Your problem it is a known issue on some 701/Savannah airplane (Savannah is a "derivative" of CH 701 design).
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First of all i will pinpoint another common problem of easy solution regarding fuel system. If you have progressive fuel pressure drop with altitude please check that the rubber cap on the back of fuel press instrument is removed (or drill a hole in it). Otherwise the instrument has not a valid "reference pressure" with false readings.
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Returning to question Your problem with pressure drop with high power settings and 5 second lapse(i know it seems more , an eternity when you are in air...) for pressure recovery after electric pump it is switched off are experienced by a very large number of users. IT IS NOT AN INSTRUMENT OR ELECTRICAL/SENSOR PROBLEM. Even in my Savannah plane, that has a mechanical fuel pressure gauge in the cockpit, i have the same problem and , at first, i was very scared.
It is very interesting the post of R. Lee of bad fuel in the circuit. I''l check asap.
Another ipothesis i have heard is that it can be a false reading by fuel press gauge circuit section due for example to fuel vapours slowly vented by anti vapour lock hole (one of the ipothesis at the date) but, i repeat, it is seems NOT A SENSOR PROBLEM.
in my case it seems very related with fuel quantity. It worsened when fuel it is at maximum level. But in my plane, as in your and in others vents are ok. Fuel discharge from right vents with full tanks (rx tank is where fuel return circuit goes into is suspect for tank "pressurization")
Another ipothesis suggested to me it is that fuel venting circuit has a form of "inertia". Therefore when electric pump it is switched off for some seconds anti vapour lock circuit has an excess flow with conseguential pressure drop until it "equalize". But this do not explain immediate pressure drop adding power...
I am absolutely not an expert but speaking with other owners of this models of plane it seems that even manufacturers know this issues but have no sure explanation for these at the date.
It will be nice if someone with data acquisition hardware will put flow and pressure sensor before the carbs , at pressure gauge instrument and in the anti vapour lock section to identify solve this issue.
In the meantime personally i use do not fill the last 5 liters in the tanks (20 minutes less endurance it is not an issue for me) to minimize the problem and to not have fuel "spilled" by the vents. (checks the flaperon for gasoline deposit beneath the vents to see if you dump gasoline)
Last advice not related to this kind of issue but very useful on light plane with hot engine compartment.If you have "winter" fuel (high volatility) in the tanks (usually you do not fly for long due to bad weather) please at the first warm day of spring dumps this (for expample in your car) and substitute with fresh "summer" fuel (low volatility). Infact fuel formulation sold at the pump changes with temperature and using winter fuel in a hot day it is the perfect recipe for vapour lock.
Nicola di Biase
