The 4MV, This is what I have done to my carb. This is some of what I have collected from the wrecking yards over the years. I have numerous choke pull offs, plastic secondary unloaders, linkages, electric chokes, regular chokes, throttle shafts , butterflies, everything and everything. I have some pics here of my carb. This will be on my motor Im building. My latest modes for it is the adding of Locktite weld to the air path to smothen out the path. Too many bumps and humps in my opinion. I have bored out the primary side with flappper wheel and drill press. I used a hammer and peened the original throttle blades to make them bigger. It took more than one set to get it right. No knife edging, just thinning and flush tapered screws. It should be a little over 1000 cfm. I removed the secondary well lugs and drilled the passageway even and smothened the corners , no more restriction for fuel. I have 5 different HP books for Rochesters, all have similar info , but each has a mod or two that the other ones dont have. So I combined them all to get the most, or dam close to the most overkill 4MV I think I can get. I think its good overkill though. My drill or not to drill is the one thing that I am perplexed on still. Two books mention it , and they are not in agrement. I have tried both set ups many times, I cant notice a power differance, The only thing is when rev it in neutral the drilled one is crispyier. One thought on it occurs to me, when the wells are refilling , and yes I have slightly enlarged the holes that refill the wells, which would refill quicker, while still on the pedal. drilled or not drilled. Keep in mind this is not the main secondary circuit. I found the tubes you see in the pics with the holes drilled in them off a turbo regal, this is the main feed for the secondaries. So please lets get some other thoughts on this
Not sure how the Pontiac airhorn got 301 nozzles in it and slotted flaps, but get solid nozzles in it and solid flaps. It could use a new cam and spring as well. If you are going to use it on a really fast car, I'd fill the slotted vent as well, or use a divider behind it. The stop looks too long, might want to shorten it up to increase cfm.......Cliff
The nozzles I got off a turbo regal, Early 4 barrel turbo on a v-6, The flaps with the slots came off a different Pontiac, was a 455 fire bird 73 or 74. When I had the solid flaps it would pull fuel from the holes just above the secondary flaps. I tried lowering the float a hair but didn't help. I know and checked the throttle blades were not leaking . But when I would open the air horn flaps a little it would stop pulling fuel at idle. So I put these flaps on and it worked great. The slotted vent in the middle with the screen, Yes I do have the divider, just not in the pic. I left the primary butterfly off to just to see the fill in material i added. What is the stop? Also I checked the cam with dial indicator with the magnetic base. I actually found the cams to be all the same, but the metal part that the cam lifts and the hanger screws on to is what gets worn the most. I found the newest one in my pile to be 12 thousandths more lift. Are you talking about the air horn sec spring. I'm sure you experienced how the spring starts to drag and wear the pin on the shaft flat. I replaced the pin but still not smooth enough. So I found that if you bend the pin slightly toward the spring its smooth as butter. Even on a worn out flat one it still smoothens out after slightly bending the pin. The carb has been on my HotRodRivi for many years, its going through an new rebuild now, I'm going close to all out on this motor. I haven't run it with the fill in material, that's my latest modification. Why do you think they put nozzles with holes on the turbo carbs.? I have always wondered about that.
I too, would like to know what Cliff means by the "stop". If he is referring to the open angle of the secondary flap, my belief is that the secondary will produce no benefit beyond 80 degrees of open angle when measured from the air horn gasket surface to the big part of the air flap. I have read that 90 degree openings produce: "turbulence"... "diminishing returns"... "blocked fuel nozzles"...& "top end lean out". Buick used an 80 degree slot and Pontiac used a 76 degree slot on the end of the flap rod for big engines. Less angle was used for smaller or lower powered engines. I assume the qjets were tested in some way by the engineers that made this complex carb. Many of our Buick guys say they use 90 degrees or straight down for the flap opening. I would like to see this tested on a dyno. I cut some 76 & 80 degree cardboard templates to measure mine. I did not trust my ability to measure them to the Cliff book recipe using a ruler, especially since I had already smoothed the edges of the bore in the air horn. Also like to get comments on the 4-6 degree forward pitch of the secondary throttle valves. Is this just for dual plane manifolds that run lean up front?
I think the forward pitch is for the secondary enrichment circuit. The 2 holes above the flap or just below on some models. I have read that you get the most power when the leading or front of the flap lines up with the divider. I think that would put the back part of the flap at 80 deg or so. Ah forward pitch , over center , I have also read its to direct some air toward the front. I dont think this works for my 455 though. I had mine set up that way and on the freeway I noticed that after holding it floored, if I backed off the pedal a little I actually accelerated. So I scrapped that idear.
The Pontiac airhorn has the POE exit holes in the correct location (best set-up). It pulled fuel at idle because you are using on a main casting that lacks internal venting (miss-matched parts). The solid flaps are a LOT better on that airhorn than the notched ones. The turbo fuel nozzles were cross-drilled due to the limited opening of the airflaps (long stop) on the turbo carb. That airhorn needs solid nozzles, all the fuel is pulled thru on the back side of the divider. ALWAYS replace the cam and spring, the cams wear down and get loose on the shaft, the spring gets tired, used parts at those locations cause more issues than any other with correct secondary operation than any other that I know of.....Cliff
The numbers off the body match pontiac nums . The only thing I changed is the throttle plate. The previous plate did have the air bypas holes , that bleed to the 2 holes on the outer edges of the main body. When i ran that bottom plate my carb sounded like a jet fighter with all the air sucking through it, also my whole carberator and even the top of the intake starting sweating like a pig, and was cold to the touch. This worried me so I switched to the throttle plate I have now. Then it had no holes, I recently drilled them and got rid of the throttle primary blades that i drilled two holes in. It all added up to too much bleed. Previous set up had primary throttle blades with one hole each, in line with the idle mixture screws, and holes in the throttle plate for the air bleeds on the outer edges, they lead to the primary side at 5 or 7 oclock. You can see it in one of the pics of the underside of airhorn, looks like a notch in the circle. That was the wet cold one. So I changed to this throttle plate which had no bleed holes and did not sweat the carb. Im trying to get it back to proper now. with what you see in my pics. I also had no idle mixture adj, car would run all the way screwed in ( but stumbly and almost die. ) ounce I got past one turn out of each needle I could remove the needles and it wuld remain the same. But it was smooth . So I rolled with it.
These 2 pics better show the air bleeds im talking about. I drilled the throttle plate holes, it had the channels but no holes, Are these the venting your talking about?
"When I had the solid flaps it would pull fuel from the holes just above the secondary flaps." Two scenario going on here. Either you have a main casting that isn't vented (required for that airhorn) or the secondary throttle plates are not fully closed and seated at idle. Notched flaps were not and should not be used with that airhorn.....Cliff
Ware are the vents for the main casting that i can verify my casting. I did address the throttle plates while i have it apart. They were fully closed but I could stil see light through them. I went through my pile until I had a pair that lined up better with no light. The slotted plates were a quick fix, that worked so I stuck with it. I appreciate the help , so keep it coming because we arn't there yet! I think this pic shows one of the holes.
That casting is already vented, so IF you see fuel getting pulled from the POE at idle, the secondary throttle plates are cocked open some. I also noticed that a lot of the venture area has been removed from the primary bores. Correct carburetor function relies on he pressure differential AND increase in incoming air velocity above/below the boosters. What power level are you trying to support? Engine specifications?......Cliff
TE stg 2 TA, roller cam, sperical dish pistons, hydrolic or solid havent madeup mind yet, sportsman rods, shorty headers 3 in exsaughst, the cam I want enought to get in the 575 HP range. Sp2 intake. I know what you are saying about the venturi, But to gain cfm you have to loose something. I actually noticed an increase in power, I have another casting that i didnt touch the venturi, It is more responsive but dosent have the sustained pull at the top. This one does.
I m sure it was the throttle blades open a little, that's why the slotted butterflies helped. The stop , is that on the secondary butterfly shaft. Because I did change that, The carb was kick ass before , but now that im correcting my blunders, I might even get back my idle mixture adjustment, and I should have more cfm, But the motor it was on is destroyed, broke the # 4 connecting rod at peak torque in second gear, The carb, timing cover, roller rockers valve covers are the only things being reused, and distributor.
Seat of the pants assessments for power production are pretty much useless. You need to get the car on the track, engine or chassis dyno to see whats going on with your modifications. Your engine build doesnt need anywhere near that much cfm, so hogging the carb to death is pretty much a waste of time/funds. I have stone stock smaller 750 cfm castings running on engines in Stock and Super Stock classes making considerably more power, some of them run into the mid to high 9s in the quarter mile, and a grinder or sanding roll has never touched the castings. I just happen to use the same carb that you do on my 455, makes up near 600hp and over 600ft lbs torque. Right on the dyno it outran a custom tuned HP950 by 2HP. Its a stone stock 1977 Pontiac q-jet, just recalibrated exactly for the application. Im not saying there is anything wrong with cleaning up the castings, but extensive modifications to them that hurt the basic design just arent a good idea. This starts with hogging out the throttle bores in the baseplate so they dont seal when the plates are closed. Pulling air past the secondary throttle plates is a vacuum leak, they MUST seal tight in the bores, so all the incoming air comes down thru the primary side of the carburetor. Id start there, fix that problem, and put the solid airflaps back in the airhorn, they are superior in every respect to the slotted ones, for a variety of reasons. Id also obtain another main casting (those are very common). The hogged out primaries are reducing sensitivity to the boosters, and efficiency. The venturi area is stepped and narrowed for a reason, as it creates three areas so the boosters can supply fuel at very light throttle openings up thru heavy/full throttle progressively. This is why you noticed a loss in power at lighter throttle openings. A good friend of mine uses one of these carburetors on a full time drag race GTO, on an engine making 735hp. It runs high 9s over 135mph. He has back to back tested the q-jet we built him many times against his custom 1000cfm Quick Fuel carb, and they run the same ET and MPH. He sticks with the q-jet because it drives better down the return road, and much easier to stage (foot brake car) on the starting lineFWIW..Cliff
I took care of the throttle blades, and I put back the solid top butterflies. I do have an untouched casting. But man i sure put a lot of time in this one. you cant over cfm a rochester as far as i have read. I have to try at least with the adding material and streamlining the pathway. I was thinking that with the double booster that was enough of a good signal. I will try just swapping the main body untouched too. But with my airhorn.
