I'm finally getting back to dialing in my BBD. I went through it about 6 months ago paying as close attention to the rebuild kit instructions and various threads on this forum. Today I connected my LM-1 to be able to see AFR numbers. I have to have the idle mixture screws 5-6 turns out to get an idle mixture around 13.0-13.2 - any leaner and idle quality starts to suffer. Any further out and the screws feel like they could rattle out. I assume that the mixture screw should really be somewhere in the 2-4 turn range when everything is right.

My question is, what changes to make to richen up the idle a little bit and get the screws into the sweet spot of their adjustment range.

More info: the tag on the carb (yes it still has one) says --> 8146S 1467 580

Throttle plates do have holes in them. Should I close 'em up before making any other changes?

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8146s- 1977 dodge truck auto trans canda, 318 v8 . If original has venturi screw 101-639 .067 idle air bleed and .055 economiser. Most slants used screw 101-569 idle air bleed [carter called the first air bleed the bypass] .063 thats about 10 percent smaller in area square inches than .067, thats ok leave alone. What needs to checked or modified is the economiser. 101-569 had .059 which allowed more idle circuit flow. The ecomiser is pressed in steel bushing, steel rusts and cloughs up. This can cleaned or made bigger with correct drill bit.

Photobucket is down no pictuires http://www.slantsix.org/forum/viewtopic ... 19&t=63938

Yes close up the holes in plate with small screws and nuts and lock washers and locktite. Also idle tubes need be checked and cleaned with bit 0295 inches .075 mm. even a small blockage here makes a difference. With the plate holes open, curb idle screw is backed out too far. Not enough off idle slot exposed under throttle plate. In theory a small part of off idle slot should be exposed at curb idle, so that some idle fuel comes from slot. May be thats why mixture screws are out so far, there having to feed all of the idle fuel. Now more guess work, that was 318 carb, so blocking off holes will make it necessary to turn idle speed screw in to get enough air to idle on, thus more off idle slot exposed.More fuel from off idle slot less needed from idle mixture screws.Turn them in.

x2, but I wouldn't use hardware that might endup going through the valve on a very bad day in the far future....

Your idle screws shouldn't be more than 2 out for start of rough tuning, and mine ended up about 1/2 to 3/4 out from seated
when on a 9:1 motor with RV cam... (tuned for best vacuum reading at idle)

Thanks guys. Just after asking my questions I realized - duh, those plate holes are obviously a big part of the problem. I've temporarily plugged them with small screws. I'll come up with something more permanent and less risky later. Any suggestions?

Now the idle is much more stable and mixture screws are set by ear at 2 around turns out. Weather permitting, I'll take it for a drive tomorrow to check throttle response and AFRs.

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8146s carb more info , Was a heavy duty emissions carb for trucks with 6100 gvw. Gross vehicle weight rating. Different emissions standard than light duty trucks. Heavy duty standard meant no cat converter, no egr valve, no heated air cleaner, no charcoal vapor storage canister. So if it really is 8146 carb. No port for egr,or heated air cleaner and no vapor canister purge port [ that,s the one in front between the idle mixture screws].This carb 8146 for 318 for auto and 8147 for manual. In the US carbs were used for 1977 and 1978. For Canda from 1978 to 1983

Your description matches what I have to a T.
Only 2 ports, one for vacuum advance and one for PCV.

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My first short test drive shows me there's a bit of work to do yet.
It goes pretty lean just off idle - AFR in the 15-16 range and throttle response is poor.
Steady half throttle is lean but close at 14.5-15
Wide open throttle is way lean climbing past 18 and misfiring.

Hmm... I figure it needs larger main jets. However, the mains currently installed measure at 0.086". Reading other posts these should be pretty close. I hesitate to start throwing bigger jets at it if the root of the problem lies elsewhere.


Other info that will have some bearing on final carb setup:
Freshly built motor with Comp 264 cam, big valves and porting. Static CR 9:1.
14" round aftermarket air cleaner. Offy intake. 904 Automatic.

Suggestions welcome.

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--> Check out my FI Turbo build <--
--> And the race truck build project <--
--> The Diesel Corvette <--

Carb info 8146s step-up piston spring 61-538 long and stiff ,10 inches of vacuum needed to pull down..This is tune able part, modern day afb springs work just fine. Will need vacuum gauge. metering rod number 75-2092 .066 -.0588 -.045. Number is stamped on rod. Main jet number 120-386 - .086 Some guess work, is there vacuum to vacuum choke pull off ? That carb may need notches in flange gasket. 8146s uses idle screw number 30a-198 the early one up to around 76. Cars and light duty trucks got a different idle screw around 1977.. Tore down the slant six heavy duty emissions carb 8110s idle screw number 30a-203., the newer one. The slant carb did have in the idle down channel just above mixture screw a brass restriction , limits flow to idle mixture screw. Most carbs did not get get this but some did. If there can enlarged or removed.

http://www.slantsix.org/forum/viewtopic.php?t=55801 the bbd manual can be useful