I will be puting together a q-jet for my truck with a 305, th350 and 2.73 gear with 28" tire. The carb I will be using is a 76 model that has had the jets, rods and hanger taken out but I have plenty of extra parts. The engine has headers, a 1972 cast q-jet intake and a stock replacement 350 cam. I need a jet/rod, air valve and float setting for fuel mileage. I have info on stock settings, I'm wondering if someone has a better combo.

If it's model with black cap choke on the side I'd start out with 72-73 main jet, 48K series primary metering rod. For the secs, use the richest rod you can find. Set the float at 5/16", make sure you have the link that runs from the choke pulloff to the secondary air valve in place. Put 3/4 turn of tension on the secondary air valve spring. If you have more than one power valve spring use the stiffest one.

The "K" series primary metering rods have a smaller (richer) power tip, they're worth finding.

Tom

Thanks for the info. What is the reason for using the rod from the pull off other than keeping the seconds closed til warm up? Do you recommend any pull off over another?

Originally posted by Tom Mobley:
...72-73 main jet, 48K series primary metering rod.Tom, does the 48K rod imply 0.048 diameter? If so, I thought the differential between rod and jet (rule of thumb) was 0.030-0.035?

Reason I ask, running XE268H in a 350 and its rich to the point that the plugs are black. Jetted down from 72 to 70 jets and now it has a stumble. Haven't been back to it again to play with it.

Marty

That link doesn't do that, a little interlock on the sec throttle shaft does. The link to the pull-off dampens the air valve so it doesn't just flop open and cause a bog. Some guys eliminate it and just use the return spring tightened up a bunch. I see this as covering up one problem with another. The spring doesn't like to be tightened more than about a full turn or so and running it overtighened will keep it from opening all the way.


Robert,

48 means .048" at the thick, cruise/economy part of the rod.

There's differences in the way the jets and rods interact between the older, divorced choke style and the newer integral choke style. The divorced choke style uses a larger spread between the jet and rod numbers. Something to do with the size and placement of the main air bleeds. Also the rods are different length between the two styles of carbs and are not really interchangeable.

The "K" series rods are the integral choke carb equivilent of the older "B" series for the divorced choke style. the power tips are are smaller, about .025" instead of around .035". This is a big difference, most performance cars will really respond to this.


Tom

Thanks Tom, you fixed a problem I did not ask about.

Marty,

That's cool, what was it?

Tom

I do not have the rod from the pull off to the av on the q-jet I'm running now and it bogs. I thought I had another problem because I was under the impression that the rod worked along with the lock thingy to keep the seconds closed til the engine warmed up. BTW, what should the setting be on the apt screw?

Wow, not many guys get into the APT screw deal. Is yours in the baseplate or in the main body?

In practical terms the only way to adjust it is with a meter that will read the CO in the exhaust. Most performance type engines will want at least 1% at 2500RPM. Higher perf engines will run better with more, up to about 3-4%. Past that are dump truck numbers. Run the engine in neutral at 2500 or so, adjust the APT. If you have a integral choke carb with the APT in the main body and are using the recommended K series rods, you'll be disappointed. the economy portion is not tapered so screwing the APT up and down doesn't do much.

early type carbs with the APT in the baseplate and the B series rods can be adjusted to whatever you want.

Mostly I'd go for about 1% cruise on mild street cars. If the car has any emissions stuff or is running on oxygenated fuel then all bets are off. The readings are farkled by A.I.R or any catalytic converter. If a guy has a 4-das analyzer he can use the oxygen reading just as well as CO. There's charts around that compare EGO with A/F mixtures but I don't know those numbers offhand.

Tom

I was wondering about how far to turn it out. I think stock is two turns and I was thinking there may be a better setteing. No smog stuff on this truck, I live in "Missippi".