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Q-jet screw settings & general question

FlatBlackBurb

FlatBlackBurb

1/2 ton status
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Santa Cruz, CA
i'm a carb noob. :mad: i've never had to mess with any of the settings on my '69 caddy (only other carb'd vehicle ive owned until now) so i don't know what i'm doing. i did a search but didn't really find anything so i'll ask the experts.

what is a good setting for all of the mixture/fast idle/idle screws on my carb? the truck is a '77 camper special with a small block 400.

so far after i put the carb back together, i screwed the two air fuel mixture(?) screws on both sides of the bottom all the way in and then out 1 and a half turns each. i did the same for fast idle on the passenger side of the carb and the idle too. the bottom primary butterfly valves (sorry if i'm screwing up the names here) are barely open right now is that how they're supposed to be?

also when i pull the throttle, the bottom primary valves and then the secondarys open just fine all the way to WOT, but the primary valves on top open about halfway and that's it. the secondaries don't even open.

all the linkage is back the way it was taken off and that stupid manual that came with the rebuild kit wasn't very good or clear at all.:doah:

if theres a good thread on ck5 somewhere about this, please send me a link. i couldn't find it.

edit: i should add that when the truck warmed up, the idle was excessively high so high that the wheels would start spinning if you were off the pavement. even with the brakes on. throttle linkage seemed to be okay when i took the carb off.
 
If your idle mixture screws have the fine threads, go 4-4.5 turns to start, if your they have coarse threads then go 2-2.5 turns as a baseline.

The fast idle adjustment. What I did was engage it, (the linkage will hold the throttle open some with the choke flap closed) Let the engine run and turn the screw to adjust the RPM's up and down. You can do it by sound or if you have a tach even better. Usually between 1000-1200 RPM.

Now the choke will be on top of your primaries opening not all the way. My old Q-jet did that, but it had the stove top choke. I ditched it and put an electric one on it. Most parts stores have a conversion kit if you don't have electric. I suggest that route. It will tell you have to adjust the choke in the kit. Hope this helps
 
I just read your edit. If the idle is adjusted correctly and the idle speed is still too high then you could have a vacuum leak somewhere.

Did you check all your idle/air bypass ports, and or idle tubes? I know you can't check the tubes but they do clog up and can be replaced.

A good place to check if you can't figure it out, give this guy a call. He is awesome and he helped me a ton on my first rebuild

cliffshighperformance.com
 
You need to get your idle down to normal before going further.

You should have a vacuum guage to set the idle mix screws properly.

Linkage sounds correct, the top secondary flaps only open under load. The engines vacuum opens them.

On cold start up it should idle fast after pumping the gas 1.5 times and the choke will be closed. After a few minutes you should be able to kick the throttle down to a lower RPM, choke will open. After it's completely warmed up you should be able to kick it down again to it's base idle speed 700ish RPM in P or N IIRC and the choke should be fully open. I'm assuming you have an auto trans, there is a specific idle speed for when it is in gear too.

Right off the bat it sounds like your base idle screw is in too far and holding the primary throttle blades open. You should have a tach to help set the idle speed properly. If you don't have a tach it can be done by ear if you know how your truck runs.

Keep in mind, after you get your idle set, once you start adjusting the mix screws the idle will change. You will need to reset the idle again. Getting this done right will take time and trial and error. Don't be worried to spend some serious time on this especially if you're unfamiliar with it. Carbs are not "bolt on and go" items. You may find yourself tweaking this thing a month from now when the weather gets warmer.

edit: Oh, wait, you're in CA. It's always warm there.
 
496truck said:
Oh, wait, you're in CA. It's always warm there.
Click to expand...

haha funny you should mention that, its actually been pouring these last couple of days. we've had some pretty big storms over here.

and thanks for the info it really helps. i'll probably go with the electric choke like the first post said if i can't screw around with the primaries on top. when i took it off the truck they opened all the way and now they won't so it's gotta be something i did. back to the workbench. after work of course.:mad:
 
You should be able to open the choke plate all the way; if not, check that the linkage rod can move freely (I've managed to jam that one before...).

The engine needs to be at operating temp to set idle, and I would worry about the fast idle speed as the last thing in the whole process. So, I would set the fast idle screw so it doesn't do anything at this point. If the engine is still revving high, check the linkage on each side so that nothing is hung up. Check that the idle set screw (driver's side) actually makes contact with the lever off the primary. I assume you've checked timing to be roughly correct? I said 'roughly' because idle and timing are somewhat dependent on each other.

It's good to have a tach (actually, best would be a timing light that can display rpm as well...), and a vacuum gauge.

Warm engine, idle speed set at 700-ish as pointed out above, start turning in the idle mixture screws for a crude "lean drop". Note how many turns in are required to fully seat the idle mixture screws. Turning both idle mixture screws all the way in should stop the engine. If it doesn't, the throttle blades are open too far, and you're not running on the idle circuit but on the primary, meaning that your idle speed screw is too far in. Turn the idle mixture screws back to your starting point. Start to turn each idle mixture screw in for 1/2 turn, and note the rpm. Continue doing that until the rpms drop. Go back 1/2 turn on each side. At this point you should have a somewhat stable idle as a starting point for further adjustments (timing/idle speed/idle mixture) to get the best vacuum at idle.
 
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