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2x4 on 383 Carb question
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chefothefuture
Posted 2021-10-12 1:39 AM (#615777)
Subject: 2x4 on 383 Carb question


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Posts: 49
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Location: Chuckanut Mountain, Washington
Hi all-
I recently had a pair of AFBs pro rebuilt. 2652 and 2653.
1) On the primary carb (2653) the idle speed screw has to be opened 3 1/2 turns before it has any effect on the idle speed. The secondary(2652) effects the idle speed at one turn.
Is this normal? I'm also having trouble getting an idle speed (at full engine temp) of greater than 630RPM.
2) When the idle mixture (not speed) screws are just adjusted where the engine runs smoothly, there's excess smoke at the tail pipes indicating poorly burnt fuel; when the screws are adjusted leaner to where the exhaust is fairly clean, the engine idles poorly.

Any thoughts appreciated :-)

Cheers,
chef
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ABloch
Posted 2021-10-12 9:19 PM (#615812 - in reply to #615777)
Subject: Re: 2x4 on 383 Carb question



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Timing? Have you tried bumping the timing up a few degrees? Might help on curb idle then back off a smidge if there is too much pre-detonation under load.
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chefothefuture
Posted 2021-10-13 12:15 AM (#615819 - in reply to #615812)
Subject: Re: 2x4 on 383 Carb question


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Posts: 49
25
Location: Chuckanut Mountain, Washington
Two things with the dizzy. It’s an MSD Ready to Run so I’ve been adjusting the timing
as well as using different springs for different advance curves.
W/o the vacuum advance connected, I have it at 12.5BTC. The vacuum port on the carb is below the
throttle plates so with the vacuum line connected it’s at @ 14.5BTC.
Yet even at this amount of initial advance, I’m still having issues with rough idle when the idle screws are lean enough for the exhaust to be fairly clean.
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57chizler
Posted 2021-10-13 2:33 PM (#615840 - in reply to #615819)
Subject: Re: 2x4 on 383 Carb question



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With the primary carb idle screw in 3 1/2 turns, you might be exposing too much of the transition slot which will cause a rich idle. You might try opening the secondary carb more and closing the primary carb some (same amount of turns on both) to allow the correct amount of air without exposing the transition slot in either.
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Shep
Posted 2021-10-13 3:42 PM (#615847 - in reply to #615840)
Subject: Re: 2x4 on 383 Carb question



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Assume the carbs are balanced, referring to throttle blade positions at idle? If not you will find it difficult to set mixtures.
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chefothefuture
Posted 2021-10-16 12:15 AM (#615922 - in reply to #615840)
Subject: Re: 2x4 on 383 Carb question


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Posts: 49
25
Location: Chuckanut Mountain, Washington
57chizler - 2021-10-13 2:33 PM

With the primary carb idle screw in 3 1/2 turns, you might be exposing too much of the transition slot which will cause a rich idle. You might try opening the secondary carb more and closing the primary carb some (same amount of turns on both) to allow the correct amount of air without exposing the transition slot in either.


The screw that's open 3 1/2 turns is the center idle speed screw; a brass screw with a 9/16" hex head. Did you mean the mixture screws on either side of that? It might be beyond my knowledge but thought the speed screw allowed air to bypass the butterflies to increase or decrease the idle speed.
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60 dart
Posted 2021-10-16 4:07 AM (#615929 - in reply to #615777)
Subject: Re: 2x4 on 383 Carb question



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With the primary carb idle screw in 3 1/2 turns, you might be exposing too much of the transition
slot which will cause a rich idle.

i did this yrs ago and it didn't take long to
learn not to do it again -----------------------------------later
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57chizler
Posted 2021-10-16 4:22 PM (#615939 - in reply to #615922)
Subject: Re: 2x4 on 383 Carb question



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chefothefuture - 2021-10-15 9:15 PM

57chizler - 2021-10-13 2:33 PM

With the primary carb idle screw in 3 1/2 turns, you might be exposing too much of the transition slot which will cause a rich idle. You might try opening the secondary carb more and closing the primary carb some (same amount of turns on both) to allow the correct amount of air without exposing the transition slot in either.


The screw that's open 3 1/2 turns is the center idle speed screw; a brass screw with a 9/16" hex head. Did you mean the mixture screws on either side of that? It might be beyond my knowledge but thought the speed screw allowed air to bypass the butterflies to increase or decrease the idle speed.


I wasn't aware that both carbs have the center idle screw, I thought you meant the butterfly idle screws. Do the carbs have both the center idle screw and the idle screw that opens the butterflies or just the center screw?
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chefothefuture
Posted 2021-10-16 7:09 PM (#615944 - in reply to #615939)
Subject: Re: 2x4 on 383 Carb question


Member

Posts: 49
25
Location: Chuckanut Mountain, Washington
57chizler - 2021-10-16 4:22 PM

chefothefuture - 2021-10-15 9:15 PM

57chizler - 2021-10-13 2:33 PM

With the primary carb idle screw in 3 1/2 turns, you might be exposing too much of the transition slot which will cause a rich idle. You might try opening the secondary carb more and closing the primary carb some (same amount of turns on both) to allow the correct amount of air without exposing the transition slot in either.


The screw that's open 3 1/2 turns is the center idle speed screw; a brass screw with a 9/16" hex head. Did you mean the mixture screws on either side of that? It might be beyond my knowledge but thought the speed screw allowed air to bypass the butterflies to increase or decrease the idle speed.


I wasn't aware that both carbs have the center idle screw, I thought you meant the butterfly idle screws. Do the carbs have both the center idle screw and the idle screw that opens the butterflies or just the center screw?


I probably should have been more specific. Both primary (2653) and secondary (2652) carbs have the center screw for idle speed. (In the 300E supplement they're referred to as "Idle By-pass air screws")
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