by 67a 100 pickup Fri Jun 04, 2010 5:21 pmI can't find anything in my service book that says how many turns the screws should be set at. Mine is running rich, and will stall at a stop light sometimes if you are not ready to "two foot it" real quick. Part of my problem is because I need to install longer studs in order to use the thick base plate gasket.
4 posters
BBD Carb mixture screw adjustments
67a 100 pickup- Number of posts : 1319
Location : ann arbor, mi
Registration date : 2009-01-29
donivan65- Governor
- Number of posts : 12248
Location : San Diego, California
Registration date : 2008-05-12
I would start at 1 turn each,,,,,,,see if it will idle,,,,,then the idea is to get the highest rpm out of each screw. And if it goes higher than your base RPM, like around 600, turn the RPM idle screw down,,,,,,and keep playing with the screws until you get to the point of where opening the screws SLOWS down the RPMs. You can use a vacuum gauge also,,,,,get the highest vacuum reading by adjusting the mixture screws and lower the RPMs if they get too high,,,,,and if turning those mixtures screws in won't slow down the idle as a test, you got carb problems,,,,,,
jkr- Number of posts : 1148
Location : prince edward island canada
Age : 66
Registration date : 2008-05-29
there might be a passage that your blocking off on the bottom of the carb too. i remember some of the base gaskets had a couple of slots to open up passages for the carb but cant say what ones and what you've got. if you take the carb off take a pic of the bottom.
RodStRace- Number of posts : 3046
Location : Chino Valley
Registration date : 2010-01-21
The reason they are adjustible is becaues there is no right answer.
For proof, remember how all the mid to late 70s cars ran? They had the screws set, then plugged off.
Each screw controls the fuel flow to one barrel idle port. For a rough start, try 3/4 to 1 1/2 turns out from LIGHTLY seated. start and let it idle. try turning each screw slowly in (cuts off fuel flow or leaner). It should start running rougher and probably die. This usually happens at about 1/4 turn out. Turn the screw back out a little and restart it. There is a touch to it, it will be smooth when right, 'roll' when rich and 'shake or shudder' when lean. Find the point where it's happiest, and repeat for the other side. A 1/8 to a 1/4 turn should make a difference. go back and forth between the two until you can't get it any smoother. If you readjust the idle speed, reset the mixture screws.
Old-timers tip: if you have a vacuum gauge, connect it to manifold vacuum and adjust to highest vacuum. It's a great visual aid to help develop feel.
After this I try to adjust them in just a little, for a slightly lean idle. It will start better, idle down to a stop better, and not burn your eyes as much.
For proof, remember how all the mid to late 70s cars ran? They had the screws set, then plugged off.
Each screw controls the fuel flow to one barrel idle port. For a rough start, try 3/4 to 1 1/2 turns out from LIGHTLY seated. start and let it idle. try turning each screw slowly in (cuts off fuel flow or leaner). It should start running rougher and probably die. This usually happens at about 1/4 turn out. Turn the screw back out a little and restart it. There is a touch to it, it will be smooth when right, 'roll' when rich and 'shake or shudder' when lean. Find the point where it's happiest, and repeat for the other side. A 1/8 to a 1/4 turn should make a difference. go back and forth between the two until you can't get it any smoother. If you readjust the idle speed, reset the mixture screws.
Old-timers tip: if you have a vacuum gauge, connect it to manifold vacuum and adjust to highest vacuum. It's a great visual aid to help develop feel.
After this I try to adjust them in just a little, for a slightly lean idle. It will start better, idle down to a stop better, and not burn your eyes as much.
