How many of ya'll have fuel pressure gauges?

I have a good Pro-systems carb on my car, but tuning the idle is proving to be a difficult task. Any kind of transition and WOT is killer as expected, but getting it to idle consistent and not be pig rich isnt working well.

A couple years ago when I got the carb I talked to Patrick about the same dilemna. He always told me that I needed to get the fuel pressure nailed down, respectively. My mechanical fuel pressure gauge fluttered so bad it was almost unreadable with the engine running. IIRC, he told me to get a liquid filled gauge to settle it down. I now have a liquid filled gauge, and sometimes when the car warms up the fuel pressure appears to creep down and down. I had the car out today for some carb tweaking and the fuel pressure was just below 2 PSI. Normally when I run the car it varies anywhere from 8 down to 6.

How can I be certain of what the pressure really is?? The fuel system I have is bare bones basic. Factory 3/8 line with the pickup sock removed to a Fram HPG1 filter (element just changed), -8 line out of HPG1 to a Carter 172 pump (fresh also), -8 from pump to carb. I've had the lines all apart and blown compressed air through, changed the fuel line style on the carb, double checked all routing of lines for sufficient heat clearance, changed gas tanks last year thinking something could be floating up in the tank to the pickup, etc. etc. I just cant nail it down.

What causes fuel pressure to drop randomly when its hot out? Is it really dropping? How can I check?

Sorry for the long post, I'd just like to bury the hatchet on this problem so I can get a nice clean consistent idle.

I have a mallory liquid filled guage for reference, I set it hot, then when its cold the fuel pressure can read as high as 10-11 psi. I've never had any issues.

Liquid gauges do this all the time...they are POS. Look for a little rubber nipple on the back side of the gauge, when the motor gets hot and the gauge drops in pressure peel pack the nipple just a little and let some air in, the pressure will come back up. I fought this on a nitrous system a long time ago and it drove me nuts.

What Ratt Vette just said is something to consider, I have never heard anyone mention this, makes sense, but on sealed gauges with no rubber expansion or fill plug, I don't know how this is handled, maybe that why the glass is half full of fluid.

I was gonna say that if you had a mechanical fuel pump,, I would be looking at the fuel pump pushrod and or the fuel pump cam lobe, on the cam.

Very common problem on those little 1" liquid filled gauges.
7psi cold. 2-3 hot. So I put in a temporary T and another Matco, vacuum/fuel psi gauge. And we have 7psi on the Matco gauge and 2-3 psi on the liquid gauge. Cold....both read 7.

Get a better gauge and your pressure problem will go away. The bouncing may be a bad ck valve in the pump though.

Thank you guys, you hit the nail on the head!

I took the car for a ride and came home. With the car idling, the gauge showed 4.5 PSI of fuel pressure. With the way the gauge is set up currently, the plug is on the side of the gauge. Heres where it gets good. :)

I took a small screwdriver and pried the corner of the plug out a little. The pressure instantly went back up to 7.5ish! :beers: However, apparently I overdid it a bit when I saw the pressure go back up because the plug popped out and I didnt see where it went... So here I stand, with the car idling and my finger over the hole so more of the liquid doesnt leak out... and the plug is nowhere that I can see. I reached out and grabbed a T-shirt that was laying there, and wedged it between the gauge and valve cover. All said and done, the gauge is only about an 1/8 full of fluid now, and its a little jumpy again.

Blue66, what would you consider a "good" gauge?

I said "peel back the rubber nipple just a little"...... ha ha.......hope it works better now.