This article is about my experience with calibrating a M4MC Rochester Quadrajet to work properly on the 225 slant six in my 1965 Barracuda.

The Rochester Quadrajet spreadbore 4bbl carburetor has a much smaller, centrally located fuel bowl that seems to work well for off-road vehicles.  Since I also had a surplus Rochester Quadrajet that I removed my 1977 Pontiac in November of 1997 (for a propane conversion I completed in January of 1998), I thought I would give it a try in August of 2004.  Just bolting-on the carb with its Chevrolet 350 calibration after sitting dry for 7 years, I found that it ran better than my AFB. After a disc brake upgrade that dragged on way too long (several years), I found that my Quadrajet's fuel inlet wouldn't seal and the carb would overflow, likely from a rusty gas tank.  I had Specialty Carburetors (SC - Toronto, ON) rebuild it and they switched it from a #77 primary jet to a smaller #72 jet.

After replacing a rusty gas tank in 2015, I ran the AFB again to ensure that any residual rusty silt didn't settle out in my newly rebuilt Quadrajet.  On November 16, 2016, I had enough of chokeless cold starts and reinstalled the Quadrajet.  After a few tankfuls of gas, I found that, even though I now have a working choke, it was difficult to start in cold weather and it has noticeable lean surge at part throttle. 

 

 

Specifications about my 750 CFM Quadrajet carburetor:

Carburetor Rochester Quadrajet 17057204, M4MC
OEM Primary Jets #77 - 0.077"
OEM Primary Metering Rods #52K - 0.052" [Single Taper (0.052 - 0.046) & Step]
OEM Primary APT Turns ?? - preset from factory, no spec
Secondary Metering Rods CH
Secondary Hanger 33797
Post-rebuild Primary Jets 0.072"
Post-rebuild Primary APT Turns 3.5

 


The Quadrajet's primary venturis are in use at all times and it is crucial that the primary metering circuit is properly calibrated for all engines.  For smaller engines (like the slant six) where the secondary air valves rarely open, it is especially important to get this right.  Although the primary jets interchange between all years of manufacture, there was design change in 1975 and the primary metering rods from 1965-1974 (PNs starting with 70) are 0.080" longer than those from 1975-1980 (PNs starting with 170).  Since the Quadrajet has been out of production for decades now and it is not as popular as modern squarebore aftermarket carburetors for performance applications, performance parts aren't as available as they used to be.

Compared with the old Carter Strip Kit (metering rods & jets), jetting a Quadrajet is much simpler because all of the metering rods have the same tip (power setting) diameter (power setting) of 0.026".  Only the newer M-series metering rods (ie, 48M, 49M, ... 55M - truck applications & Edelbrock Quadrajets) and the 50D have a tip diameter of 0.036" along with Edelbrock's . The part throttle cruise section of the metering rod comes in a variety of diameters and tapers (transition between between lean and rich).

Quadrajet Tuning Steps

Lars Grimsrud (LG) is very knowledgeable about Quadrajets and has a Quadrajet tuning paper that is available upon request.  He may be reached via the Corvette Forum and I would ask him for his paper before trying to tune your Quadrajet.  Assuming you are starting with a freshly rebuilt carburetor, LG's Quadrajet tuning procedure has the following steps:

  1. Verify that that the float level is correct.  Adjust as required.
  2. Determine main jet size.
  3. Determine main metering rod size.
  4. Set secondary air valve spring initial tension.
  5. Determine secondary metering rod hanger length
  6. Determine secondary metering rod size.
  7. Adjust secondary air valve spring tension.

The best way to determine the optimum jet diameter is on a chassis dynamometer.  Since this can be quite expensive due to the time required to remove & replace the carburetor while changing primary jets, I opted for the much cheaper time-trial method.  This involved multiple timed runs from 0-60 mph.  While a drag strip would have been better, I found a nearby straight length of road with very light traffic and got my wife to take several measurements with a stopwatch for each jet..  To keep the times as consistent as possible, I left the transmission in Drive where it shifted from 1-2 around 4500 RPM.

Sticking with the 52 metering rod, I have the following results for a variety of main jets:

Main Jet Average 0-60 times 1,2
seconds
77 not tested
76 not tested
75 15.2
74 15.2
73 14.7
72 15.6

Notes:

  1. APT adjustment @ 3.5 turns, Secondaries locked out, transmission automatically shifting @ ~4500 RPM
  2. 0-60 times reflect that this car has highway gears (2.76:1) and larger tires (P225/70R14) than stock (P185/75R-14)

Besides having the quickest 0-60 time, the #73 jet also had the best seat of the pants feel.


One of GM's better ideas was to include a way to fine-tune the part-throttle fuel mixture with the Adjustable Part Throttle (APT).  The APT is an adjustable lower stop for the metering rods, located beside the power piston on newer Quadrajets.  A pin sticks out of the side of the piston, which hits the APT stop at high manifold vacuum.  Having the metering rods sit a bit higher above the jet effectively enriches the part-throttle fuel mixture as the APT is turned out from the bottom.

The APT has a rectangular head, which requires a special tool.  Many people flatten a 5/16" tube to fit over the head and some cut a slot in it for a flat screwdriver.  SC made me a tool in their shop.  There is plug in the Quadrajet's cover that is removed to access the APT without having to disassemble the carb.  SC drilled a cone into this plug so that it can be removed with an ordinary wood screw.  I've also seen a kit for replacing this press-fit plug with pipe plug but I think that this is way more work than necessary for the few times the APT needs to be accessed.

 

Quadrajet APT

 

APT Adjustment

Although a Corvette club has posted the write up How to Adjust a Q-Jet Power Piston with LG as the author, LG recommends against its use.

According the above document , the top of the economy step of the metering rods should line up with the bottom "lip" of the jet.  Using my digital vernier caliper, I measured the length of my 52K metering rods to be 2.41" and my jets to be approximately 0.087-0.088".  The length of the non-metering portion of the metering rod was 2.01" and the length to the top of the power step was 2.026".  With the power piston pulled down all the way into the jet (economy zone), the length from the top of the metering rod to the top of the jet should be approximately 1.922-1.923" (ie, 2.010" - 0.088").  After screwing in my APT all the way, I found that the top of the metering rods were about 1.95" above the top of the jets.  This confirmed my expectation the factory used 0 turns of the APT adjustment starting point.

Since no specification exists for the number of APT turns of any carburetor, I presume that the factory set each carburetor individually on a chassis dynamometer when each car reached the end of the end of the assembly line.

My understanding is that Specialty Carburetors sets each Quadrajet they rebuilt to 3.5 APT turns as their rule of thumb,. From my internet survey of APT adjustments, I believe that the APT may be turned out to a maximum of 8 to 8½ turns.  If turned out this far, the metering rods would be sitting very high out of the jet so that the economy fuel mixture would be fairly close to the power fuel mixture.


With the popularity of aftermarket carburetors and fuel injection, the supply of Quadrajet tuning parts has dramatically shrunk.  It is impossible to exchange early metering rods with late metering rods.  Only truck rods (M-series) are now available new so all other series rods must be sourced from the used market.

Since the APT limits how far the primary metering rods can be pulled down in the jets, small changes to the part-throttle fuel mixture may easily be made.  Many passenger car metering rods (ie, with a 0.026" power tip) have a 0.006" taper.  That means, for example, that the part-throttle cruising fuel mixture of my 52K may be enriched from 0.052" to 0.046",  if you can't get your part-throttle fuel mixture correct within the 0.006" taper of your metering rods, you'll have to source a better set of rods or switch to M-series rods.

M-series (0.036" power tip) and P-series (0.026" power tip) have a 0.012" taper.  If I were to replace my K-series rod with a M-series rod, I would need to go up 4 jets (from #73 to #77) to compensate for the larger power tip diameter.  Since the primary jet is larger, I would need a larger diameter metering rod as well but only the following Edelbrock M-series rods are now available:  0.048" / PN1941, 0.050" / PN 1943, 0.052" / PN 1945, 0.054" / PN 1947.  If my 52K metering rod was adjusted the bottom of its travel (ie a diameter of 0.046") and I needed more enrichment (252.3 E-5 in² flow area), the best M-series rod would be the 52M with a leanest flow area of 253.3 E-5 in².


My first challenge after reinstalling the Quadrajet in November 2016 was hard starting in cold weather.  After much trial and error with choke adjustments and APT adjustments, I learned that both have a great effect on cold starts.  If the part-throttle fuel mixture is nowhere close to being in the ballpark, no matter how rich the choke is set, the fuel mixture mixture will be too lean until the intake manifold is fully warm and will therefore require much gas-pedal pumping during warm-up.

While I was driving the car in 2017 to get my APT setting right, I found that I had a persistent lean cruise and eventually reached point where I had the APT turned out 6 turns.  That left very little travel in the metering rods.  While taking the car out for ride in May 17, my car suddenly started having a poor idle and subsequent drives became worse.  It felt like I had an intermittent severe vacuum leak, which made the car a bear to drive in heavy traffic.  After some troubleshooting, I noticed that only my left (choke side) primary venturi was flowing any fuel and the right-side venturi was completely dry.  After taking the carburetor apart twice on July 3 and blowing out the passages with carburetor cleaner did I once again have a smooth idle.

Another challenge I discovered was with carb idle and ignition timing.  The OEM specifications for my engine are:

  • idle speed 550 RPM in Neutral
  • ignition timing: 2.5° BTDC @ 550 RPM
  • distributor PN 2444648 (distributor: 12° @ 2200 RPM centrifugal & 7° @ 13" Hg vacuum)

The commonly recommended performance timing for my car is:

  • total mechanical timing (initial + centrifugal) 30-32° BTDC, all in by ~3000 RPM
  • total advance (mechanical + vacuum): 45-50° BTDC (leaner part-throttle mixtures require more advance)

By advancing my initial timing from 5° to 8° BTDC, the advance curve in my DC distributor (P3690788) provided the recommended advance (distributor: 11° @ 800-1200 RPM centrifugal w/ single red spring & 7° vacuum).  However, advancing the initial timing also increased my idle speed and I could only reduce it back down to ~600 RPM in Drive with the throttle completely closed.  Without recurving my distributor, an additional 2° of initial timing would cause my idle speed to be even higher.  I got the distributor through Mopar Performance (formerly Direct Connection) and it was used for performance (drag racing).  For better highway fuel economy, I would still need a bit more vacuum advance.


Having found the jet that gives me the best acceleration, my next task was to maximize my highway fuel economy by adjusting the APT.  Leaner fuel mixtures tend to burn slower, which is why many people mistakenly believe that leaner mixtures are hotter.  The slower combustion rate can result in the fuel charge in the combustion chamber still burning when the exhaust valve opens, thereby causing burned exhaust valves.  If combustion occurs at the right time, the pressure peak in the combustion chamber will occur when it can exert the maximum torque on the crankshaft, thereby maximizing power and fuel economy.After fixing the plugged primary circuit problem, I started off with my APT turned in all the way (ie, 0 turns) and found that my fuel economy was poor and I had a lean surge.  To eliminate this surging, with my transmission in Park, I set my idle speed to 2000 RPM so that I would be off the idle circuit and on the primary circuit.  I then turned out my APT 1/2 turn at a time and found that my idle speed kept increasing.  With every APT 1/2 turn, I would reduce my idle speed back to 2000 RPM.  After raising the APT to 5½ turns, I felt that the engine had reached the practical limit of part-throttle enrichment so I used that as my APT adjustment starting point.

When I previously worked on tuning my AFB, I found that my best fuel economy occurred when the economy step on my metering rods was just rich enough to not have a lean surge.  I was a bit surprised to see that my Quadrajet idle speed responded so linearly to APT enrichment.  As I drove my car and reduced my APT turns (1/2 turn at a time), I found that my fuel economy improved as I leaned-out the fuel mixture.  I suspect that this improvement comes from the reduction in pumping losses with increasing throttle opening (lower manifold vacuum).

 

1965 Barracuda APT Tuning Results

 


Information

http://garage.grumpysperformance.com/index.php?threads/q-jet-info.1144/
http://garage.grumpysperformance.com/index.php?threads/looking-for-q-jet-index.7763/
http://www.route66hotrodhigh.com/PowerTuneQjet.html
http://www.gmcmi.com/wp-content/uploads/2014/05/Quadrajet-Carb-Primary-Metering-Rods.pdf
http://www.corvetteforum.com/forums/c3-tech-performance/1023501-updated-q-jet-listing-shows-available-parts.html

Parts

http://www.cliffshighperformance.com/
https://quadrajetparts.com/
http://www.edelbrock.com/automotive/mc/carburetors/access-q-jet.shtml
http://jetchip.com/product-category/performance-carburetors/rochester-quadrajet/rochester-quadrajet-parts/

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