Well I did the little shop vac deal stated in the other thread . The fuel pattern looked the same except the thined shaft on the right in the pic in the other thread flowed about 75 to the rear of the divider and 25% to the front of the divider, the untouched side was all to the rear of the divider, The other thing and what i think is the most significant thing is the air flow or cfm was noticably higher on the thinned shaft. I could tell by the sound of the shop vac. You could say that as cfm increases velocity decreases. I used brake parts cleaner, filled the bowl up 4 times, all with the same result. Beign that this carb will be used on a 290H cam, Ta stg2 se head, spherical dish piston, B4B intake, shorty header 3 inch collector and TA 3 inch exh , i think it will be benifical.
Now I'm thinking a little more about it , the throttle body hole for sec remains the same so there is no decrease in velocity just a gain in cfm, and more flow to the front without having throttle blades going slightly forward , slightly forward you loose a little flow. I have found the best flow just slightly back, so I don't know why of 4 Rochester books not one mentions thining the air horn shaft. But I'm very sure it's a positive to do so. Thank you very much
That is all I removed so far. I'm going to go all the way across and try again. It's not a flow bench but pulling air through the bottom is the same .By looking at the pic u can see how restrictive the shaft is.
We have the smaller "750" cfm q-jets in service on legal Super Stockers running into the 9's and at Tech they'll grumble if you have nicked the throttle shafts grinding the backs of the screws off! I really see no need for these type of modifications, aside from maybe a FAST Class car where the only thing that has to be stock is the part number on the carb. For most folks with street/strip engines under 750hp or so, you're not going to find any big improvements in ET or MPH with these sort of things......IMHO......Cliff
One definite thing I have noticed about grinding out and actually adding locktite weld to smothein things is I can cruise at 100 mph with just the primaries . Not that that happens often. I think it's mainly i have a hard time spending 900 bucks or more on a carb. Most cfm gains are found on the primary side, but I have to correct something above. It seems it was the position of my hand that directed fuel forward of divider, wasn't really forward before divider , it was curving around the bottom. BUTT! This pic does do just that! The holes added to the discharge tubes do in fact direct fuel before the divider. Which will help even out fuel distribution.
To see any benefits from those modifications you would need to do some direct testing. I've tested carburetors on the street and at the track with paper thin shafts, ported extensively above and below the venturi/booster areas, etc. There is absolutely NOTHING to gain with these mods that my engines can see in the 1/4 mile. I think this simply happens because the restriction or choke point is at the boosters/venturi areas. What does work quite well and will show significant increase in cfm, power, and track performance is to remove the outer booster rings, or to obtain a 1971 Pontiac 455 HO carburetor that didn't have them. Back to back at the track my car picked up 1.5mph with no other changes. The down side to that particular modification is that you loose sensitivity to the main nozzle, less atomization, and I'm certain fuel economy would suffer some. As far as thinning shafts, NOT recommended. They are too thin right to start with, and very delicate. We've had a number of q-jets sent here over the years with this mod and it become very difficult to get the throttle blades fully sealed and correctly aligned in the bores. They also get twisted very easily, and simply tightening or removing the end screw that retains the fast idle parts can twist the shaft before they are thinned down. Not trying to put a big black cloud over this topic, but these sort of modifications are best left for big HP and "max effort" applications, where there are no rules applied......IMHO. The factory cross-drilled the secondary fuel nozzles on some q-jets intended for small CID applications and they added a long stop for limited air flap opening. This was done because the long stop increases a much higher percentage of the airflow down the area in front of the divider plate. Once the stop is ground down for higher flap opening, the holes are not needed, and using or adding them can actually hurt engine power. Coincidentally we ran into this last summer on a carb we supplied a kit and tuning parts for, built elsewhere. The dyno shop testing the engine couldn't get it to make good power, and replaced it with a Holley carb which worked fine. Long story short, the carb ended up here, it had the cross drilled nozzles in it. We replaced them with solid nozzles, sent the carb back, and it ran right with the big Holley carb everyplace.....FWIW......Cliff
Before I further elaborate on this I must say thining the sec butterfly shaft is a no go , Did not help , less draw on the fuel circuit. But at least I learned on it
Thanks fir the input Cliff it took u long enough. Could have saved me some time and a shaft. Haha. After another can of brake cleaner I have seen that that narrow gap at the sec nozzle is crucial fir suction on the sec nozzle. So I have switched it back to a non hacked shaft . As far as carbs cfm n fuel go I had a vw 73 bug that would lug up hill in 2 gear, with some drilling I just made it rich , then I thinned shafts , cleaned up casting. , I had the bug going up the same hill almost toping out third gear. On a storm burg carb. So I gave that car away and been messing with Rochesters 4 barrels . I have no track or dyno or the money to do so. But as far as the qjet goes if u optimise the ability to pass fuel you can make it better.
As far as Qjets and exceptional mileage, the mods I made were limited to getting the carb to feed the power requirements (fuel flow) and the response. The mileage enhancements were really with the engine and getting it to draw the carb effectively ie. airflow: VE, airspeed/velocity, piston speed, then tuning...ignition curve, etc. It already does a good job vaporizing at the point furthest down the intake tract possible.
This is what I have done to my 4MV roch Remove sec well plugs and drill file sand the leading up narrow passage, smothein sharp turn . fuel inlet behind filter, I opened rounded the turn up into needle n seat , use the biggest hole open side window seat, 76 jets , with 52 m needles. Biggest needles I could find. Thin shafts and use flush screws. Some people say they break shafts but never hapened to me. I lock tite the screws , remove them still never break shaft. I don't knife edge blades. use plastic / metal pull off sec and drill hole in pulloff for easy fast open, then adjust spring for just open smoth n hard but not the big DaWooop break tired loose way. A lot of primary porting , then use locktite weld to smoth over the bumps , drill air bleeds, some say has neg effect but I disagree , maby if that is the only thing u do. Also sec discharge nozzles , add 2 holes in middle , with bigger sec passages this gets more fuel to the front cylinders without opening throttle over center, again if that is all u do it may not help. An electric pump with return line regulater with 6 to 8 psi, and you'll have at least. 1000 cfm kick ass Rochester.
I will also add this is my third main body , made many mistakes along the way but I think I have this one rite , I have run this carb but without the added locktite weld smothin ing
No just feel . And freway . motor is nothing close to stock. But if I had any feeling of doubt that anything I did did nothing, I would not be mentioning it on this forum. I would instead start a How to make your Rochester suck thread.
As it relates to testing, not just to this thread, "seat of the pants" testing is more often than not completely USELESS to evaluate engine/vehicle performance and efficiency. This is why we have dyno's, chassis dyno's, drag strips, or at least we can mark of a measured distance and get a stop watch to see I we've helped things out? There are tests that can be done to check tuning efforts. Fill up a the same station at the same pump, drive a given course/distance at the same speed and return to the same pump and fill up. If you consistently use less gallons after one modification compared to stock or another modification, then it will have been beneficial, and improved engine efficiency, etc. Simply leaving the driveway after any particular modification and simply driving around and trying to "feel" improvements, not very scientific and subject to many flaws......IMHO......Cliff
I can say for being scientific I am not, instinct feeling of inertia, I'm in tune with, I have done primary testing by locking out the secondary's with the lockout lever . I know that before full primary it would take quite longer to get to 100mph. I can maintain 100 with it backed of full primary a little now . It also gets there a lot faster. A lot if mods get over done and become negatives , if you don't have lots of spare parts , your not likely to try it again , I gave gotten very good at over shooting and tailoring back just rite. Like filling a trany on a fresh build, would almost always overfill it, then suck pump just enough out and be on the money
