71 Idle circuit question

Currently working on my 71 GS455 and have a few questions. The car had a 73 vintage qjet that had one 75 primary jet and one 77 primary jet. Go figure. The car ran ok with that carb but not great. I have the original 71 carb and recently rebuilt it. It has both primary 72 jets which I believe are correct for 71, but not sure. I did not build this motor and am not sure if there is a cam in it. I can hear a bit of something that just may be the car running crappy. And this brings me to the main point. The carb I rebuilt to stock spec will not idle on the idle circuit. I have to turn the idle screw way in for the car not to die. At that point the entire transfer slot is exposed. I pulled the carb and looked at the throttle blades. Needless to say, the car idles like crap. It also idles very lean. I hover my hand over the carb and the idle goes up. If I cover it completely, it will die as it should. Idle mixture screws will also kill it if turned all the way in. The only vac leak I was able to find was the secondary throttle shaft. Not major, but rpm does rise when I spray it. I was also only able to squeez a max of 15 hg of vac out of it. So the question is...what would cause an issue like this in a stock motor? I know large cams can cause this, but I dont think that is the issue here. It is a mild one at best. I know putting holes in the blades will allow me to close the blades and get back on the idle circuit but would like to explore why this is happening first. Thanks for the opinions.

 

72 jets are way too lean fro a 71 455 carb. Stock was 73 I believe. I forget what metering rods were in a stock 455 but the Stage-1 in 71 had 75 jets and 45B metering rods. I think you should step up to the Stage-1 specs if you are looking for nay kind of performance. My guess is you are way to lean. Could be other problems as well. I can check service manual to see what was stock for that carb. Check float level as well.

 

You guys are right on the money I think. I got the car to idle on the idle circuit by drilling small holes in the primary plates. Can now get a pretty good idle at around 850 in P and 650 in gear. Still not perfect, but much better. This is a band aid I am sure but at least it runs decent now. My plans are to have a carb built by a reputable builder so until then, this will do. And you guys are right about the 72 jets....they are lean. Car has a pretty good off-idle stumble. I will see if I can find some 74, or 75 jets in my qjet collection. Thanks very much for the replies!!

 

I have been chasing a similar issue on my 71 Stage 1 GSX with a 72 GS 455 carb. I opened the secondary throttle plates by a few turns on the adjustment spring and that made a big improvement.

I've already gone to bigger jets, rods and changed hangers as well. I'm taking is one small step at a time and making careful notes before I drill holes. I may bump the base timing a few degrees and see how much that changes next.

 

The reason you have an off idle stumble is that your idle mixture screws are not optimized. Set idle mixture screws to get highest vacuum readings with wheels blocked car in gear with vacuum advance disconnected and plugged.

 

Guys, Something I do not see mentioned in this chain is the setting of the ignition timing. I have never seen an engine with an aftermarket cam not respond to a slightly advanced initial timing setting, especially if the cam moved the torque peak higher in the rpm band. The reason is that there are two types of compression: the first involves the ratio of the cylinder's volume at the bottom of the piston travel, and the volume of the cylinder at the top of the piston's travel. This is the mechanical compression. The other type is that of effective compression, and this is where the ignition timing comes into play. If, for example, the cam used moves the power peak up in the rpm band, the ability of the engine to properly fill the cylinders with mixture at lower rpm's is compromised. If the timing is advanced slightly to compensate, the ignition point occurs slightly earlier, allowing the mixture extra time to expand during the burn. This extra time relates to extra pressure, raising the idle slightly, and increases the idle vacuum. Though this may require other changes to the timing advances, it is part of the tuning process, and should be done BEFORE changes are made to the carb. On a related note, the mixture delivered to the main system in a Qjet is the result of the combination of the rods and the jets, not just the jets. I have never seen a properly calibrated and built engine for the street require stagger jetting, (where the primary or secondary jetting is different side-to-side), unless something was affecting the intake's mixture distribution.
I think all of you are on the right track here; I am not sure, however, that all of the bases are being covered. Let us know... Ray

 

Any time you have to bump the idle speed screw in to help the idle, you greatly increase the risk of nozzle drizzel and weaken/shorten the sudden acceleration fuel supply. MINE STAYS ALL THE WAY OUT AT ALL TIMES.

The fix is in the modification of the idle system AND the initial timming.
Drill the adjustment needle holes to .096 straight through AND... from the top down of the base plate. Drill the baseplate air bypas holes to .096-.100 from the bottom up (not the throttle blades). Drill the idle pickup tubes to .037 and down channel to .052. Drill the upper and lower idle bleeds to .070.
Weld the distributor mechanical advance slot up a little to narrow the slot (or try a brass collar first) and limit the timming advance to about 20 degrees total. This will give you 12 initial to 32 total or 14-34, etc.

Know what your drilling before you drill it!!!
The numbers I gave you are for a mild-medium street cam with 13-16' vacuum.

Idle will be easly adjustable at the needles and you will not see your nozzels dripping and have no stumble-bumble-rolling from idle up to 1000-2000 rpm and the idle should remain strong when you drop the tranny into gear, and the carb should hit like fuel injection when you slam the throttle down.

Buy Cliff Ruggles book on q-jets.