One of the more intimidating aspects of tuning a Rochester Quadrajet carburetor is getting the air horn back on without damaging the primary metering road assembly, which must be seated as you assemble. The problem is it wants to come up – there’s a spring-loaded piston – and if does and the metering rods become unseated and you press down on the assembly, it will bend – and ruin – those fragile metering rods.
The trick is keeping the assembly seated while you reassemble. Without further ado, here is my redneck – but effective, so don’t laugh – technique for getting it together without breaking anything:
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The Q -Jet is a miracle carb. Spread bore design keeps velocity constant and the throttle response crisp.
There are a LOT of tricks to tuning a Q-Jet and they require detailed research and skill PLUS application experience.
My ’71 Eldorado with 520 CI uses a stock ’71 Cadillac QJ but has many little tweaks. I have metering rods that are so fine in diameter that a fumbled touch will warp the pins. ’70-71 QJs had factory metering rods that were thinner than the solid lifter 427 Chevy units!
Smoothing the fuel emitters in the bores helps a lot too. A Dremel tool or 400 grit wet sand paper on a phillips screwdriver shaft will make a great hand polishing tool.
A small diameter rod bent in such a way to stop the air doors from opening too far is a great fix too- anything past vertical and the doors actually restrict incoming airflow.
The best Q-Jet man in the country is a guy named Paul Becker from Rodchester NY now Charlotte NC (where all the hot rod guys are moving to these days). He can talk for half an hour detailing the mods he makes and you can’t keep up as his ideas are so innovative. He developed a way to get signal on a small cammy motor like the DZ302 by a #8 drill hole in the plates at a certain place. Ever had an idle mix screw that wouldn’t change the idle? He has a fix for that.
A Paul Becker Rodchester is a grand- and totally worth it if you want your motor to run like it’s a modern EFI engine. No joke- the QJ can be tuned that precisely – at half the cost of an EFI unit and a totally stock looking engine- if that’s what you need on a concourse engine restoration.
Hi Auric,
I will amen all of that. Even a fumble-fingers like me can tune a Qjet to near-EFI smoothness/cold-start performance. It does take patience to learn how to work on one, of course. And then time to acquire that intangible feel so necessary to dialing one in. But once you, you will be amazed at what a gem you have on your hands… no wires or computers required!
In NHRA stock/superstock circles, many simply leave the primary metering rods out.
Probably wouldn’t work well on a street vehicle.
Hi Aljer,
Of course, in NHRA racing, idling isn’t a consideration! With no rods in the bores of the primary jets, fuel would be dumping into the engine. Which works if you keep the revs up and the thing is very hungry for gas. But the Q Jet’s systems were designed specifically for a smooth idle/transition to WOT. It is why – arguably – it’s the best street high-performance carb ever made. Tuned right, they deliver immediate starts, no hesitation and superb WOT performance – all on par with EFI.