Either clogged fuel tank vents, fuel strainers or pinhole leaks in the fuel line somewhere (There have been incidents of pinholes developing under the friction tape used to secure the fuel lines in various places).
A float problem in the carb will not cause your fuel pressure to decrease with RPM. That's a symptom of fuel starvation. Sunk floats tend to cause a rich fuel mixture and usually the aircraft still runs under full throttle. It just won't idle- it floods the engine out.
There's a good article on this on Cy Galley's Bellanca website. Go here and look at the tech articles.
http://www.geocities.com/CapeCanaveral/Hangar/5511/
Cy- if you read this, I hope you don't mind me copying your article about fuel tank vents.
Here it is:
Tank Vents
(all)
by Cy Galley
The following is a letter I wrote after a phone call for help from a member. I thought it was good information for everybody.
Symptoms: his fuel pressure drops under full throttle. Although he has a Cruisair, it applies to all aircraft, cars, motorcycles, and boats, too. If air can't get into a tank, you can't get the gas to come out either. He had already checked the pressure relief bypass by plugging it.
"I have given your fuel pressure problem much thought. From your description of the pressure dropping off, I think you ought to check your tank vents. It sounds like they are partial plugged. I have always wondered why that there was a vent hole drilled in both of the caps on my airplane even though there is a vent line that comes out of the back of the each tank. If the air can't get into the tank, you can't get the gas to come out either.
This is my theory of the situation. With full tanks, the regular tank vent may be covered by gas. It is not a very large tube so that this provides a restriction to the fuel pick up by the engine driven pump. The wobble pump being closer to the tank can overcome this restriction. Even Cessna retrofitted vented caps on the 172. If these vents become clogged with dirt or paint, then there is not enough venting for the tank and the pressure drops off. There also could be some thing floating in the tank that gets washed up to plug the back tank vent.
Also, mud daubers like small round holes. This can happen to any open hole over night. Another reason for vents to plug, airspeeds to quit.
Before you start digging into the pump once again, I would most definitely check the vents. You could even leave the cap of the tank in use a little loose instead of tightening it down completely and then make a flight check. (There is a secondary cover on the Cruisair so the cap won't fall over board.)
You can clean out the vent tubes by pushing an old tach or speedometer cable through them.
Hopefully your tanks have NOT been sloshed. This coating sometimes comes loose and causes all sorts of problems. The last thing could be some dirt or rust in the bottom of the tank that is sucked up onto and restricts the screens.
Notice that I said "screens" as there is a screen in each tank, two screens in the gasolator, a screen in the engine pump, and a screen in the inlet of the carb. There might even be a screen in the wobble pump. All these must be clean, free of all lint, dirt, sand, rust, paint flakes, and other nasties.
If you have ever used any silicone rubber (bathtub caulk) or teflon tape as a sealer, you need to disassemble each joint, carefully clean and reassemble without any goo. If the joint leaks then parts are damaged and need replacing
Remember, lineboys have been pumping junk along with gas into your tanks for several decades. This junk needs to be removed. In my own plane, I have carefully drained all the gas out of my tanks through a metal tractor funnel. These funnels have a very fine mesh screen to catch junk before it goes into a tank. You would be surprised at the amount of sand, rust, paint flakes, and other solids that came out of my tanks that was caught in the funnel. After draining, I removed the quick drain. More gas will come out so that all draining of gas should be into metal tanks, grounded to the airframe. Do NOT use any plastic hose, funnels, or tanks. It only takes one spark to set it off. Make sure all electric devices are off. All flames extinguished, and the hangar door is open for ventilation. Gas fumes are very explosive. Take all precautions. Have a fire extinguisher handy. Burned corpses are not very pretty... Besides it ruins a good airplane. After I had everything drained, I used mineral spirits, (paint thinner or Stoddard solvent) to flush the tanks until I stopped getting junk out of the drain into the funnel screen. You can use a pump-up garden sprayer to agitate the crud to the drain. If you run it though the funnel, just dump the filtered mineral spirits back in and flush again until it comes out clean. I would still check any rubber hose connections. They might have swelled from the solvents in the either autogas or 100 LL. The old mil-spec 6000 hose does not resist solvent degradation. 100 LL is especially bad as it has toluene as an ingredient for octane enhancement. As a side note, I would also replace all the 6000 hose connections from the engine to the oil cooler on a regular basis as they harden and get brittle from heat and age. After all tanks, lines, and vents are clean, you should have trouble free flying. Good Luck."
