|Performance Tune Ypur Q-jet Secondaries Part 2|
Written by: Damon Nickles
Secondary metering rod measurement and performance is a tough one. You can look at the specs all day long and you won't know what's gonna work or not unless you try a few and start to get a feel for what works and what doesn't. Basically, secondary rods are tapered. They start out at a large diameter and work down to a small diameter at the tip. The large part of the rod's taper is what meters fuel with the air valve closed and as the air valve opens you work your way lower and lower on the taper until you get to the metering tip- at which point the A/V is all the way open and you have max air and fuel flow. Once you're on the tip, no further enrichment is possible with that rod. Rods with "Short" tips reach their maximum enrichment (power tip) at approx. 90+ degrees of A/V opening, medium rods reach maximum enrichment at approx. 80+ degrees and long tips reach maximum enrichment at approx. 70+ degrees of A/V opening. Medium tips are usually a good way to go and are the most "tunable" with a selection of hangers. Speaking of which.....
The hanger does nothing more than determine how far down
the taper you START on initial air valve opening (by how high or low the little holes in
the hanger are where the rods attach). So the hanger can richen or lean the entire
calibration by starting the rod either higher on the taper (leaner) or lower on the taper
(richer). The only part of the calibration it DOESN'T affect is once the A/V is
fully open and the rod is down on the power tip (maximum enrichment). So the hanger
really affects the a/f ratio AS THE A/V is OPENING, but not once it is FULLY open.
The long and short of it is this- almost ALL Q-jet rods are way too lean in the initial 30 degrees of A/V opening (DR rods being a notable exception). Also, the hangers don't make a very large difference in calibration- definitely not enough to make up for a bad set of stock rods. Play around with hangers from "B" (full rich) to "J" (middle of the road) but don't even bother with the leaner ones- they're useless for performance.
The vacuum can is the vacuum, um....., well.... CAN on the side of the carb, passenger side near the front. It uses a little pushrod to connect it to the A/V door shaft arm near the rear of the carb. This vacuum can uses a TINY LITTLE vacuum restriction at its inlet so that the vacuum can releases very slowly. This in turn slowly allows the secondary air door to open. Slowly because if you let it "snap" open the thing will bog badly when you open the throttle quickly. Stock cans usually take 2 seconds or more to open. For performance applications you want one that takes about 3/4 of a second.
Getting a faster opening can is a matter of either
modifying your own (TINY TINY little set of drills can do it IF you can even get to the
restriction point on your can) or finding one that opens quickly and will bolt onto your
particular carb. No easy answers here. You can comb junkyards and look at old
Q-jets or you can try putting a vacuum can from an Edelbrock newly manufactured Q-jet
on. The vacuum can for an Edelbrock #1910 performance Q-jet should fit your late