Did a plug change on the 70 GS- Some of them, maybe 3, came out real white. The rest looked normal. Why would only a few run lean? I think it was 1, 7 and maybe 2 going from memory. Plugs didn't look worn, but definitely improved how it ran.
Hey Jason. Stock intake and what Quadrajet? Do you know the specs? How is it driven? Lean at idle or cruising speed you think? I had heard that the stock intake design had this type of issue with distribution. Some people compensated with richer jetting, but depending on how you drive it, it may not solve all of it. You richen those up and then your other cylinders may be too rich.
Its a 7040246. its got 69 jets up front with 42 metering rods and an APT screw kit. Factory cast iron manifold. I thought I feeling a lean miss at cruise. I was going to adjust the APT screw but Im unfamiliar with how to do it exactly. I drive it the way a 70 Stage 1 4 speed is supposed to be driven! :laugh:
The APT only gives you a richer mixture at idle, by lifting the metering rods a bit. Go back and look at the plugs, to be sure which side of the carburetor feeds them. I'd bet that the four fed by one side are actually all lean. You'll have to open it up to see what is happening.
The APT sets the cruise mixture by setting the lowest position of the primary rods. Cliff Ruggles describes how to check the cruise mixture using the "Tip In Procedure" on page 99 of his book. With the engine fully warmed up, set engine RPM to 2000-2200 using the fast idle or idle speed screw. Then gently tip in the choke flap or partially cover the choke opening. You are looking for a 50-100 RPM increase. This indicates that the primary settings are just on the edge of lean. Use the APT adjustment to achieve this. If there is no RPM increase, or RPM falls off some, pull a manifold vacuum hose. If RPM increases that indicates the mixtures are rich. Jason, you don't have Cliff's book?
The car definitely has a lean condition at cruise. I can actually feel it at 2500. Today I tried the tip in procedure. I was able to adjust it so that the rpm increased slightly at 2200 rpm. Still didn't help. started unscrewing the APT screw till the tension got loose on the spring. I have both mixture screws out 3 1/2 turns as well. Everything is telling me the jets need to go up a size or two. I thinking about just building a 71 and up 800 cfm.
69 main jets and 42B rods will work with that carb number if the APT system is being used. Turning the APT screw clockwise raises the metering rods in the jets. The B series rods are tapered slightly on the upper section, then taper quickly between steps down to .026". You may have to raise them up pretty high to get the A/F on par at part throttle, just make sure the PP hanger arms are exactly even and the adjustments will be accurate for both sides of the engine. It's difficult if not near impossible to read plugs with this new fuel, so make changes based on actual results instead of plug reading....Cliff
So by turning the APT screw out/ counterclockwise I was going the wrong way?? ou:o No: To get me back to square one on this, what is the baseline setting for this adjustment? Is it all the way in and back out 2 1/2?
There is no "baseline" setting with the early APT system. With the APT screw backed out, set the fast idle at 2000-2200 rpm's (engine fully warmed up and heat soaked). Start turning the APT screw in until the engine reaches the highest rpm and going in further makes no change. Then very slowly back it out until you hear the engine slow down slightly or about 50-100rpms slower. Work from this as your base setting......Cliff
Got it adjusted, this time turning the screw the right way:grin: Lean condition at cruise cleared right up. Big improvement in driveability. Thanks! Im gonna take it on an extended cruise over to car night in Bellmore and hopefully meet up with the Wizzard and the King. Should be a good night!
It was a good showing of Buick muscle! Place was jamming too. Since I straightened out the ignition issue and now that the carb is adjusted properly, its a night and day difference. Car ran great the whole way there and back. Amazing what a night and day difference a few turns of a crew can make.
