Very lean at Idle

[equidr]

I have a 68 chevy truck 402 ci big block.  204/214 @.050 on a 112 LSA with .501 lift cam and 3:42 gears.  Carb # 7040511 #71 primary jets #43 primary metering rods.   converted to small air bleeds & adjustable APT screw.  The throttle shafts were bushed when we aquired the carb.  Truck won't Idle well and only with 11 to 12" vac.  All vac. connections have been plugged with no change in idle.  Base plate sprayed with carb cleaner and no change.  Tip in procedure (hand over the primary air horn increases idle speed & creates nice idle with good vac. that can be adjusted down.  Mixture screws will kill the engine when turned in past 2 turns.  Open vac. port will kill engine.  Where do I go next?  I need more Idle fuel.

[Cliff Ruggles]

Measure the upper and lower idle airbleeds, idle tubes, and idle down channel restrictions?  Some of those carburetors had huge upper idle airbleeds, which limits idle fuel to the mixture screws.  This requires large idle tubes to compensate, or reducing the size of the airbleeds, or both.

Idle vacuum is also pretty low for that camshaft.  What is the compression ratio of the engine, and where was cam installed (ICL)?

What intake manifold are you using?.....Cliff

[equidr]

With a Holly 770 vac secondary we had good vac.  over 16".  The compression ratio is supposed to be 9:1.  We are running a 68 396 stock intake.  When placing a hand over the choke stack everything gets good.  I will tear the carb down this week and measure all of the Idle passages and tubes.  Which of your idle recipes would you recomend for this carb.

[equidr]

I also just noted your last post.  The exaust passages on the intake ore plugged with screw in plugs.

[Cliff Ruggles]

Even with the channel plugged, you still have to use the correct gaskets to avoid a vacuum leak.

I'd go to the 2nd recipe for idle fuel.  The big block carbs from the early 70's used a LOT of idle bypass air, and often pretty large idle airbleeds, with pretty large idle tubes.  The fix could be as easy as reducing the size of the idle airbleeds, and a tad less idle bypass air......Cliff

[equidr]

Thanks Cliff,

Will tear it down later this week and modify per recipe 2.  We are using the correct base and stainless gaskets. 

Roger