Quadrajet Conundrum

[Figuarus]

Guys,

I have a Quadrajet on my 283. This 283 has 1.92 Fuelie heads.

I have a stock cam.

I need to rejet this thing since it came from Los Angeles, and It's running WAY too rich.

The problem I'm running into is two-fold. First, While I CAN rebuild a carb, I know next to nothing about the Q jet. Second, I'm having trouble finding information on what size jet to use for my elevation.

I'm now at about 4500 feel elevation.

Has anyone done this before, or know of a chart to find the right size jets?

thanks again guys!

[geezer#99]

Running too rich is what you want at your elevation,
You need to fix that before you consider re-jetting.
First you need to put as much initial timing into your motor as you can and still have it start good when hot.
Then turn your idle down and get your carb back on the idle circuit. Likely it's idling on the power circuit.
What's your initial timing set at?

[68Gold/white]

www.cliffshighperformance.com

You will need the carb number. You need to send him an email first. Cliff stays about one year behind on carbs builds. Phone calls slow him down. He COULD talk on the phone all day...He WILL help you, if you let him...

[jayoldschool]

A quad is a BIG carb for a 283. If it were me, I'd move the quad along, and go with a nice fresh Edelbrock or Holley 500. I prefer the Edelbrock, but I'm crazy like that.

[66Submarine]

Originally Posted by jayoldschool

A quad is a BIG carb for a 283. If it were me, I'd move the quad along, and go with a nice fresh Edelbrock or Holley 500. I prefer the Edelbrock, but I'm crazy like that.

The secondary circuit is big, but the throttle bores of the primary circuit are actually slightly smaller than the OE 2 barrel setup.

[HumphreyMurdoch]

I don't know diddly squat about Quadrajets. But, if you run an Edelbrock carb, get a 1/2" phenolic carb spacer to help with hot soak starts.

[cortcomp]

Some q-jets have an APT setting letting you adjust mixture with a screw on top (primary side.) You likely don't need smaller jets as much as larger rods.

[geezer#99]

Originally Posted by cortcomp

Some q-jets have an APT

All quadrajets have an apt. Two different styles. One in the baseplate or one by the airhorn.

[bonehead]

the rich running could be leaking well plugs, google it. This is a common issue with Q-jets, I redo the well plugs on all my q-jet rebuilds.

[Figuarus]

66Submarine, you are correct, the primaries are smaller than the 2gv this thing used to have.

geezer#99, Im not sure where my timing is at the moment, but I will find out in the next day or so. I need to go get a timing light.

I DO need to rebuild it since it's pretty evident.

So, just so i'm clear, The carb running too rich at this altitude is NOT a problem? I was always under the impression that I needed to lean it out for altitude since it's been tuned for Sea-level operation.

I was under the impression that changing the timing will affect power, but not necessarily mixture.

I also did some reading on the carb number, turns out this was made in 1982, and was made for Automatic trans applications.

[geezer#99]

You need to up your jet size 1 number per 2000 feet elevation. That's richer.
Timing affects your mixture because with max initial timing you can get your carb idling on the idle circuit. The timing speeds up your motor allowing you to close the throttle to get back to the idle circuit.
A quick check to see if you're idling on the power circuit is to adjust your mixture screws. If you can close one or both off all the way then you're idling on the power circuit.
When you get your timing bumped up and your idle turned down, you might find the richness goes away.
That's why the rule has always been "Timing first, carb second'.

[awbrown]

While I build a pile of Q-jets and have great success with them , I don't know a whole lot about jetting , except I used to change the jetting in the truck I drove to school ,a 180 mile trip on weekends .. The jetting that gave the best power was also the jetting that gave the best gas mileage .. in my truck it was 74 in the primaries.. Q-jets have more than one problem that can make them run rich , bad float , improperly set floats , leaking welch plugs under the primary wells ,and a few others I wont get into .. My advise is to get a another carb from a small cubic inch engine that you know runs well , other than that get a junk one and start getting good at assembling it ,they sure are not a holley or carter .....

[awbrown]

Also as altitude increases the air gets thinner meaning you need less jet ,as air gets denser such as a cold day or lower altitudes you need more jet .. here is a guide ..

ALTITUDE
As you increase your altitude the octane requirement decreases 1-2 octane per 3000 feet elevation. This is because the density of the air is reduced or there is less air available for your motor to burn. The higher the altitude, the richer your motor will run, making it necessary to re-jet the motor in order to lean it out. The fuel volume remains the same and the air volume goes down. If you have a vacuum advance, as the altitude increases, the motor makes less vacuum and the air fuel ratio becomes richer due to the decreasing air to fuel volume.

TEMPERATURE
When the temperature goes up, the air density decreases, thus you have less air available for combustion and your air fuel ratio becomes richer. The same works in reverse. As the temperature goes down, you end up with more air per cubic foot, and without re-jetting your carburetor, the engine will run leaner.

AIR DENSITY
As the air density increases, your engine will lean out. As the air density goes down, the engine runs richer. Like driving up a mountain, at the top, the motor has less power because you have less air to burn.

HUMIDITY
When the humidity increases, octane requirements ease. The formula is something like... for every one gram of water increase per one kilogram of dry air the octane decreases by .25 to .35. WWII aviation engines used water injection and it worked well for a short time by cooling the cylinder temperature. As temperature goes back the effect goes away.

[awbrown]

Originally Posted by bonehead

the rich running could be leaking well plugs, google it. This is a common issue with Q-jets, I redo the well plugs on all my q-jet rebuilds.

I usually peen them with a hammer , then use a good epoxy on them before I reassemble it ...

[danheit]

To the OP, a q-jet is a great carb. Most of the guys that are against them, don't understand them, or have had qjets that were improperly set. It is capable of great fuel efficiency, and can still flow up to 800cfm with little to no modification, when needed. As stated earlier, call Cliff. Another great place to start is Rochester.com. There's tons of replacement parts out there. Most likely you have a secondary electronically controlled power valve (the plastic plug on top of the air horn) and it probably isn't hooked up correctly. I would also guess that the choke and choke linkage is incorrectly or just not set up. Go on YouTube and start watching videos about rebuilding them. There's tons of info, and they're incredibly simple, once you understand them. If you look at stock class drag racers, a lot of folks still use them. Please know, this is a little of me talking out of my behind from memory, and a lot of personal experience. I grab one every time I find one in the junkyard. That and York AC compressors.....

[geezer#99]

Here's a quick read on all things quadrajet!!
Study hard!
There will be a test!! LOL!!
http://www.digitalcorvettes.com/foru...ad.php?t=88376

[1971 GMC SHORTY]

Put a Holly 500 4412 thing will Pull Hard!

[Figuarus]

Ok, I'm gonna start with Timing as suggested.

Changing out the carb isn't an option for me right now. I don't have much money to invest into it other than a rebuild.

i need to get my hands on a timing gun anyway, I will start with that and see where i'm at.

thanks for the info guys! i'll report back with results

[geezer#99]

When you get a timing light, get a dial back to zero or digital type. You might need to read your timing higher than the 12 degrees available on your timing tab.

[luvbowties]

Originally Posted by geezer#99

When you get a timing light, get a dial back to zero or digital type. You might need to read your timing higher than the 12 degrees available on your timing tab.

I've seen & bought some good tune-up equipment on local Craigslist. Local lets you take a knowledgeable friend along to weed out the junk. Good and used quality equipment often is much better investment than brand new, low quality stuff.
sam