Engine will not make power after carb install, post-overhaul

Connor Wilkinson wrote:

Propeller is a sensenich, model W68T2ET70J, so a 68” dia. With a 70” pitch. Found the yokel who put this “special mixture” system ok had removed the fuel vent lines so we put new ones into the Rotax aluminum intake air box that’s on the aircraft, and rpm went from 5750 to 4860 at WOT. Adjustments to idle mixture screw have no effect, enrichening or leaning. At this 4860 RPM, fuel flow indicates 5.2 gallons per hour with boost pump OFF.  This seems like a high fuel flow value for low rpm, possible the prop is too coarsely pitched? 

Then don't feed the vent lines into the air box and tuck them into the bowl clips like stock and it sounds like you'll have the rpm back... 

Nickolas,

"Then don't feed the vent lines into the air box and tuck them into the bowl clips like stock and it sounds like you'll have the rpm back... "

I do not understand your reason for the above advice.

The main function of the "vent" lines are as breathers, to equalise the pressure in the float chamber, with the  carburettor inlet air pressure .

If the air box mentioned is an OM Rotax unit, the float breathers should be connected, so that the air pressure/carb inlet, within the air box, is the same as  a float chamber.

If the air box is not a Rotax - principal remains, however result may not be as for Rotax.

Conner,

."....idle mixture screw have no effect, enrichening or leaning.....4860 RPM,.fuel flow indicates 5.2 gallons per hour with boost pump OFF."

Mixture screw is only for idle and should be set to 1.5 turns from seated - in most instances there will be no benefit in playing with/changing this setting.

5.2 Gals/hri (19.7L/hr) normal for WOT BUT what is not normal is 4860 rpm, at this power setting UNLESS your prop is very coarse.

Static RPM is a hotly debated topic - Rotax recomend 5200 rpm, many operators opt for a lower RPM, in the hope of a higher cruise speed- this impacts negatively on TO performance but may be acceptable when always operating from long strips. Climb out should always be above 5200 rpm.

Boost pump should have no effect on / off to the fuel flow. In most 912 initiations, the Boost pump is a back up for the Mechanical, used in TO/Landings, in case of failure at a critical flight stage.