Fat Hack
08-11-2004, 07:54 PM
Okay, here's the running update. After seeing that my Carter electric fuel pump looked to be delivering too much pressure to the carb, I installed a Holley regulator to dial it down a notch. It still blew past the needle and seat and made a big mess!
Okay, so I dug up a Sun fuel pressure gauge and plugged the line going from the regulator to the carb into it. 5 psi on my first test. The regulator is supposed to have a 1-4 psi range.
Holley regulators are adjustable, so I dialed it down as low as I could get it...4.5 psi.
Too much.
That got me to thinking...I bought the 12-804 regulator, which has a 1-4 psi range...but suppose what was in the box was the more common regulator, which has a 4.5-9 psi range? That would jive...as the lowest pressure I could get out of it was 4.5!
So, I pulled the regulator off and returned to the speed shop with it. We pulled another and opened the box to inspect it. Here's a tip for ya...they look IDENTICAL, and are built using the same housings, so visually, there is NO WAY to tell the difference! I took another 12-804 home and hooked it up.
Following the counterman's advice, I put the gauge on the line comming from the pump and gave it a test. WOW! It jumps around between 4 and 7 psi! Now I see exactly what the regulator does!
Next, I hooked up the regulator and plumbed the output into the gauge. This time, I was able to dial it down to just a hair under 3 psi. Better than nothing, I suppose!
Now, this will probably seem overly basic to many of you, but me having always used mechanical pumps and never being one to run a fuel pressure gauge, I found it kinda cool to see how the regulator not only steps the pressure down, but smooths it out to eliminate the "pulsing" effect...delivering a constant pressure reading. Neat!
Problem is...the regulator can only step the pressure down X amount from what the input pressure is. I'm gathering that the most it can knock off is 4 psi, since my pump can deliver 7 and I can only dial it down to 3.
I may yet have to step down the voltage to the pump just a pinch...it's got battery voltage now, but with the car running, it'll have up to 14.5 or so. That could spike the pressure more than the carb will allow. Cutting the voltage to maybe 10 volts might keep the output pressure down to 6 psi, meaning one regulator can step it down to 2, which is where I'd like it to be.
Interesting stuff, actually...kinda cool to learn new things and engineer answers to different problems...makes this hobby a little more fun and challenging!
Now, on to the carb questions:
On those boxy little two barrels found on early 2.8 V6 engines, what is the fuel pressure specification? I don't have a manual that goes newer than 1979, so I can't look it up here.
Also, just to be sure...is the 3/8" nipple that comes out of the top of the carb a bowl vent? It's pressed into the carb body and my guess is that it could only be a vent, but there is another vent or vaccum line in back of the carb, too.
(The fuel line actually enters at the bottom of the float bowl).
Thanks...might be some older GM dealer techs out there who've dealt with these carbs before!
Okay, so I dug up a Sun fuel pressure gauge and plugged the line going from the regulator to the carb into it. 5 psi on my first test. The regulator is supposed to have a 1-4 psi range.
Holley regulators are adjustable, so I dialed it down as low as I could get it...4.5 psi.
Too much.
That got me to thinking...I bought the 12-804 regulator, which has a 1-4 psi range...but suppose what was in the box was the more common regulator, which has a 4.5-9 psi range? That would jive...as the lowest pressure I could get out of it was 4.5!
So, I pulled the regulator off and returned to the speed shop with it. We pulled another and opened the box to inspect it. Here's a tip for ya...they look IDENTICAL, and are built using the same housings, so visually, there is NO WAY to tell the difference! I took another 12-804 home and hooked it up.
Following the counterman's advice, I put the gauge on the line comming from the pump and gave it a test. WOW! It jumps around between 4 and 7 psi! Now I see exactly what the regulator does!
Next, I hooked up the regulator and plumbed the output into the gauge. This time, I was able to dial it down to just a hair under 3 psi. Better than nothing, I suppose!
Now, this will probably seem overly basic to many of you, but me having always used mechanical pumps and never being one to run a fuel pressure gauge, I found it kinda cool to see how the regulator not only steps the pressure down, but smooths it out to eliminate the "pulsing" effect...delivering a constant pressure reading. Neat!
Problem is...the regulator can only step the pressure down X amount from what the input pressure is. I'm gathering that the most it can knock off is 4 psi, since my pump can deliver 7 and I can only dial it down to 3.
I may yet have to step down the voltage to the pump just a pinch...it's got battery voltage now, but with the car running, it'll have up to 14.5 or so. That could spike the pressure more than the carb will allow. Cutting the voltage to maybe 10 volts might keep the output pressure down to 6 psi, meaning one regulator can step it down to 2, which is where I'd like it to be.
Interesting stuff, actually...kinda cool to learn new things and engineer answers to different problems...makes this hobby a little more fun and challenging!
Now, on to the carb questions:
On those boxy little two barrels found on early 2.8 V6 engines, what is the fuel pressure specification? I don't have a manual that goes newer than 1979, so I can't look it up here.
Also, just to be sure...is the 3/8" nipple that comes out of the top of the carb a bowl vent? It's pressed into the carb body and my guess is that it could only be a vent, but there is another vent or vaccum line in back of the carb, too.
(The fuel line actually enters at the bottom of the float bowl).
Thanks...might be some older GM dealer techs out there who've dealt with these carbs before!