secondary stumble

[sscargo]

'78 chevy Q-Jet set up like #2 recipe.
 Road racing at WOT. The secondaries open great through transition and then power is lost at just under 4000 RPM. almost like hitting a rev limiter. It seems like the float bowl is emptying. I removed the round phenolic insert and installed a slightly larger needle & seat.  and replaced the ignition module. and removed the fuel suction sock from the tank. No change at all. I would have at least expected a change in the time or rpm of the onset of power loss.  Any thoughts? Fuel pump?

Thanks
Mat

[Cliff Ruggles]

Matt, if it comes up clean thru transition, then power falls off, the bowl is going low or empty.  Pretty common with mechanical fuel pumps fed by stock steel lines, and worse on some set-ups than others.

What fuel pump are you using?  Size of the lines coming up from the tank, etc?....Cliff

[sscargo]

Thanks Cliff.
 It is a "84 Camaro 3/8" suction line. with a stock replacement pump. with no return line.  So the pump is likely the issue. I just don't understand why  removing the phenolic insert to increase the bowl volume didn't put the stumble a little further down the straight.?

Mat

[davis95]

I went through the same thing awhile back. I wound up putting a Carter electric street pump and the problem was gone.

[Cliff Ruggles]

Leave the insert in place.  It's a splash guard and needs to be in there.

Bowl capacity is meaningless with any high performance carburetor.  The fuel delivery system must be good enough to keep the bowl full at all times.  As the bowl goes low, the calibration, or fuel delivered to the engine changes. 

Same deal with aftermarket carbs, they must be kept full on hard runs.  The only advantage some have over a q-jet is two needle/seat assemblies for the power level.  Once you figure out how the keep the q-jet full, it makes a great high performance carb......Cliff