I know there are a few carb experts on the board and was hoping someone could give me a baseline for modifying my Rochester for my combo. I got my rebuild kit and number drill set today, and have been aquiring a pretty good selection of secondary rods and hangers. I've read Doug Roe's book cover to cover about a dozen times but still am not sure where to begin. Any professors want a new student?

I know there are a few carb experts on the board and was hoping someone could give me a baseline for modifying my Rochester for my combo. I got my rebuild kit and number drill set today, and have been aquiring a pretty good selection of secondary rods and hangers. I've read Doug Roe's book cover to cover about a dozen times but still am not sure where to begin. Any professors want a new student?

well...gotta have a baseline to start from. so far you haven't provided any info on your engine, cam,car,intended use,etc..

Try contacting Karl Boe or Cliff Ruggles. They've both got a clear understanding of the Q-jet basics and much beyond!

I can provide some basic info that will apply to all q-jets. Install bronze shaft bushings for the primary shaft. This will insure all incoming air goes thru the carb. Any play at the shaft will allow the engine to draw in unfiltered air and make tuning more difficult. Obtain a .136" fuel inlet seat. Pull the idle tubes, even if you do not need to enlarge them. Lots of debri and corrosion will be found in and under the tubes. Enlarge the baseplate idle holes to at least .090". This will keep you from having to back out the mixture screws the typical 3.5 or more turns for a decent idle. Leak test the bottom plugs and seal them up as needed. Obtain a brass float and a good kit with a new pump. NAPA sells both. Most low end kits will not have an accl pump, just a seal for the old one. Specific modifications to the idle tubes, airbleeds, channel restriction, jets/rods etc will depend on the carb number and original calibration. The original calibration must then be compared to the new combination and the carb modified accordingly. A good rule of thumb is to install secondary rods with about .040" tips and leave the primary jets/rods stock. Test the carb and go back and make changes to the areas that need help. If you post the entire engine/drivetrain combo and the q-jet number I may be able to provide some specifics for the initial set-up so at least it will be close enough to run well for initial evaluation....Cliff

http://www.tbns.net/qjetcarbs http://www.thecarburetorshop.com you can e-mail GTOKarl at gearhead@ponchopower.com

[ February 15, 2002: Message edited by: Robert Williams ]</p>

Thanks for the help Cliff.
The carb # is 7043264. The combo is as follows:
+.060 400/zero decked
6X-4 heads 250/190 cfm @ .550/milled .030 9.5:1
C.C. XE 274 230/236 @ .050 110 CL installed at 106
1.6 rollers give .520 lift
Torker
1 5/8" headers/ 2 1/2" pipes/ glass packs
ST-10 4-speed/3.08 safety track
3650# T/A with 26 1/2" tire
My kit has the pump,needle and seat and float.
The car runs OK now, but I have no adjustment with the idle screws and seems to be running lean in the mid and top end. I also didn't know you could pull the idle tubes.Can this be done without destroying them?

What Cliff said. Also, here is a link to a long discussion about idle tubes. I posted many pictures and describe how to remove them. The pictures will be there for another few days, then they are coming down as I prepare to post my website at the location they are currently being stored. Once I post my web site, the links will break and only the prose will appear in the threads.

http://performanceyears.com/ubb/ulti...&f=19&t=001107

If you remove them, post the orifice size, your air bleed diameter, and your air bypass diameters, and Cliff or I should be able to give some good recomends on changes.

Later

GTO Karl

Karl, glad to see you are moving along on your new web-site. Your old site was great, i can't wait to see the new one. [img]tongue.gif[/img]

Yea, what Cliff said.

Also,

Use a Top-plate with part-throttle enrichment circuit. I believe the 7043264 has it.

JB-Weld a decent fuel inlet to the centerbody, minus the internal filter.

Consider running dual Q-Jets...build two carbs, then try both on a singleQuad intake. Then you'll be more willing to buildup and try the Dual-Qs.

H.I. Stud

Having no control of the idle mixtures can have many causes. First make sure the baseplate holes are at least .090". Also make sure that the springs are not coil binding prior to the needles seating. Adjust the idle speed to about 800rpm, back out the mixture screws about 4-5 turns. Turn each idle mixture screw clockwise very slowly and listen for a speed change or the engine starting to run rough (lean). If they can be seated and no change is noted, look into the carb and see if there is fuel coming from the primary venturi (nozzle drip). Also gently tip in the choke flap or place your hand over the choke housing and see if the engine speed increases. Don't cut off all the air and choke out the engine, just richen things up slightly. If you get an engine speed increase then the idle system is not supplying enough fuel. It may be difficult and involve lengthy discussion to make specific recomendations concerning idle system mods. There are several different styles of q-jets, some use idle airbleeds in the airhorn, some do not. The main system calibration will also have some effect on the idle system, so there are variables involved with setting up each carb. I'll try to be of assistance, but it often gets complicated.

Have you expiriemented with different secondary rods? What size are you currently using? Do you know the primary jet/rod sizes? Are you still using the stock power piston spring?

I reassembled the carb yesterday and in the process I plugged the air bleed for the choke tube since I've filled the crossovers in the heads and am now using an electric choke. I also plugged the hot idle compensater and drilled out the idle holes to more closely match the needles. I left it at that and ran the carb. Idles much nicer and I can lean the idle mixture enough to kill the engine, but still can't richen enough to cause any change. Problem I did have was very weak pump shot and found the well wasn't filling unless I raised the pump up enough to clear the fill slot.I don't have enough pump travel to clear the slot and bottom the pump in it's well. I assume the fill slot must need to be lower than the pump cup to allow the well to fill. Is this correct? As for rods and jets,I've just changed the secondary rods from DB on N hanger to DR on G hanger to richen somewhat. Haven't driven this way yet.Primary jets are 72 and rods are stock as I have nothing to change them with right now. I did cut 1 1/2 coil off the power piston spring to keep it from coming in too soon.Thanks for any suggestions.