Fuel Pressure Question

[peterpsg]

Does anyone else have a fuel pressure gauge on the supply line by their Carb?
(Holley 750 in my case, so only ~1-14 ish psi)

Does the pressure needle dance all over the place (hi-lo-hi-...) constantly while the engine is running?

My fuel lines have been joined by a T, then feed thru a single large capacity pump, then via an inline filter up to what I think is a Reg? (smallish cylindrical metal thingy) , then on to the carb...

Clive, if you set this up can you elaborate a bit ???

Cheers,
Pete

[steve_d]

I have both a mechanical Gauge by the carb and a pressure sensor for the Stack dash, both stay steady. Holley carb should be running at about 6psi 14 is too high.

Steve

[gtrclive]

The Float needles will open at anything above 8 psi, always set mine to 6-6.5 psi....

[peterpsg]

Ok, I'll take a video of the pressure gauge doing it's thing.

I am now wondering if the fuel pump might not have an internal return bypass? thus causing the pressure to 'pulse' or surge.

Perhaps I might need some kind of fuel swirl pot to maintain a steady volume, with a suitable return line to the tanks?

I haven't even considered pulling the carb apart to adjust the needle pressure yet, I'll leave that bit up to PK !!!

[davefiddes]

Sounds like the fuel pressure is way too high. The fix is to fit a regulator of some sort. Bypass ones (like EFI) are the best but the dead-head ones work OK too. What sort of pumps are you using? The Ultima spec Carter pumps produce a nice steady 6 psi and don't need a regulator. As clive says more than 8psi is bad for a carb.

My brother has an 85 Camaro and the fuel pressure guage on his car and the needle kicks about as you describe. The car has a mechanical fuel pump (with an electric primer oddly enough - all stock GM) though so you'd expect it to kick with each turn of the crank. The upper limit is 6 psi when the regulator in the pump kicks in.

[andygtt]

sounds like the regulator has gone.... it was set to a steady 6.5psi.

I found the regulators unreliable and only knew as I had the guage on the carb.... but I thought you had a better one than the usual?

[peterpsg]

Is that the smallish cylindrical thingy after the pump/filter and mounted on the bulk head?

[andygtt]

yes

[peterpsg]

Swapped the pressure gauge around with another one, and the needle still flicked around but only +- 1psi... maybe the gauge is a bit naffed???

Does any one else (who's holley carbed and has a fuel pressure gauge by the carb) get any flickering, even just a little???

Cheers,
Pete

[peterpsg]

Spoke to the man (PK) about the issue, looks like it's quite common for these cheapo fuel pressure gauges to go AWOL, so I ordered a couple from real-steel as they're pretty cheap; will fit and report.

Note that these gauges apparantly have an 1/8th NPT taper thread, but I think you can get adaptors from places like DemonTweaks etc if you want to change it over to a remote one...

[Graham-P]

Buy an oil filled gauge, should damp any eratic movement, about 25 quid.

[andygtt]

I didn't think yours was the usual cheapo one. the holley ones are rubish imo and I used a few.

graham is probably right and you just need to top up your guage to stop it bouncing.... the vibration does tend to make them leak it out over time.

[peterpsg]

andygtt said:

... top up your guage to stop it bouncing....

From what I can see the flexible metal tube is open to the fuel flow, so I can't imagine that it's full of anything but fuel, so unless you mean topping it up with petrol...!?

The gauge is pretty damn simple, just a flexible curved hollow metal bit attached to some leverage setup that twists the needle around the dial, no fancy bits here!

[Graham-P]

I have this one (oil filled) I damaged my previous one fitting the carb ..

[steve_d]

peterpsg said:

andygtt said:

... top up your guage to stop it bouncing....

From what I can see the flexible metal tube is open to the fuel flow, so I can't imagine that it's full of anything but fuel, so unless you mean topping it up with petrol...!?

The gauge is pretty damn simple, just a flexible curved hollow metal bit attached to some leverage setup that twists the needle around the dial, no fancy bits here!

You do not have an oil filled one if you have inspected the flex tube. On an oil filled gauge all the mechanism at the back and the dial face is full of oil to damp out rapid movement.

Steve