Q-jet carb Gods, need adjustment pointers

[dukcaln]

Ok guys, eally starting to figure out this truck...Think the timimng issue is done and set, got the exhaust and header leak fixed, now I want to takle the carb adjustment..... let me first say it is the original 4b q-jet from 1972 and was recentley rebuilt when the motor and tranny got built (less a year ago and 750 miles)... using the stock manifold too.... any ways, I had a friend adjust it but I think he had no idea what he was doing. So here is what I know r have done..

checked the mixture screws by turning them in all the way... left side took 3 less turns to close than the right... So once I closed them i turned them out 2 1/2 turns and fired the truck up....idles and sounds great.. took it for a ride just around the block cause it got dark but seems good.... thats all I did so far...

So tomorrow how do i go about dialing it in with out a vacuum tool. I was told to turn the screws in until motor starts to idle down then turn out until some thing happens. Once that is done then turn in 1 1/2 turns and that should be good....... is this right?

Or I thought since the scews are out equal turns (2 1/2) to drive it and keep turning out 1/2 turn until I find what runs best.....

so am I going in the right direction or can you guys steer me the right way.. really all comments and suggestions will be great... i am a rookie at this...

[geezer#99]

Get a vacuum gauge and plug it on to a manifold or full time vacuum port. You want to adjust for highest vacuum. Do one screw at a time. Go slow about a 1/4 turn at a time.
2 1/2 turns out is a good start point. Watch gauge and screw in one screw till vacuum drops then go back out 1/4 turn. Do same to other screw. You might need to to fine tune back and forth to each screw to get it set right. Make sure the motor is warmed up when you do this.

[406 Q-ship]

2 1/2 is a good starting point and turn the screws slowly when adjusting for highest vacuum. Give the engine a few seconds to stablize when you turn the screws the 1/4 turn and before jumping to the opposite screw. Take your time and it will work great.

[dukcaln]

great thanks... I will have to do it by feel and sound until I can get a vacuum gauge from my father in law...... he will be back from vacation in a week or so....

[Wrenchbender Ret]

Turn one in slowly till the engine starts to slow down. Then back it out till the engine picks up to the speed it was, usually about 1/2 turn. Do the same with the other screw. Go back & check the first one again. You wont get it any closer then that with a gauge.

[dukcaln]

Originally Posted by Wrenchbender Ret

Turn one in slowly till the engine starts to slow down. Then back it out till the engine picks up to the speed it was, usually about 1/2 turn. Do the same with the other screw. Go back & check the first one again. You wont get it any closer then that with a gauge.

is this method using the 2 1/2 turns out as a starting point?

[Wrenchbender Ret]

Yes

[DT1]

I agree with Wrenchbender on this one.
I also believe that a Rochester Q-Jet is absolutely one of the best Carbs for Chevy. Anyone that refers to them as "Quadrajunk" doesn't know how to tune them...Period.
My 2 cents... A vacuum gauge is a must. A healthy stock engine will pull approximately 17-19in vacuum at sea level.
Connect the gauge to manifold vacuum. Make sure that the engine is at normal operating temperature and leave the air cleaner on. If you have an automatic transmission, (Which I'm assuming) Set the emergency brake, chalk the wheels and have an assistant hold pressure on the brake pedal. With the engine running, put it in drive. If you have a manual transmission, skip this part.
Turn one idle screw in very slowly until the vacuum just drops off. Turn the screw out until you see the vacuum rise. Continue to turn the screw out 1/8 of a turn at a time and wait for a few seconds between increments observing the gauge. Now your checking for stability. You want the needle to fluctuate the least. Repeat this on the other screw and repeat again for both screws. Never adjust for the highest vacuum. This will always be too rich.
You also have to remember that you have a carbureted engine, so don't be alarmed if it doesn't idle perfectly all of the time. The idle will be affected by ambient temperature, elevation and humidity. FWIW

[dukcaln]

DT1 thanks very much....... all of you who posted I appriciate it and thank you.... I will visit this thread in a couple of weeks when i get access to a vacuum gauge...