going lean at part throttle

[poncho2]

just finished a qjet for my granddaughters truck (7044202) and put it on a freshly built 350 thats pretty much stock.  the qjet has the following specs idle tubes .036, down channel .046, upper and lower idle bleed .070, bypass air .080, mixture screw .090, primary jets 69, rods 42 with .026 tips, main air bleeds .046 & .046,  secondary hangar L, metering rods DA.  Using the wide band shows that the idle is pretty close at 13.5 but it seems fat at cruise (12 to 12.5) WOT is around 11.5 and it goes really lean when at part throttle at around 15. I raised the APT to get it to where it is now but the tendancy to go lean still persists when trying to accelerate.  all of the parts came from Cliff and im just looking for some suggestions where I might start trying to correct this. Thanks

[77cruiser]

15-1 isn't too lean unless it stumbles. But 12.5 cruise is too rich. Did you make a change to the power piston spring?

[Cliff Ruggles]

How does the carburetor actually work?

Throttle response/transition right off idle?

Cruising at varying vehicle speeds at very light throttle.

Heavy part throttle only?

Transition to full throttle?

Power at full throttle?

I ALWAYS tune for results first and start with idle mixture and "heavy part throttle". 

This establishes whether or not I have the idle system up to par for the application, and that I'm using the correct main jet for it as well.

Once the idle system is correct, and correct main jet, tuning the "part throttle" via the APT/primary metering rods, and WOT with hangers, air door settings and secondary metering rods is relatively easy.

Don't forget timing either, the distributor is equally as important as the carburetor, and the more timing you add when the engine wants/needs it, the LESS fuel you will need from the carburetor.  The carburetor by design is also a "load sensing" device, and it will vary A/F based on engine requirements, throttle angle, air flow past the venturi, and pressure differentials above and below the venturi.

Two other load sensing devices are the distributor vacuum advance and the power piston.  The distributor mechanical advance is engine speed controlled, and a player here as well.......Cliff

[poncho2]

Well I went back into the carb and put a stiffer spring under the power piston and that got the light part throttle back to around 13.5 afr. I reset the initial timing from 8 deg. to 12 deg. with a total of 31 all in at 3000.  modified the vacuum advance to restrict it to about 10 degrees. the truck runs great with a good transition from idle to part throttle, no stumble anywhere in the power range including going to wide open throttle. secondarys kick in instantly with no hesitation. Thanks for the info.

[Cliff Ruggles]

Good news!.....