Need help--falling fuel pressure

[DFRESH]

Just put a new system in the car---750 Holley DP (Proform), Ricks Tank, -8 hose, mechanical pump (Holley 9 pd). I am not running a regulator. Here's the story---on the guage (right at the carb) it started off strong right at 9--but as the car got warm, it was dipping down to like 3 pds.

Now, I am used to FI--my pressure on the race car never moves---well some but only on a pass and very small movement at that. This is idling and me trying to tune the carb a bit. I know heat can affect the pressure, but down to 3 pds? I took the guage off and checked for debries---nothing. After it cools down a bit--it will jump up a few pds--but drop back to 3. I haven't dealt with a mechanical system in quite awhile---one other thing--Hector told me to block off the vent on the tank, and vent through the cap--he gave me a part # to use. Do I need a regulator with this mild of a system?

Doug

[Vegas69]

Replace your fluid filled gauge with a regular.

[DFRESH]

Friggn cars. :lol:

[70rs]

Replace your fluid filled gauge with a regular.

Hey Todd. I am NOT in disagreement with you at all here. But what does the fluid filled gauge have to do with it? I am only curious so I don't make life difficult for myself down the road.:cheers:

[Vegas69]

The SOLUTION heats up and causes the gauge to be innacurate. Refer to page 16 of my 189 page thread.:D My gauge would go to 0 and the car still ran. They don't work well in a high heat, low pressure situation. My standard auto meter fuel gauge has been working fine ever since.

Liquid
http://i200.photobucket.com/albums/aa251/Payback1969/IMG_0349.jpg
Dry
http://i200.photobucket.com/albums/aa251/Payback1969/008-1.jpg

[DFRESH]

Todd, swapped the gauge---holds steady now---makes sense--the liquid gauge is sealed and is affected by barometric pressure--since it is so close to a major heat source, this whacks its accuracy---thanks for the tip--saved me a lot of further disassembly and evaluation. I can get on with the rest of it now--my off idle stumble for instance---thanks again bro.

Doug

[GregWeld]

Hate off idle stumbles... there's so many little things that can cause that!

#1 - Get EFI! LOL

Just tossing out some items that you probably already know...

Check the plunger for the accelerator pump... that gap has to be just right... forget what the number is but .015 comes to mind...

Here's something that could be causing your stumble.

There are two and sometimes three holes in each pump cam, numbered 1, 2 and 3. Placing the screw in position #1 activates the accelerator pump a little early, allowing full use of the pump’s capacity. Generally, vehicles which normally run at lower idle speeds (600 or 700 RPM) find this position more useful because they can have a good pump shot available coming right off this relatively low idle. Positions #2 and #3 delay the pump action, relatively speaking. These two cam positions are good for engines that idle around 1000 RPM and above. Repositioning the cam in this way makes allowance for the extra throttle rotation required to maintain the relatively higher idle setting.

The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump’s capacity and the profile of the pump cam. The period of time that it will take for this pre-determined amount of fuel to be delivered is affected by the pump nozzle size.
A larger pump nozzle will allow this fuel to be delivered much sooner than a smaller pump nozzle. If you need more pump shot sooner, then a larger pump nozzle size is required. During acceleration tests, if you notice that the car first hesitates and then picks up, it’s a sure bet that the pump nozzle size should be increased. A backfire (lean condition) on acceleration also calls for a step up in pump nozzle size. Conversely,if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size is required.
Holley accelerator pump nozzles are stamped with a number which indicates the drilled pump hole size. For example, a pump nozzle stamped “35” is drilled .035". Pump nozzle sizes are available from .025" to .052". Please note that whenever a .040" or larger accelerator pump nozzle is installed the “hollow” pump nozzle screw should also be used. This screw will allow more fuel to flow to the pump nozzle, assuring that the pump nozzle itself will be the limiting restriction in the accelerator pump fuel supply system.

Also - the transition from idle circuit to the mains.. don't want to have those idle circuit slots uncovered... Idle speed set too fast & mixture is too lean...

Float level too low...

This can also be caused by a vacuum leak...

[Vegas69]

Todd, swapped the gauge---holds steady now---makes sense--the liquid gauge is sealed and is affected by barometric pressure--since it is so close to a major heat source, this whacks its accuracy---thanks for the tip--saved me a lot of further disassembly and evaluation. I can get on with the rest of it now--my off idle stumble for instance---thanks again bro.

Doug

Good deal....:thumbsup:

[70rs]

Thanks for the clarification Todd. I had no idea those gauges were affected that way.:thumbsup: