Last evening I finished rebuilding my '76 Q-jet. I used a kit from NAPA for the job. While the carb was apart, I also used my numbered drills and opened up the idle discharge ports as described in Doug Roe's book. Using the shank end of the drills to measure, I determined that the ports were .071-.072 (#49 .070 fit, #50 .0730 didn't), I opened them up with a #42 .0935" bit. The book called for .095", so it's pretty close. I'm using the .125" needle valves. I left the float level alone, it was at 5/16". I kept the stock #75 jets, and 50K primary metering rods. My junkyarding yielded a set of CP secondary rods (.0567 power tip), I installed those with a J .560" hanger. I set the choke linkage up just as it was stock. I installed the carb on my 462, and it fired right up. The motor was cold, so the closed choke allowed a quick start. One thing I am missing, is the proper tube to come from the heat stove in the intake, to the choke housing. Choke pulloff is a bit slow. I'm trying to find some tubing now to fix that. The only problem I'm having with the carb is the same as before - the idle circuit is still too lean. It's a little bit better than before, but not much. If I cup my hand over the primary side of the carb, or partially close the choke, the motor idles smooooth as glass. When the choke is fully open, however, it idles quite rough, and the motor won't even idle in gear until it's nearly at operating temperature. Further opening of the mixture screws has no effect. According to Doug Roe's book, the next place I need to look at is opening the idle feed restrictions. Looking at the diagrams of the idle circuit, I don't see how I would do that. This is one place the book doesn't explain something very well. As much information as it contains, I've noticed a lack of procedure in a few areas. Other than the idle circuit, this carb blows the doors off of the worn-out 1970 carb I've been using. Now I see what people mean when they say "quick throttle response" with a q-jet... Throttle tip-in is sooo quick, it reminds me of the 1000cfm race demon I had on my 464ci!! Part throttle driving is great, and I haven't gotten the carb to bog at all when opening the throttle suddenly. The transition into the secondaries is very nice. If I can just get the idle straightened out on here, I'll be thrilled with this carb. If anyone can shed some light on how to work with the idle feed restrictions, it would be much appreciated! Thanks!
If you are familiar with the carb, you will notice that the fuel comes from the bowl, through the restrictions in the bowl casting, and down into the idle passages, which are controlled byythe adjustment screws. If the idle restriction is the cause of the fuel mixture's leanness, the screw openings will not add fuel. You may recall that I mentioned this possibility. You may need to open both restrictions as this combination is unique to the application. If the combination is used on a particular engine package, the metering will be similarly lean for the entire low speed package, including the off-idle, which will carry the engine to about 30 mph. opening the idle restriction may be all you need to do as the combination of the two will feed fuel at this throttle position. These restrictions are essentially part of the bowl casting, and will require drilling to open them up. If you elect to do this, do it in small increments; the changesd will be dramatic, and you don't want to overshoot the best mixture. You might also want to try different idle mixture screws; the depth of the taper can have an effect on the fuel delivery, at smaller adjustments.
