Carter 3645SB AFB

Discussion in 'Carter' started by hobbby, Oct 6, 2014.

  1. hobbby

    hobbby Well-Known Member

    Front Carb #3645SB
    Rear Carb #3925SA

    I have a dual carter AFB setup on my Buick 455. The issue is when I get home and park in the garage I get a strong raw fuel smell. I tracked the smell down to fuel leaking into the intake manifold from one of the carbs. What normally causes this on a Carter AFB?

    I have no driveability issues other than extended cranks after 30-60 minutes. Of course with the intake flooded with fuel I expect this.

    Another question: Between the two idle mixture screws is a big adjustment knob. What's that do? I've head Idle speed adjustment but wasn't sure why there would be two idle speed adjustments so, I thought that was BS.
     
    Last edited: Jan 20, 2015
  2. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    hi .
    pic of "extra" carb knob and carbs etc ? carb numbers ? ( oops , just saw title , forget that )
    how do the plugs look ? rich exhaust look ?
    well if it is flooding or whatever i guess i would suspect either some lower level casting flaw or crack allowing a fuel leak but then it seems like it would do it no matter what and would have been doing it for quite awhile . a top warp or cracked gasket may allow a little overspill ? engine rock a little at idle ? i'm just guessin but maybe a smooth run at cruise but a lumpy idle in the garage just before ya shut it off ? probably not with correct float level . but incorrectly set or heavy floats would let the fuel level rise . or maybe a heat soak right after shut off could perk the fuel and let it go up n over ? pressure build up in the tank after shut off ? gas cap or vent plugged ?
    probably would not take alot to pop em off and rebuild n inspect .
    at work now - just wingin it .
    edit : hey - nice ride by the way !
    edit #2 : quick internet check looks like no choke and front carb ? i'll check carter book when i get home .
     
  3. hobbby

    hobbby Well-Known Member

    EDIT:

    Front Carb #3645SB
    Rear Carb #3925SA

    The plugs are a nice light brown and I actually get fairly good fuel econ 10-13
    The idle is a bit rough but it has a mild cam in it so its hard to tell. I have the idle adjust down to 800 out of gear about 650 in gear warmed up.
    The choke has been removed completely even the shaft. (previous owner)
    I'll snap a photo of that adjustment between the idle mix screws when I get home.

    I did just have the tops off to install new accelerator pumps (they didn't fit and the original ones are back in but that's another story lol)...maybe I knocked the float adjustment out of whack.
    I'm liking the idea of the floats being to high and the fuel perking, it would be an easy fix to tweak them down a wee bit.

    Thanks for the compliment and the help. I love it!
     
    Last edited: Oct 10, 2014
  4. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    MORNIN .
    wed am - busy last night .
    anyway , carter book says :
    '64 - front #3645 - rear #3634 ( manual trans ) #3646 ( auto )
    '65 - front #3645 - rear #3924 ( manual trans ) #3925 ( auto )
    '66 - front #4050 - rear #4051 ( no trans dif )
    i don't think carb number mix match will cause your problem but there ya go . i think front carb may have been oem no choke .
    my regular float setting i use are 7/16" float level , 15/16" float drop . if using a kit i'll use the kit specs if dif . check floats carefully for dings , leaks , internal liquid , heavy feel . and not sure on the carters but the edelbrocks have a dif pump number for 600's and 750's . if your carb has the removeable baffles between float n needle/seat be sure it is turned correctly - "bend" toward the needle/seat . but i think yours will be the baffles that are one piece with the top .
     
  5. hobbby

    hobbby Well-Known Member

    Here's a shot of the adjustment knob, no idea what its for...

    20141007_180653_Android_zps73855deb.jpg
     
    Last edited: Jan 20, 2015
  6. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    knob : hmmm , not ringin a bell . will check some info n pics when i get back later . if i was gonna guess i'd say being in the center like that it may be for the pump shot ?
    floats : yep , same i'd say .
     
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Looking through my Carter Carburetor book by Dave Emanuel, I see from the AFB exploded diagram (pg 110) that some AFB's have a Bypass Idle Air adjustment screw. Basically, it must allow you to adjust idle air bypass. That allows the throttle blades to be near closed as the engine idles, insuring the engine runs on the idle system of the carburetor. The book also mentions an adjustable idle air bleed that controls both left and right idle circuits (pg 37). My guess is that knob is one of those 2 things.
     
    Last edited: Oct 8, 2014
  8. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    yeah I saw similar info - some of the carbs have a center idle adjust but not with the two separate idle screws . so maybe the bypass .
    edit: did a quick computer pic search last night using the front n rear carb numbers . nothing really came up for the rear . but saw an e-bay ad for front number - no idle adjust screws in front ? yours like that ? just blank .
    so it idles on the rear carb ? I guess that makes sense maybe . will have to check closer next time I see a set up .
     
    Last edited: Oct 9, 2014
  9. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    so - I found an old hygrade/standard rebuild sheet that I had saved from doing a mid 60's buick afb . two things - it had a pic I think of the center adjustment knob and float set info . float level 7/32" but did not give a float drop measurement . other info was pump - hole "B" , choke piston .026 , choke valve 3/32" , fast idle valve .014 , unloader 7/32" , idle rpm = 525 .
    actual wording of the figure #7 idle pic says : " note : on some models idle speed is controlled by an air by-pass adjustment screw (3) and throttle valves remain seated .
    adjust as follows - (1) open air bypass screw approx. 2 full turns from seated position . (2) start engine , adjust air bypass screw for specified rpm . (3) turn mixture screws to obtain smoothest idle . (4) correct idle speed by readjusting idle by-pass air screw . then readjust mixture screws . if needed - repeat .
    duh ! as soon as I recharge my camera battery i'll load up a pic hopefully .
     

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    Last edited: Oct 9, 2014
  10. hobbby

    hobbby Well-Known Member

    WOW cool that's great info thank you very much!

    Yes my front carb has no adjustments, Idle control is all performed by the rear carb.

    I'll mess around with it in the coming weeks and post my findings here. For the moment the car is out of commission with a blown up rear end. The center pin in the posi sheered. : / parts are ordered.
     
  11. hobbby

    hobbby Well-Known Member

    The rear end is back in and running good.

    I'm going to start working on this carb issue again. I'd like to get a set of accelerator pumps and top gaskets before I pull the tops off. Where's a good spot to get my parts?
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

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