gas for a good many years to combat this issue. My engine combos has changed some thru the years but now I can run straight pump gas again. 2 years ago I swapped out the deadhead style regulator for a bypass style as I finally learned it would help with the issue. Not wanting to do a lot of replumbing, I have my regulator before the carb. So far I haven't had any issues.

Yeah, I think pump gas is the big problem since pump gas is formulated to work with EFI rather than a carb. So hot rod guys running pump gas with a carb need to be more careful with fuel routing than EFI guys. Pump gas isn't a super good choice for carbs or performance use anymore so you really have to watch it.

Thanks for all the replies,


My reasoning for regulator after the carbs is that the pressure before the regulator can spike to whatever the pump is capable of free flow till the needle and seat closes again.
Where as after the regulator the pressure is always limited.

Guess I need to wet test the regulator and see what pressure it holds before the regulator and after the regulator with the flow dead head.

Your are overthinking it, put the regulator after the carb, as long as the pump is big enough the pressure will stay the same at the carb no matter what the needle & seats are doing

I misspoke on the lower pressure fuel regulators for carbs, if you're running more than 12 lbs. of fuel pressure between the pump and fuel pressure regulator you're going to have flooding issues with most needles and seats in todays carburetors
I have always mounted the fuel pressure regulator (sometimes two regulars if needed) before the carbs and set the pressure on them based on a fuel volume free flow test through one of the fuel bowls on Holleys carbs and with the top off the Carter, Edelbrock AFB, AVS style carbs Be very careful when doing any testing with race fuels

Depends.

The Magnafuel 9945, Product Engineering, Aero and others go in front, i.e. bypass the fuel before it is supplied to the carb.

Pick whose stuff you want to use and follow their recommendations.