17059216

[Hammered]

My first post and a little history.  Bought a 1979 Corvette as a father/son project a little over a year ago.  The car had been sitting since 1988, but had only 28k miles on it.  It had the original carb with the anti-tamper plugs still installed.  I bought a rebuild kit from Cliff and idle down tubes because it is known that these carbs run too lean from the factory.   We rebuilt the carb and decided to try it without the mods.  Of course it ran lean and had to idle high.  Then we fixed some minor vacuum leaks and rebushed the primary shaft.  For a carb with only 28k miles, it had enough wear on the driver side to make a big difference.  After all that, the car ran better, but still lean at idle.  This weekend we finally took care of the down tubes.  I'm glad we bought spares because we could not drive the tube down through the sleeve and had to drill them.  We jumped the down tube diameter up to 0.035" from a factory 0.031"  It made a big difference, but it still runs a bit lean at idle and I can place a rag over the air horn and pull the idle speed up 100 rpm and the idle mixture screws need to be out 4 turns.

So what should my next move be?

Here are the specs which are factory unless noted:
Down channel restriction - 0.052"
Upper air bleed in main body - 0.0635"
Idle down tube - 0.035" *modified from stock
Lower air bleed - 0.076"
Idle screw - 0.081"
Main Jet - 71F
Primary Rods - 40
Secondary rods - CH

Thanks in advance for everyone's help.

[Cliff Ruggles]

Back out the idle mixture screws more.  4 turns on the metric threads isn't really very far out with those as they only move about half as far as the early carbs with the 10-32 threads.

Try your test again and see if it's still lean at idle......Cliff

[Hammered]

Thanks Cliff - we finally had a chance to work on it yesterday and turned the screws out 6 turns each.  Those extra two turns did just enough to get rid of the lean condition.  Of course we had to bring the idle speed down a bit after that and now the car runs much better.

Six turns out on the idle screws seems a lot, but I'm not familiar with the metric carbs.  Should we be happy and leave well enough alone, or is there something else we should consider doing? 

[Cliff Ruggles]

You either have enough fuel to the mixture screws or you don't.

Some of those later carbs also have tiny holes under the mixture screws, so they reach a limit and need to be opened up some. 

I would take a look at them and open them up to around .080-.090" if you don't have full control with the idle mixture.  If that doesn't do the trick you will need to increase idle tube and DCR size to get more fuel to them......Cliff