Quadrajet

[jstables]

Im planning on running the stock quadrajet on my blazer and was wondering what modifications you guys that have run them in the past have done. Also I picked up the Doug Roe book and was just wondering if these numbers on cfm seemed right?



750-800 cfm for a stock carb seems to be really good.

[VerticalTRX]

Terryd is really the q-jet man (he built mine), but I'll give my $0.02 worth since I've been running one on my '79 F-150 for a few years now.

First off, yep those numbers are right, around 800cfm. The reason they can flow such huge numbers and still work good on smaller/more mild/low rpm applications is the small primaries and huge secondaries. Small primaries make for good port flow at low RPM, which is needed for good crawling (my motor still is pulling hard as low as 400rpm).

IIRC, the only main mod that we did to my carb was remove the choke assembly. Other than that it is just special tuning and tinkering for off-road use. You don't want too much fuel in the bowl and don't want too high of fuel pressure. Those will cause flooding in the rough stuff/high angles. The various float are something we've experimented with, but found the standard (Advance Auto) replacement float for the '75+ q-jets seems to work quite well. There are others out there that may be better, but aren't readily available. Sorry I don't have more details on off-road tuning, it's been several years since we built the carb and did the mod's, so I'm probably forgetting some stuff. I'm sure Terry can fill in the details.


Tuning the Q-jet isn't as easy as others, but when you get it right man does it work well off-road. Probably the best off-road carb made to date, IMO.

[jstables]

Terryd is really the q-jet man (he built mine), but I'll give my $0.02 worth since I've been running one on my '79 F-150 for a few years now.

First off, yep those numbers are right, around 800cfm. The reason they can flow such huge numbers and still work good on smaller/more mild/low rpm applications is the small primaries and huge secondaries. Small primaries make for good port flow at low RPM, which is needed for good crawling (my motor still is pulling hard as low as 400rpm).

IIRC, the only main mod that we did to my carb was remove the choke assembly. Other than that it is just special tuning and tinkering for off-road use. You don't want too much fuel in the bowl and don't want too high of fuel pressure. Those will cause flooding in the rough stuff/high angles. The various float are something we've experimented with, but found the standard (Advance Auto) replacement float for the '75+ q-jets seems to work quite well. There are others out there that may be better, but aren't readily available. Sorry I don't have more details on off-road tuning, it's been several years since we built the carb and did the mod's, so I'm probably forgetting some stuff. I'm sure Terry can fill in the details.


Tuning the Q-jet isn't as easy as others, but when you get it right man does it work well off-road. Probably the best off-road carb made to date, IMO.

Sweet I had read they were good but I had no idea the real cfm ratings but that is why I kept it for my motor and also why I got a spare one from my buddy so I would have plenty of parts . Mine are both the old style with the manual choke but are different model numbers. I would love to have someone show me a few tuning tips once I get my truck up here since I can only understand so much from the book.

[alk1174]

You don't want too much fuel in the bowl and don't want too high of fuel pressure. Those will cause flooding in the rough stuff/high angles.

I dont know anything about a guadrajet but the above statement has been the key to keeping mine running decent at angles. An adjustable pressure regulator would be nice so you can drop the pressure for the trail if needed.

[TerryD]

Regardless of them being a 750-800cfm carburetor, they will only open up to allow what your engine needs to flow through them. The small primaries improve signal to the boosters and maintain velocity through the carb at low RPM, giving better throttle response at lower RPM like Matt said.

For stock engines, your basic tuning is easy enough. What I did with Matts was eliminate the choke system. The choke horn and shutter are both still in place and required for proper operation of a Quadrajet, we just eliminated the linkage and locked the shutter open. We also lowered the float 2/32 (I think) to help with fuel splash and flooding offroad.

I've only worked on one of the 70 series carbs as opposed to the 170's on my K5, Matt's F150 and Doughboy's Cruiser. The principals are the same, just a little different design. I've heard that the older models hold a little more fuel and I believe they take a longer primary rod. The kids Edelbrock sells to tune Q-Jets is for the later 170 carbs ('75+). Oh, and there is a difference between the base plate of a truck carb and a car carb. The truck carb has the vacuum signal for the distributor below the throttle plates and doesn't (I feel) control the advance properly. The car is above and causes the timing to spike when the throttle opens rather than drop like the truck carb. I'm not sure why this was done, but all the carbs I've built have had the car style timing slot.

We had to do more to Matt's carb than most because I built it out of spare parts to get exactly the carb we wanted for his truck. It had to be completely recalibrated for his engine (and probably could still use a little work for absolute perfection).

I personally prefer a Q-Jet to any other carb on a street engine. They are versatile and dependable when the right carb is used on the correct engine.

[BadAssEddie]

Terry/Grub,

What's the point of not having the choke operational? What did you do to keep the shutter open?

[TerryD]

Ease of operation on Matt's. It kept us from having to pull a circuit in to operate an electric on or try to rig up stuff to run a mechanical one on his Ford. The one in my K5 works.

To lock it open, we simply removed the operator rod, re-attached the little arm on the shutter shaft and drove it against the horn with a small ball peen hammer. It bent over into the contour of the horn and keeps the shutter completely open and still.

[jstables]

To lock it open, we simply removed the operator rod, re-attached the little arm on the shutter shaft and drove it against the horn with a small ball peen hammer. It bent over into the contour of the horn and keeps the shutter completely open and still.

Do yall have a picture of this I would do the same thing since I dont have a choke cable at the moment either?

[TerryD]

What are you putting this Q-Jet on?

[jstables]

a stock chevy 350 that is goin into my s10 trail rig.

[TerryD]

You have the factory wiring to run an electric choke on yours. That's what I would do if I were you. It's what I ran on my 82 S10 with a 350 swapped into it as well. All the wiring is there already so there is no good reason not to.

[jstables]

You have the factory wiring to run an electric choke on yours. That's what I would do if I were you. It's what I ran on my 82 S10 with a 350 swapped into it as well. All the wiring is there already so there is no good reason not to.

I know but both the carbs I have dont have the electric chokes.

[TerryD]

Simple answer is: Change carbs. Most people (except me and a few of the other faithful) will give you a Q-Jet to haul it off. Go looking and find one with the electric choke, preferably from a car with the advance slot over the throttle plates. It's not like you've got hundreds of dollars tied up in two 70 series Q-jets. You can probably trade them for a couple 170 series if you find someone looking specifically for the older carbs.

[jstables]

alright and yeh the only tied up money I have is in the book cause my buddy at home has about eight of them laying around his garage.

[jstables]

anything specific in the model number to look for other than those two things you said about the advance and electric choke?