I have a complete 70 qjet (750cfm) and an 800cfm carb body. Is it possible to use the throttle plate and air horn off the 750 to put on the 800 body? If not, I have an 800 carcass to use. I just need to figure out how to remove the seized screws on top of it.ou:
TTT Does anyone know this? They look identical. I was just wondering if there were any extra ports or things in the air horn or throttle plate with the 800 as opposed to the 750.
Throttle blades are the same size. The 800's were BB carbs so they have the fixed idle air bypass. Take close look at the throttle plate for additional ports. you might be able to use them, not sure.
I looked pretty hard at my big and little q-jets, and could only see one difference besides the bigger venturi ring, but my vision ain't what it could be.. The 800cfm q-jet has larger air bleeds (fixed air bypass?), look at the air horn, there are little press-fit brass "tubes" that open top-to-bottom, above or beside the "bases" of each venturi, they go in from the bottom, and meter air into one (per side) of the fuel wells that feed the main venturi. A picture would be worth a thousand words here... I've swapped them out for the smaller 750cfm air bleeds, so I could use the 800cfm airhorn on a 750cfm carb body. Seems to work ok while I'm shipping the real 750cfm off to John O. I'd think if you put the smaller air bleeds from the 750cfm carb onto the 800cfm carb body, you'd go way rich, as the smaller air bleed would pull more fuel / less air into the main venturis. But you should be able to swap them out, just get a small punch and go easy. I've heard that either the idle ports or idle mixture passages are bigger. I wouldn't doubt it, I put an 800cfm carb on my mostly-stock GS350 engine, and with the throttle screw backed clear and the throttle blades CLOSED, it still idles at 700rpm. Turning in the mixture screws will idle it down and kill it, however, but that's the only way... Any Q-jet experts out there? Does John Osborne haunt the board?
I also swapped out the throttle plate (the entire bottom piece) between carbs. Right now, I've got the air horn and throttle plate from a 750cfm and the body of an 800cfm on my 350, it runs fine, just won't idle down like I'd prefer. Of course, I'll soon have a 750cfm John Osborne q-jet! :3gears:
I have torn down both carbs finally....and will post some stuff later tonight if I get a chance... But I need new shoes and hair goo. So I'm off to run errands in the drop top:bglasses: It is 70degrees out today!
Adam I will through you one in the mix. I have dealt with an experienced carb builder that also races Buicks. He says that a set up 750 runs the exact times of an 800. He bases this statement on runing them back to back on a number of occasions on the same car at the strip.
I seeeeeee... I need to look at the 750 carb body to see if I can clean it up. I just realized today that the 800 body I have doesn't have any jets in itou: So now I'm trying to pull a set from one of my other cores to plug into it. James, so you're saying that the top of a 750 doesn't work well on an 800 body? (which is exactly what I am planning on doing)
Ok, I did a little photgrafee with my new digital bad boy camera. James, here are the holes you were referring to: 750cfm air horn: vs the 800cfm air horn: I think we can tell which one I cleaned up tonight:laugh: Ok, now here comes the fun part....I am led to believe that the air horn on the 800cfm carb that I have is a later model smogger air horn on top of a 71 carb body. Check these pictures out for the following reason: Now look at the stacks on the 800cfm unit: (I've never seen a qjet with 6 tubes sticking out of it) And the 750: (The earlier 800 air horn should look just like this, am I correct??) Now, would the smaller hole size referred to in the first two pictures be a factor when installing the 750 air horn on an 800cfm carb body?uzzled:
Adam, I've never seen those extra tubes before either. I'd start another thread in the engine tech forums, and ask if somebody there knows what they are, or has a later-model chassis manual and would look. Offhand, I'd think that you could block them off, but... As far as the rest of the airhorn, you can swap out the air bleeds (WOW, that's a CLOSEUP!!!) Just be careful, and use a small punch, about the size of a finishing nailhead (not that I'd use a finishing nailheal - - - ) to drive them in/out. I've marked the below pic to confirm where the bleeds I'm thinking of are located.
Yep, those are the bleeds. Do you really think that minimal size difference is going to cause problems? I started a new thread about the extra tubies on the 800 air horn here: http://www.v8buick.com/showthread.php?s=&threadid=37854
Yours don't look to be as different as mine, I wonder how many different sizes there are...but offhand, I'd think it would make a difference, as precise as the other jets / orifices are. I think I'd give it a try as is, see what happens, and if it goes way rich, then I'd swap out the air bleeds and compare. But then, they call me O.C. (Obsessive-Compulsive)...:Brow:
