Using different Quadrajet Metering Rods

[impalamansgarage]

I was watching the "Rare Classic Cars & Automotive History" youtube channel
and in one of his videos he mentions that he uses primary metering rods
which are .004 thinner than stock to improve the Cadillac 472 500 off idle
and part throttle performance..... basically making it more peppy. The reason
he gave was that he had driven many large V8 cars from the early 70s and the Caddy motor
always felt somewhat sluggish off idle but when you nail it that is when you really
get the motor to shine.

I wanted to know the opinion of those in this forum on this and whether or not
you all were running different metering rods and why.

Cheers,

Mike Harris
71 CDV

[Gene Beaird]

Most of what determines proper jetting on a carburetor is the atmosphere you're in.  With jets/orifices and metering rods, the Qjet makes up for a fair amount of atmospheric changes.  Sometimes, you can do better, sometimes you can do worse.  The only way to know is to try it. 

I used to be very much in the Qjet building hobby, building several for friends and maintaining my own carburetors.  I did this mainly because I wanted to show others what can be done w/o immediately throwing a Holley on their car.  I've had good success. 

I think all I changed on my Grandfather's 68 Calais over the years have been the rear hanger and rods.  I do need to go in and seal up the metering wells, as the carb is original and I'm sure they leak now. I doubt I'd change the jetting, as it's pretty peppy (well, for a 55 year old car, anyway), and I'll not be racing it anytime soon. 

A quick test to see if your car is about jetted properly is to pop the accelerator pump lever off and rev a fully-warmed engine just a bit.  If the RPM follows the rate of throttle opening, you're jetted (and rodded) about right, or maybe already just a bit rich for the current conditions.  If there's a bit of a lag, but the engine eventually catches up, it's a sign you may be a bit lean and perhaps could enjoy a slightly richer primary rod. 

This is all considering everything else is in shape, though, good general tune, and the timing is set right.  So, as others say, YMMV. 

["Cadillac Kid" Greg Surfas 15364]

Mike
If you saw it on U tube it absolutely must be correct. Seriously though these Quadrajet equipped Cadillac motors when new had the carburetors tuned to absolute perfection for the motor configuration they were going on. With variations in emissions came variations in tune which included carburetor and ignition specs.  Given the changes in gasoline since the late 70's when the last of the mechanical Q jets were used it might be safe to say rod,(primary and secondary),jets, idle air, rod hangers and APT settings can all be looked at to bring performance up a notch or two.
HOWEVER, just changing say rod sizes is not the way to go. All aspects need to be considered. Cliff Ruffles has a book out that goes through all the Q jet systems and talks about performance modifications.
Right now if you are getting decent mileage say 10-12, and the car performs smoothly I would say you are probably good. I've built Q jets for my "race" car that have gotten me into the 450+ HP range but that was with a lot of trial and error work. If it ain't broke don't fix it.
Greg Surfas

[klinebau]

With the new fuel, I am most concerned with being too lean at wide open throttle.  To correct this condition, slightly larger jets are needed.  The small end of all of the rods are the same, so bigger jets are needed under low vacuum (high load) to make it richer.  If you can access the adjustable part throttle (APT) of the power piston, then raising or lowering the piston slightly can make it a bit snappier right off idle.  The APT changed throughout the years, so you need to research your year to find out how to adjust it.

Throughout the years, I have read a few threads on this forum regarding 472/500 failures.  I have always wondered whether these failures were caused by running lean at high load. 

["Cadillac Kid" Greg Surfas 15364]

Fuel value with 10% Ethanol works out to be about 96% the value of straight gasoline. That plus the fact that since my '76 will run 85 MPH all day still in the primaries WOT would normally be a momentary condition for "normal" operation. The APT is just that "Adjustable part throttle" and has more to do with cruise conditions than acceleration.
For conversation's sake here is a Matrix of rod and jet combinations I put together a while back.  The free area is tha area available for fuel flow.
Greg Surfas

[klinebau]

Increasing the jet size can compensate for a lean condition at WOT. Real world experience indicates a 10% drop in mileage rather than the 4% predicted  (see link https://www.thecarburetorshop.com/Ethanoluse.htm).

Raising the power piston slightly using the APT adjustment can improve performance off idle.  This allows for more fuel when the power piston is seated (high vacuum).  For me, this eliminated a slight hesitation when taking off from a red light.  Not a huge difference, but noticeable.

["Cadillac Kid" Greg Surfas 15364]

4% difference in fuel energy value in BTU PER GALLON. A lot of the poor mileage (poor compared to what the cars got new on the then standard 100 octane "Ethyl") comes from the differences in fuel blends. When I was back in California I documented a difference (in my 72 Eldo) between 91 Octane Richfield (now ARCO) and 92 octane Standard. To be considered motor fuel gasoline the fuel value can range from 116,000 to 126,000 btu per gallon. That in itself is an 8% diffenence. I take exception with "the car can run on 87 octane" pendents an use 93 Octane MAJOR brand in all my 70's vehicles. 
Greg Surfas .

[impalamansgarage]

Some excellent ideas rummaging around here! Consequently I am now running a CRT Performance HEI distributor, coil and wires and 87 octane pure gas with 12 degrees of base timing. Engine is stock otherwise. No pinging. Runs wonderfully. I stopped running Ethanol gas a while back in this rig since we have a pure gas source close by now. If I want higher octane I throw in a bottle of Lucas octane boost.

[Chopper1942]

In case you don't know, the APT is behind the plug between the idle mixture screws.

[Jon S]

I recall changing the Jets and ports on a Carter carburetor on my 1966 Pontiac and all it did was guzzle more gas with no difference in performance.

[impalamansgarage]

Well I sent the Qjet off to National Carburetors in Florida. It will be back tomorrow. They
are pretty much restoring it. I did a standard rebuild on it a couple of years ago because
the idle was shaky. I also added bushings to the primary throttle shaft at that time.
Off idle performance has always been fine. No amount of tuning I ever did
made any difference to the idle. So we'll see what we get this weekend.