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2bbd Mixture Screws

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MIPS View Drop Down
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    Posted: Feb/02/2023 at 3:58pm
I was getting a lot of shaft play and swapped the throttle base out of my feedback 2bbd with another one that was a lot better but now I have to reset my mixture screws.
I did not do this the last time me and a friend rebuilt the carb because I had to sit inside with my foot on the brake as he fought with two screwdrivers. I'm trying to get my head around what the TSM wants you to do but it's a little confusing.


https://amceaglesden.com/guide/images/e/ee/82tsm1J46.jpg - 1982 Eagle TSM, 1J-46 "Idle Mixture Adjustment"

Step 1 - Sure that makes sense. It wants everything installed to prevent biasing.
Step 2 - Easy enough. I have several.
Step 3 - This is where he had me sit in the car leaning on the brake because we don't trust ourselves, or the parking brake. ;)
Step 4 - I'm assuming "Set-to-engine" RPM for me is going to be 500rpm
Step 5 - I'm assuming for this you have to make sure both screws are both in or out the exact same distance, so I should completely remove both, clean and put them both back in exactly one turn, then start turning both clockwise as described. Is "perceptible" implying an audible loss in RPM or from needle drop on the tachometer?
Step 6 - No questions about this.
Note 1 - Will this still apply when the screws are in an unknown state relative to the last complete idle adjustment performed on the carb?
Step 7 - So it wants me to turn both screws the same amount to drop the engine by 50rpm and then call it good, correct?
Note 2 - Might as well perform a full idle adjustment again anyways.
Step 8 - Makes sense, though I probably won't.

Just want to be sure here.
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FSJunkie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Feb/03/2023 at 3:46am
They make it sound more complicated than it is. 

All you are doing is starting out with the two mixture screws at the same position from fully seated, so something like 3 tuns from seated, and the engine somewhere around 600 RPM in gear. 

You then turn the screws together while watching the RPM. You set them to whatever position creates the highest idle speed, and then you turn them lean (clockwise) from that position 1/4 turn at a time until the RPM drops from that highest idle speed by more than 10 RPM, which means the engine is now running lean. You then return the screws back counter-clockwise to the last position they were in before the idle speed dropped. This should give you the highest idle speed that you saw before, but anything more than 1/4 turn clockwise will drop it. This is called a "lean best idle" setting because it's the best idle, but on the edge of lean. Most engines idle best at this setting. 

The "lean drop" or "idle drop" that you see in your manual means that AFTER you have found the "lean best idle" setting, you turn the mixture screws clockwise (lean) to drop the idle speed down by the specified amount. This deliberately runs the engine lean at idle. It is an emissions setting, 100%. I do not recommend doing it. My CEC 258's really do not like idling that way. They tend to stall during braking like that. 

I suggest using a "lean best idle" mixture setting, or even a 1/4 turn richer than that. It should definitely be a "best idle" mixture. Too rich or too lean will drop the RPM. 

Only after you are done noodling with the mixture do you set the final idle speed. 500 RPM in awfully low. My AMC sixes idle rough at that speed. They are happier at 550 to 600. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote wittsend Quote  Post ReplyReply Direct Link To This Post Posted: Feb/03/2023 at 10:44am
FSJunkie said it well. A lot of times following a manual seems confounding but having it explained a different way, or seeing the process done is helpful. In essence the target is to the "best idle" but inclined towards the lean side. Maybe this chart will help. The "hill" is the rise in RPM that will drop down if too rich or too lean. The goal is to set the mixture just before it goes towards lean.



Edited by wittsend - Feb/04/2023 at 1:30pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Feb/03/2023 at 11:02am
If you still have the old base plate and it's untouched, counting the turns clockwise to lightly seat will provide a good starting point to count out on the new base plate. FSJunkie is right on for doing this, my suggestion just get's you closer faster in most cases.
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