My son's 1969 Special 350 4 bbl with Quadrajet is running really rich.is there a thread on the right way to adjust these carbs? :Smarty: Maybe give me a starting point?Engine is stock except for a .30 overbore with stock type pistons and has the early(68-69)heads.Thanks.I can build engines all day long but can't tune a carb. :rant: :af: :ball: ou:
carb adjust Goodguy, you and I both. Make sure engine is warmed up and the choke is off. Lightly tap the carb to be sure the float isn't sticking.I've always checked timing first.Double check the dashpod and then turn a mixture screw out till the engine runs rough and then back it in till it runs rough again. there will be a sweet spot between the two rough spots. Next do the other screw the same way. You may need to do the first one again.You should hook a vacuum gauge up to see where the highest point is. Good luck. :beer
Hey beatlebuick I was reading what you wrote about the procedure for adjusting the mixture screws and I was wondering what it is that your looking for with the vacuum gauge. I just got mine going after having the engine rebuilt and sent the carb to Daytona for rebuilding. Now that it's back on I was wondering about setting the mixture to get everything dialed in. Dave
A good rule of thumb for a starting point is 1 1/2 turns out on both screws. That should get you pretty close. After that fine tune them using a vacuum guage to get the highest vacuum reading. On a bone stock 350, you should be able to get 19 inches with the engine warm and idling. Make sure you re-adjust the dwell after timing. The more you turn the screws out, the leaner you make it. Jason
Carb tune Thanks 64203, that's what inpresses me about this site, everybody helps out.Thanks for jumping in. :beer
I thought I was reading that wrong... Make SURE your timing is correct. Stock setting is 4* I believe.
My question regarding the richness, and the issue on the previous post is when the richness occurs, and why you think it is running rich. The correct method of adjusting the idle mixture is using a tach, adjusting for the highest speed. Be sure that the timing is properly adjusted, and that the speed is properly set. Though the vacuum gauge method will work, there is some risk that subtle changes will either not affect the gauge, or that you will not see the difference. The tach is the most accurate as well. When does the engine run rich? Does this relate to the rich post previously? A heavy float could cause your problem as well. Food for thought...
Ooops, my bad. I wrote that the wrong way. Yes, turning them in leans it out. Sorry guys, didnt mean to screw you up (no pun intended)
Actually, you need to adjust dwell, then timing. Dwell affects timing, timing changes do not affect dwell. Idle mixture needles control the volume of air/fuel emulsion entering the intake, not the air/fuel ratio. Air/fuel ratio is set by the various idle feed restrictions and air bleeds. Turning the idle screws out discharges a larger volume into the manifold resulting in an overall richer condition. Do use a vacuum gauge to set your best idle. Like Jason said, start at 1 1/2 -2 turns out, and adjust for the highest vacuum level. If the engine is running that rich, I would expect something like a high float level, or plugged PCV valve. Check for that.
On the Q-jet, the two screws on the bottom of the carb, on the throttle body, control the volume of fuel entering the mixture, adjusting the mixture at idle. The volume of auir entering the manifold is controlled by the throttle opening. As I previously stated, the dwell tach will be slightly more responsive, and easier to read. The factory manual recommends the use of a tach, and the use of the lean drop method of adjustment, setting the mixture at the fastest idle possible, then leaning the screws to obtain a lean drop, typically 10 rpm per screw. The issue here is to remenber that the screws control the fuel volume entering the mixture. The various air bleeds and fixed orifices control off-idle fuel and part throttle metering. They are not adjustable, but their dimensions will vary, based on the application. On earlier carbs, before the Q-jet, there were screws controlling the amount of air bypassing the throttle plates, allowing a fixed plate orientation. With the advent of the Q-jet, these screws disappeared, allowing the adjustment of mixture and speeds only. A quick note in this regard, dwell and timing adjustments will affect the final throttle plate position with respect to idle speeds. Changes to timing, which allow improved throttle response, will affect this throttle plate orientation, and possibly create hesitations or a stumble. Ray
Well Thanks for all the input Guys-i have a Pertronix kit so i don't have to set the dwell but i will start at 1 1/2 turns and go from there.
