Here is the problem, when I swapped from tpi to carb I used a mallory 3 port afpr. I am still using the stock TPI in tank pump.

When I set the psi to about 6 and then make a pass. I check the psi and it show maybe 4.5 to 5. Then when I adjust it back up to 6 and make a pass. I check it and it will be like 7.5 to 8 psi.

Is it possably the mallory regulator or maybe the guage? Is my fuel pump going south? Would voltage drop to the fuel pump do this?

I am running a bypass line, so all excess fuel is going back to the tank.

What do you guys think?

 

Try begging/borrowing/stealing another gague first before you decide there's a real problem. If the fuel pump's output changes quite a bit it will have some effect on the final fuel pressure, but that's quite a change. I would be more concerned about the fuel pressure DURING a run. I always duck-tape a gague to my windshield (temporarily!) to see what kind of pressure I'm getting at WOT, under full load, throughout the RPM range. That's what really matters.

If there's a weak link in your fuel system this kind of testing will definitely find it. Unmistakable- fuel pressure will nose-dive in the upper RPMs at WOT.

 

There was another topic I posted on with your similar problem. More than likely it's the fuel pressure gauge itself. The underhood regulator mount style are very sensative to heat and can affect readings both ways. We had this issue in our pits with a few buddies running both summit nonliquid filled and liquid filled fpg, they would come back after a pass and be reading 0-2psi at this low of pressure the car would be lean stumbling all over itself but they ran great 11 second passes.

In my case I have a Holley liquid filled gauge mounted on my reagulator as well as a Autometer phantom fpg mounted under my cowl away from engine heat. The one under the cowl reads rock steady at 8.5psi whereI have it set no matter if the cars cold or been driving on the street for awhile, while the gauge under the hood will show 8.5psi when first fired, but after a pass it's common for it to jump into the 11-12psi range.

 

my summit non-liquid fuel guage has turned to crap from the engine heat too. it reads 8+psi even if i'm only running 2 psi through my autometer guage. i wouldn't trust that guage at all, but i guess that's why it's $13.

 

Yeah, I have a summit liquid filled gauge. It is on the fuel line beside the carb.

Maybe I should put it in the afpr I have mounted on the strut tower? It would have less heat than beside the carb?

 

Use the correct fuel pump for carburetor, get rid of the mallory regulator, and you'll have no more problems with pressure.

 

I have the same problem, as soon as I close my hood the psi drops from 7psi to 3psi but if I leave the hood up it stays the same. Mine is a liquid filled so when the guage shows the presure dropping I get a spray bottle w/ water and spray it on to the gauge and it brings the psi were it should be.

 

Gauges that are liquid filled are sealed, so when they get hot and the silicone/glycerin expands there is pressure built up in the case that counteracts the expansion of the bourdon tube thats reading the fuel pressure. (the little piece that moves the needle up the scale)Non filled gauges are referenced to air pressure and don't do it as much. They would have to get real hot to createthe same effect. The cheap non filled gauges just break from the lack of smooth movement and vibration.
Just thought i'd throw that out there for ya.
Jp

 

So I should set my fuel pressure when the car is cold. Then check it again after everything cools down. Just to make sure the psi is staying the same.

Still wouldnt be a bad idea to take it off the dual feed line and put it directly on the apfr.

 

Definately set it when the motor is cold that way radiant heat wont affect it as much and dont be surprised when it starts showing different readings after enerything is warmed up, just the nature of the beast unforetunately. This is an old pic, but shows how I had mine set-up. Actually still using the same system but now also have the line going to the fpg mounted under the cowl of my hood.

 

Then here is pic (during construction) but you can see the same gauge used in above pic, but the new regulator has a port that tied into for the cowl mouonted fpg-that's what the little braided line is for coming out of the regulator

 

thanks for the pics.

My regulator is over on the driver side strut tower. Then the guage is on the dual feed line at the carb. It would probably be cooler if the gauge was over at the strut tower. I might try that.

Should read the same at either place. I guess I just need to set it cold and leave it alone.

 

I had a liquid filled holley guage, around $42.00 and from the start the thing seemed kinda shaky... I dont really know how it happened, but the little rubber cap fell off the back side of the guage and the guage was shot.... Right now I am using a non liquid filled Mr. Gasket guage that i got from the local parts store for around $12.00 and it works fine. It bounces around a little bit like a non liquid guage would expect to do. As far as the regulator placement, mine is about an inch above the feed hose. I used the mount that came with the holley AFPR to mount it on the side of the engine bay. Looks decent...

 

The main thing is that I want to make sure the gauge is reading right when I set the PSI. Even if it is just when it is cold.

I have a fuel pressure tester. I guess I could set the afpr and try it. Then I can be sure the afpr is working right.