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too big a carb = lots of smoke?

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The PO of my 67 put a Holley double pumper (750cfm I believe) carb atop the 289. When starting it, the car will run for about 10 seconds and die. I've been told too rich a fuel mixture will do this and the easiest fix here is to put a 600cfm or smaller carb on the 289 (I have a rebuilt Holley 600 lying around). More than one person was taken aback by a double pumper on a 289...

When the car was running, it was emitting quite a lot of smoke. This was something I specifically looked for when I bought the car and there was none. The PO didn't have much trouble getting it to idle but then again it was 30 degrees cooler back then and the car has been sitting for a month now.

Think the smoke is likely a combo of too much gas and a change in temperature? All that has changed since I saw the car running last is a new gas tank, gas, oil and filter change will a little Marvel Mystery Oil thrown in.

All thoughts welcome!


Houston, TX - 67 Fastback in restoration. A-code, C-4, 9" Traction-Loc rear.

"If you wish to make an apple pie from scratch, you must first invent the universe" - Carl Sagan
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From the front of the choke air horn of the carb what is the List number?

Greg B
Aaron, if you have the 600 cfm carb sitting around it shouldn't take you but a short while to swap them and try it out presuming the fuel feed lines etc are a straight swap). I don't claim to be a top mechanic or anything but that sounds like an awful lot of carb fo a 289, to much fuel going in will cause it to flood and stall and also I gess, if you aren't buring it all it will smoke. Changing to the 600 cfm carb would be a good start. Are you sure this is the carb that was running on the car when you saw it at the PO's place? The difference in how it is running now sounds to much like night and day.

69 Sportsroof, 351C, 4spd toploader, Restomod BOSS 429 clone - the one and only (?) 1969 BOSS 351
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sounds like the carb may be dirty..or something lodged in the idle circuit..the double pumper is too much of a carb for that motor, but it should only affect it when you stomp on it. I assume the secondaries are mech?..I would wire the secondaries closed and see if that makes a difference..if not the carb could use a rebuild more then likely

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CFM of a carburetor has NOTHING to do with the idle circuit. A properly functioning double pumper will idle as clean as a 600. Look for a closed choke or trash in the needle and seat.
As far as a 750 DP being too big for a 289, that depends on the equipment in the engine.... A D.P. is better suited for a car with a 4sp., big cam and low gears..... I say; Try it, you might like it!

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thanks all for the replies. i won't be able to get to the carb until my return from florida (work) on thursday.

the secondaries on the carb were wired open. closing them made no difference. i'm going to put the 600 on (it's in better shape regardless of the CFM) and see what happens. the only problem here is i have NO IDEA how to tune a carb.

any advice???

thanks once again,

Houston, TX - 67 Fastback in restoration. A-code, C-4, 9" Traction-Loc rear.

"If you wish to make an apple pie from scratch, you must first invent the universe" - Carl Sagan
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Could be a blown power valve. Or maybe the secondaries are not closed at idle....could be many things but what you describe doesn't sound like it's because the carb is too "big". It's because something is wrong with the carb.


65 GT Coupe, dismantled waiting for resto.
67 Restomod Coupe, in assy.
69 Mach 1 S code, SWMBO say don't take it apart!
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