Well, today I totally lost it! A few weeks ago I bought a rebuilt q-jet from one of my local parts houses. Installed it and it ran great with no adjustments. Idle was smooth and sounded great. Only problem was that it seemed a little flat when given more throttle at cruise. Did some checking and found that they had sold me a carb jetted for a 305. Talked with them and they ordered the correct carb for me and told me that the guy who sold it to me did not know enough about carburetors to know that q-jets were not all the same. The new carb came in and I installed it last night. Had big problems starting the car and keeping it running. Figured that it was late and I was too tired to adjust it last night so I went to bed. Got up this morning and started asjusting the carb. Nothing worked, even with idle mixture screw turned all the way in it still poured gas from the main venturis. After a few hours of dicking around with it I got pissed! It took 5 minutes to take the carb off the car and head for the parts house. Told them that this was a brand new (rebuilt) carb and it shouldn't require major adjustments to make it run correctly! I guess the counter guy could tell how mad I was and told me he would refund my money and sell me an Edlebrock 750cfm with electric choke for less than I had paid for the rebuilt q-jet. So, new carb will be in Tuesday and they are throwing in the stuff I need to mount it because of the problems I have had with carbs they sold me. Pretty good deal, new carb and all I need to get it running for under $350!

 

Good luck with your new Edelbrock. I am pleased with mine.

 

Are you going to replace your intake manifold with a squarebore unit? An adapter plate will work, but a new manifold is better...

 

M<y $0.02

IMHO, parts store Q-Jets are generally junk. There are so many Q-jets made in so many configurations that the stores cannot keep them in order. The best way is really to get the Dog Roe book, Rochester Carburetors and Lars' tuning papers from www.corvettefaq.com .

The Edlebrock is basically a new Carter AFB. Good carb but don't expect miracles. Also, a 750 cfm is probably too big for a stock 350. The Q-Jet design is good because it has a secondary air door that controls flow based on demand.

 

The parts place you are dealing with sounds like a decent company. Even if the first guy didn't know what he was doing, they care about your satisfaction. There are SO many jet/rod combos you can put in a Qjet. The rebuild company might not change or even look at that, and assume it was set up as stock. If improperly jetted/rodded the adjustment screws will have little effect.

 

hope it is a up-dated 750 some of the early one had adjustment problem's.. they worked better than the people's old carb's so many didn't know that they were off till a tuner got ahold of them

 

Parts store Q-jets are generally junk Maybe, the Q-jet itself is not.

A 750cfm carb is too big for a stock 350.

People who say this about Q-jets really do not understand them. Q-jets are 750 cfm carbs and have sat atop of corvettes and other passenger vehicles and trucks. The Q-jet is a mechanical secondary
carb. A properly tuned Q-jet can help an already stout engine perform
better. You ust got to understand them.

A 750 is not too big a carb for a stock 350. Once again this 'carb is too big' myth comes from individuals who do not know how to properly
tune a carb.

 

page62 - yes, I plan to now go with the air gap RPM manifold since I no longer need an adapter plate to make it work.

SteveG75 - found out the hard way that your right about parts store junk. BTW, the Edelbrock carb has an adjustable secondary air valve above the secondary throttle plates as well.

isosceles - found out that the rebuild companies only replace the parts inside the carbs. They do not check or repair any wear on throttle shaft holes in the baseplate. Got a hard education in rebuilt carbs that I never really wanted.

comp - supposed to be new, how do I tell. With all the trouble I've been through I'm sure they will do what ever it takes to make me happy.

Store manager called me today at work to tell me that the one they said will be in tomorrow had not been found in the warehouse by 9:30am today. He told me that he had already ordered 2 more from different places to make sure that if the 15 people he had looking for it couldn't find it, worst case would be Wednesday morning. I think they are trying to make me happy.

 

a 750 is fine on a 350, if it is "tuned"

 

Oldguard7,

I run a Lars tuned Q-jet on a 350 that DD2000 says is pushing 400hp. I rebuilt this thng myself. Lars helped me tune it and then I have tuned it many times since. I can floor it at 1500 rpm in first and the tires break loose and pulls straight to redline.

Q-Jets are great carbs when tuned right, HOWEVER, they have so many variables that getting them right is hard. When GM put in different rods/jet combos for an L-48 manual, L-48 auto, L-82 auto, etc, it is hard for the average guy to get it right.

My comment about parts store carbs is that they are generally a configuration that will work OK on multiple applications but will not be tailored specifically for any one.

Yes the Q-Jet is a 750 (or more) cfm mechanical secondary carb. However, that spring loaded air door (that also controls the secondary rods) is the key to the carb's genius. If set right, the carb will only flow as much air as the engine needs, whether it is 750 cfm or 580 cfm. Remember, when people loosen the air door spring (presumably for more flow), they get a carb that flows 750 cfm when the engine needs 580 and thus the dreaded Quadra-BOG.

The trick with getting a 750cfm crab to work on a mild 350 is tuning. That Q-jet tuned for a 305 just need some larger jets and maybe some rods to run right on Fanuc's motor.

If Fanuc is getting an Edelbrock with a secondary air valve, then it is probably a Thunder series AVS carb. That air valve makes the AVS carb act like a Q-jet in that it will only flow what the engine demands.

I misspoke when I made a blanket condemnation of 750 cfm carbs on a 350. However, I am looking at Edelbrocks catalog right now and per Edelbrock's info, the 600 cfm is jetted for small blocks with .098" primaries and .095 secondaries, while the 750 is designed for 402" or larger motors with .110" primaries and .107" secondaries. Both carbs uses .075x.047 metering rods so the 750 will be fatter both at WOT and cruise.


Trust me, I understand Q-Jets. I am not in Lars category (is anybody) but I have tuned my carb for 3 different engine combos so far. Anything that can feed a 400hp motor and get 20mpg on the highway is great in my book. The only carb I would consider instead of a Q-Jet is a 750 cfm Speed Demon with mechanical secondaries tuned by Lars but since a new manifold, carb, aircleaner, fuel lines would be push $750, I'll think I'll stay with my Q-Jet for now. (At least till I go for fuel injection )