Qjet Adjustment

[skokie]

How do you adjust a qjet? I guess what I am asking is what do the screws on the qjet do? Sorry guys for the dumb question. Never messed with a qjet before

[Solver]

basically 4 screws
1 on drivers side sets idle speed
1 on passenger side sets fast idle speed
2 on front set idle mixture
If it is 4 barrel.

[skokie]

Yes it is a 4 barrel. I should have mentioned that. To set idle mixture do you do that the same as a holley? Turn the screw inwards until it misses and back off 1/4 to 1/2 turn?

[jimfulco]

Yep...........

[724wdcopper]

and just so you know, all q-jets are 4 barrels. the q is for quad = 4. rochester 2-jets are what they sound like - two barrels. but there is also an altitude adjustment screw under an aluminum plug on top in front of the carb. if you were moving from , say, sea level to denver you would need to adjust the altitude screw. if you truck was born at about the same altitiude that you're at currently, you should be fine.

[71-longbed]

its best if you use a vacuum gauge and tach to tweak the idle mixture screws

this is from gmc pauls site ..great info

Connect vacuum gauge to base of carburetor to full vacuum source at idle. Then monitor your readings and compare readings to chart below.
1-Steady gauge reading from 17-22 indicates normal engine in good condition.

2-A intermittent fluctuation at idle indicates a ignition miss or sticking valves.

3-Needle fluctuates as you increase engine speed indicates ignition miss, blown head gasket, leaking valve or weak valve springs.

4- Steady needle that drops at regular intervals indicates burnt valves or improper valve clearance, (the needle falls when bad valve is in operation).

5-A drifting needle goes high then drops goes high then drops in a steady movement indicates minor intake leak or improper carb adjustment.

6- A low reading but steady from 10-16 indicates late ignition or valve timing, low compression, stuck throttle valves, leaking carburetor or intake gaskets or a big cam.

7- Needle vibrates heavy at idle but becomes steadier as engine speed increases indicates worn valve guides.

8-A gradual drop in reading at idle indicates blocked exhaust system or excessive back pressure in exhaust system.
That's troubleshooting with one. Now for fuel mixture adjustment with one.
Disconnect vacuum advance at distributor and plug vacuum line, start motor with timing light and a RPM gauge attached and set idle and timing to correct settings. Connect vacuum gauge to full unported vacuum at base of carburetor, now shut motor off and turn both idle mixture screws all the way in but be sure that you only use light pressure as you can turn the idle mixture screw past being lightly seated by twisting hard and then you are damaging the idle mixture opening with the needle causing it to flare the opening trashing a good carburetor body so please only lightly seat the needles. now back each out 1 1/2 turns, restart motor and check idle and timing and adjust as necessary to correct settings, now back each needle out slowly a 1/4 turn each alternating between the 2 until you reach the highest vacuum reading, check and adjust idle speed and timing, now repeat step one of backing idle mixture screws out alternating between the 2 a 1/4 turn at a time until you have reached highest vacuum reading again. Continue the process adjusting timing/RPM/idle mixture screws until you have no changes occurring and have reached highest vacuum reading possible. You have now optimized your timing idle mixture adjustments. Reconnect your vacuum advance while disconnecting the RPM gauge, timing light, and vacuum gauge.
Ok now for monitoring fuel consumption with a vacuum gauge.
Connect the vacuum gauge to full unported vacuum and run vacuum line long enough for gauge to be in cab. Go for a drive monitoring the vacuum gauge. When you have high readings you are getting the most fuel economy possible, when you have low readings fuel consumption to efficient combustion ratio is lowered. By easing into your gas pedal rather than moving it rapidly you will notice vacuum readings stay higher which increases MPG

[skokie]

Thanks for the info. I will print that out and take it with me once I get the qjet on.

[71cadc10]

Do you know which model you have, the 4MV (earlier) or M4M?

What year did car did it come off of, or did you buy it new / rebuilt?

The 4MV earlier model will not have the extra adjustment screw on the front/top area that was mentioned above.

The secondaries can be tuned as well, by a set screw and spring tensioner. It's on the right side (looking from cab) on the shaft of the secondary airdoor (top flap of the carb above the secondaries). Adjusting the tension of this spring controls how quickly the airdoor opens, and will have a big effect on what happens when you floor it. Too quick and it will have a bad bog or be very lazy.
If you bought a rebuild kit for it there should be instructions for setting this, as well as float level and other things.

[skokie]

It came off of a 68 327. Its the 4mv. Had it rebuilt.