Would this carb work?

[Babaganoosh]

I need a new carburetor, mine is a piece of junk. I have been watching this one on ebay trying to figure out if it's what I need. The seller says it's a 750CFM and will work on all transmissions. If I recall the linkages are different, if needed could I use the linkages I have now on my quadrajet to make this one work?

1970 Chevy Carb in perfect shape No. 7040503 1970 Cars, all trans, all engs. 1970 Truck 350' & 396' engs. All trans. Comes with helpful information sheet. Flow tested for quality.

I have decoded the number and came up with this

70 Rochester products division#
4 1970-1975
0 last digit of model year
5 Quadrajet (4 bbl) California standards
0 Chevrolet
3 Manual trans.

My current engine specs are a bone stock 78 350 with emissions standards for a 1982. Would this carburetor work just fine?


Thanks
Ben

 

[GNERGY]

I just found this.

http://phoenix.craigslist.org/evl/pts/826222514.html

Tarey

 

[Babaganoosh]

That is also a option, but I would just like to replace it with a quadrajet instead of messing with linkages and mounting it.

 

[Randy92782]

Stock motor?? 750 sems awful big if so. I thought the quadrajets were 600 or 650?? Guess they coulda made a 750 also

I'd say it's too much if it's a stock motor. Just my opinion

 

[TerryD]

They made 3 Q-Jets, 730, 750, and 810. The 810 was the BB Chevelle Carb.

 

[Thunder]

I dont think That carb is exactly a direct fit.
Check your choke. The older Q-Jets like that one are divorced choke. I believe the choke on your 78 is an electric choke. I think they make a kit to convert the older divorced choke to electric. You will need it to make the choke work. If not you will have to put a manual choke on it.
Other than that I think everything else should hook up. You may need some of the linkage parts on your old carb. Q-jet linkage is pretty standard.

 

[Randy92782]

They made 3 Q-Jets, 730, 750, and 810. The 810 was the BB Chevelle Carb.

learned something today

 

[Thunder]

Randy. Q-jets are a very unique carb. They only deliver as much air as the engine needs. They have an air valve on the secondaries that only feeds air based on demand of the engine. Even tho it is capable of 750 cfm. When installed on a stock 350 for instance. At WOT It may only be giving 600 -650cfm to the engine.
Because it has small 380 cfm primaries Q-jets can be adjusted to work on almost any size engine.

 

[Randy92782]

Randy. Q-jets are a very unique carb. They only deliver as much air as the engine needs. They have an air valve on the secondaries that only feeds air based on demand of the engine. Even tho it is capable of 750 cfm. When installed on a stock 350 for instance. At WOT It may only be giving 600 -650cfm to the engine.
Because it has small 380 cfm primaries Q-jets can be adjusted to work on almost any size engine.

ya know if i learn too much my head hurts right?/ you do now...

But i did NOT know that. Maybe the actual numbers (cfm's) you posted are what i read before

 

[Thunder]

My head hurts too. I had a long intense discussion with that well known intellect, Adolf Coors last light.
I must have learned a lot.

 

[Hogback Fabrication]

They made 3 Q-Jets, 730, 750, and 810. The 810 was the BB Chevelle Carb.

Never heard of a 730 or 810 CFM Q-jet.

As I understand it, there are 750 or 800 CFM units. The 800 CFM units had 50 more CFM in the primaries and were used on lots of different big block motors. The 750's were put on everything from L6's to (sometimes) big blocks.

 

[Babaganoosh]

I dont think That carb is exactly a direct fit.
Check your choke. The older Q-Jets like that one are divorced choke. I believe the choke on your 78 is an electric choke. I think they make a kit to convert the older divorced choke to electric. You will need it to make the choke work. If not you will have to put a manual choke on it.
Other than that I think everything else should hook up. You may need some of the linkage parts on your old carb. Q-jet linkage is pretty standard.

Yes the choke i have is electric. I have been trying to pinpoint the electronic part on the choke in the picture, I don't believe it has one. I'm going to have to look into this manual to electronic kit to convert the choke.

 

[TerryD]

When installed on a stock 350 for instance. At WOT It may only be giving 600 -650cfm to the engine.
Because it has small 380 cfm primaries Q-jets can be adjusted to work on almost any size engine.

Here's a neat calculation from Holley to help with carb sizing. Now, any carb with vacuum (or like the Q-Jet) will not open past what the engine can use, which is why they work better than mechanical secondaries.

(cid*maxRPM)/3456=Max CFM of a given motor.

Q-jets only come in so many sized, but the adjustable "air valve" on the secondaries allow a 750cfm carb to act like the 500cfm your engine needs.

Never heard of a 730 or 810 CFM Q-jet.

As I understand it, there are 750 or 800 CFM units. The 800 CFM units had 50 more CFM in the primaries and were used on lots of different big block motors. The 750's were put on everything from L6's to (sometimes) big blocks.

I do not remember where I read that the Q-jet came in those sizes, but I think it was a Rochester manual my Dad has from the 70's. The next time I'm over at his house, I'll check in some of his old manuals and see what I can find.

 

[Babaganoosh]

I got a reply about the choke, This is an early model when the choke thermostat was in the intake manifold. Is there anything I can do to make this work? I wouldn't bother, but he has the buy it now for a stupid price.

 

[Hogback Fabrication]

Here's a neat calculation from Holley to help with carb sizing. Now, any carb with vacuum (or like the Q-Jet) will not open past what the engine can use, which is why they work better than mechanical secondaries.

(cid*maxRPM)/3456=Max CFM of a given motor.

Another correction here:

that formula is for the recommended CFM carburetor to use, not the max CFM that the motor can use. A bigger CFM carburetor will always make more power on any sized motor. Less intake restriction means more power.

Any time you hear about a carb being "too big" for a motor its because at part throttle and idle, large carburetors run horrible on smaller motors. They simply don't have enough adjustment.

Large, vacuum secondary carburetors with small primaries (like the Q-jet) can sometimes get around this because the primary and idle parts are smallish (and work well on most 200-400 CID motors) while the secondaries are large and can support lots of power, supposedly well over 500HP (and the vacuum secondaries insure that only the air that is needed is passed through the carb).

 

[TerryD]

That calculation is for the Maximum CFM an engine can "pump" at 100% volumetric efficiency. Considering that most engines can only achieve ~80% VE, then the numbers it spits out is already more than the engine is capable of moving.

I understand what you're saying about the larger carb, as I run a 750 Holley on my camaro motor and that equation says it only uses ~688CFM. However, running an over sized carburetor (like a holley or edelbrick design with relatively "square" bores) changes where the engine will run well also. A correctly sized carburetor will run better throughout the power band, giving you maximum power across the power band. A smaller carburetor will give you better throttle response and more low RPM grunt. This is because you have maximum signal to the boosters due to the reduced space the air if flowing through. A larger carburetor will really bring the engine to life on the top end of the RPM band, but can cause lacking performance on the lower end. My Camaro with the 750 holley wont spin the tires, but turns ~6800 RPM. The same motor with the 600 holley I have, melts the rear tires on the starting line, but will max out around ~6000RPM.

What I'll dispute with what you're saying, is that RARELY does an off-road go over ~3000 in crawling, where you need the grunt. Telling guys who don't know how a carburetor works that you can make more power with a bigger carburetor on their engine is asking for trouble. I gave the calculation so that they could figure out how much air their engine is capable of moving and pick a carburetor that will work best with it.

With that said, this does not apply to Q-Jets because of the ultra-small primary size. They provide the best combination on the market. The only reason I don't run a Q-Jet on my Camaro is because I don't have the manifold vacuum to keep the primary metering rods down in the jets at idle and off-idle was suffering. (I did break the motor in on a Q-Jet because I had one already assembled and my holley's were scattered all over the table). IMHO the Q-Jet is the best carburetor made, hands down.

 

[Hogback Fabrication]

That calculation is for the Maximum CFM an engine can "pump" at 100% volumetric efficiency. Considering that most engines can only achieve ~80% VE, then the numbers it spits out is already more than the engine is capable of moving.

Carburetors are rated at XXX CFM at a certain pressure drop. 750 CFM is by no means the max a Q-jet can flow, its the amount of flow that creates a 1.5" Hg pressure drop (the standard by which 4 barrel carbs are measured).

An 850 CFM carb may only create a 1" Hg pressure drop while 750 CFM of air is moving through it. Hence more air makes it into the motor and the bigger carb will make more power.

What I'll dispute with what you're saying, is that RARELY does an off-road go over ~3000 in crawling, where you need the grunt. Telling guys who don't know how a carburetor works that you can make more power with a bigger carburetor on their engine is asking for trouble. I gave the calculation so that they could figure out how much air their engine is capable of moving and pick a carburetor that will work best with it.

Your advice for picking a carb is spot on, I was simply making a point that a lot of people overlook.

I'll agree, in general its better to err on the small side for carburetors (with Q-jets being the exception like you said).

 

[TerryD]

Your advice for picking a carb is spot on, I was simply making a point that a lot of people overlook.

I'll agree, in general its better to err on the small side for carburetors (with Q-jets being the exception like you said).

Gotcha!

I'm not used to talking to people who know how a carburetor works. There are so many guys around here with stock 302's and 350's with 750+ CFM carbs its not even funny. They all complain that their carb isn't big enough or doesn't put in enough gas. When you start talking to them about how a carburetor actually works and proper sizing/performance, they get mad and you get the "well, my daddy says..." speech...