1970-W30

Finished hooking up the AEM O2 sensor and gauge, here's the results on my 403 with #5 heads, a very mild cam, performer RPM manifold and fulltime outside air only.
I'm using a 1970 350 #7040250 Qjet, #72 primary jets (stock was #71) and AX secondary rods. Idle was 13.8, easily adjusted to14.3 with the mixture screws. Full throttle dropped the A/F ratio to 11.5 which is too rich, I have a set of CH secondary rods which are leaner which should get me closer to 12.5 at full throttle. So far easy-peasy. The puzzler is this, at easy and medium acceleration the gauge reads 15.9-16.1 which probably explains why it doesn't feel as crisp under normal acceleration as should. Am I correct in assuming that the primary jet power valve spring could be too weak thereby allowing the power valve to keep the primary rods in the jets to much?
The primary rods are the stock #52C 3 step unique to the 1970 350 Qjet. I'm hesitant to start messing with the spring because it's
hard to get back to base when messing it. The other option I'm thinking of is to go one step larger, a #73 primary jet (which I have in my carb parts bin) or would a different power spring be the best course of action? And lastly, what should the A/F ratio be during medium acceleration?

Cliff R

72 jets are huge in one of those carburetors. I would go back to the 71 jets, and install smaller primary metering rods, 43B or 44B would be a good starting point.


I would also install an external APT screw in the baseplate to fine tune the metering rod height in the jets. Although difficult to get loosened up without breaking it, that carburetor has an APT screw between the mixture screws under a metal cover. We remove it, and install an external screw with a spring on it (custom made on our lathe). This provides the tuner with full control of the part throttle A/F without taking the carb apart.


I attached a pic of the APT and the external screw. The best method to remove the factory screw is to remove the cover and apply plenty of penetrating oil, then some heat to the bottom of the baseplate to free it up. Attempting to turn the screw without heat usually snaps it off


This is an excellent tuning feature in those carburetors if you can get the old APT screw out and replaced with an external screw......Cliff

1970-W30

Thanks for the reply Cliff, I'm going to try that modification. I do have a small lathe in my garage to make what is needed. I'm finding that an AirFuel ratio meter is a great help in tuning a carb accurately......

455man

I'm going to follow this thread. Let us know how it turns out. I'll be fine turning my quadrajet in the future.

cdrod

X2. I'm gonna do the same thing as 1970-W30 when I get my engine assembled. Will be looking for updates to this post in the future!

oldcutlass

Hey Cliff welcome to the site. It's nice to have your expertise around. For those who don't know who you are, why not get in the newbie forum and give a brief intro.

cdrod

Oh, that Cliff! I bought your book on Q-jets many years ago - excellent resource. Thanks for contributing to this thread and welcome to Classic Oldsmobile.
Rodney

steverw

Welcome Cliff
I spent a little of my hard earned money with you not too long ago.
Nice to have someone with your knowledge here
Steve

Cliff R

Thanks guys. I'm down for a few days (had knee surgery) so found some time to put up a few posts. With the workload we have in the shop, I usually don't have the opportunity to help out on most of the Forums. A/F meters can be a good tool, but I don't use them here. The best way to tune with one is to set the carb up exactly for what you are doing, fine tune the idle mixture, primary jet, and secondary metering rod. The APT system is tuned last, using a metering rod that is full taper, so you will have control from rich to lean. It only takes a few short drives to get close, then a tank or so to nail down the tune. At this point hook up the A/F meter and see where the numbers are at, and use them as a baseline for further tuning. There are so many variables involved with tuning that any numbers are simply for reference. In any and all cases we are looking for best results in all areas, not for specific readings on the A/F meter. Also make sure to include distributor tuning as part of the process, as it is a very important part of engine efficiency. Any engine that is street driven should be using the vacuum advance, as it will allow for much leaner mixtures at part throttle, and improved engine performance/efficiency......Cliff http://www.cliffshighperformance.com/

billmerbach

Cliff wrote a book? We have a famous author here?: D

1970-W30

Update, I ordered and finally received a pair of #43 primary rods and installed them along with #71 primary jets as per Cliffs suggestions. I also removed the cover and exposed the screw that allows for fine tuning the primary power valve (controls the distance the rods can go into the jets). That in turn of course, determines how rich or lean the mixture will be under part throttle conditions. Luckily for me, the screw was not stuck at all, and came loose easily. This is where stupid comes in, I forgot to check how many turns in it was when I took it out to match the threads for a longer duplicate to make later on and had to guess when I screwed it back in, a quick test drive showed the A/F meter indicated that now it’s running rich at light load, easy cruising, low RPM mode. This is opposite of my original problem. I then backed the APT screw out a half a turn to lean it out and will try for another test drive later today to see how much change a half a turn makes. I also modified an old screwdriver so it will fit into the hole where the adjustment screw is located. You can see how much the little cam moves up or down to control the power valve.

Fun71

Cool, this is all great info that a lot of us will be able to apply to our vehicles. Thanks!

Cliff R

I would make or get one of our external adjustment screws with a spring on it. Couple of reasons. First, the factory screw is hardened, and the ears snap off easily. Second, it needs to be loctited in place or it can move. Last, having a large external slotted head makes it much easier to turn.


It is also a good idea to install new jets for tuning, as most used jets will be worn some on the insides from the metering rods.


The correct metering rod should be a tapered rod, or 43B, so the APT system works like it's supposed to.


Also be aware that there are at least 4 different types of jets that fit the Q-jet, but only one is correct. The correct jet has a pretty deep orifice compared to the other three varieties......Cliff

1970-W30

I used Edelbrock #1944 rods from Summit, They are a tapered rod near as my 66 year old eyes can tell. A longer spring loaded adjustment screw is going to be made, I just wanted to try it first, turns out the thread size is the same as the body to top assembly screws, I'm going to modify an extra assembly screw off a junk Qjet and use a spare idle mixture spring for tension. For the guys that do not have a lathe, better have Cliff sell you an adjustment screw......
I was going to try the car out today, but after my dentist appointment, I think I'll lay low till tomorrow.

classicmuscle442

Also along for the info, welcome Cliff.