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Idle bypass air - what I've noticed (lengthy)

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Bailey28:
I have built and ran two carburetors on my stock 350 Chevy.   Both carburetors were originally released on 350 Chevrolet engines in the 1970's.  The carbs are 17059217 originally for a '79 Camaro, and a 17056202 originally for a '76 Camaro.   Both are large main air bleed carbs. 

Getting either carb to idle correctly with the bypass air fully open is impossible.  Both have baseplates with .080" air bypass holes drilled from the factory in them.   Both bodies are fully open for bypass air. 

The 9217 has .033" idle tubes, .046" idle down restrictions, and .080" idle screw holes in the base.

The 6202 has .036" idle tubes, .046" idle down restrictions, and ,095" idle screw holes in the base. 

I wound up going from fully open bypass air where I had the idle screws out 6 1/2 turns and it still wouldn't stay running to closing them off fully.  (like the rebuilder guys do).  I got the car to idle smooth but low at 475-500 rpm.  I turned the idle stop screw in lots to get it to idle at 650ish, but had no nozzle drip. 

The throttle blades were right at starting to uncover the ported vacuum advance slot.  With a vacuum gauge connected to the ported source I could see 1-2" Hg at where I had bumped up the idle screw.

Cliff said to give it more fuel in an earlier post, hence the 6202 with larger orifices. 

Still, not happy at idle unless I plug or restrict bypass air.   

I wound up taking two plastic rods and made my own baseplate air bypass restrictions at .050".  The car now idles at 550ish warm, but still wants more fuel as evident by placing my hand over the choke horn and hearing it speed up a little. 

I have 18" of Hg at this point at idle. 

Basically, it is known that:

'70s Chevy 350's were around 8:1 compression and had a small camshaft producing high manifold vacuum. 
GM used the ported vacuum source to raise exhaust temps at idle for emissions.
Initial timing on these Camaro's were between 6 and 8 degrees BTDC
Dual plane intakes
Cast iron manifolds
Catalytic converters

Yet they had carburetors with the bypass air fully open and they idled at 600 rpm.  As well as having restrictive idle circuits. 

It puzzles me that I have more fuel and more initial timing yet I cannot get the car to idle with bypass air open fully, as the carburetors were originally delivered.

Cliff Ruggles:
Removing the idle bypass air opens up more transfer slot so you get more idle fuel at any particular setting with the mixture screws.

Bypass air is used to lower throttle angle at idle speed, and may or may not be needed for all applications.

In any case if you don't have nozzle drip and still see an improvement in idle speed and quality covering the choke housing or gently tipping in the choke flap, you need MORE idle fuel to the mixture screws.  This assumes you have the engine warmed up, fully heat soaked and have back the mixture screws out for best idle quality.

For the mid to late 70's emission calibrated carburetors in almost all cases they respond very nicely to opening up both the idle tubes and DCR's slightly to put more fuel to the mixtures screws/transfer slots.

Initial timing is also a player, as is the engine CID, compression ratio and camshaft used.

Really "low" compression engines with larger than stock cams are seldom happy with later model carbs having "stock" specs for idle fuel delivery.

Even a "stock" low compression 350 Chevy engine will enjoy a little more timing at idle and idle fuel available.

Here's a case in point.  Several years ago a carb was sent here for rebuilding, the owner MANDATED that we leave it "stock" since it was for his L-82 low compression Corvette 350 engine.  So instead of opening up the idle tubes .002" or so and slightly larger DCR's, stock idle airbleeds etc we sent the carb back.

Almost IMMEDIATELY the owner calls up here bellyaching about crappy idle quality and lack of control with the mixture screws.  His engine "builder" clamed that we needed to install bushings for the secondaries as when he sprayed carb cleaner around the baseplate the engine would speed up and smooth out slightly.  He saw the same thing when he placed his hand over the choke housing.

I had them send the carb back, opened up the idle tubes .002", DCR's to .055" and sent it back.  It cured all the issues they were seeing and everyone was happy. 

Lesson learned with rebuilding emission calibrated carburetors using this new fuel even if they are going back on "stock" applications.  They were a little lean anyhow, and respond nicely to adding a little more fuel to the idle system.......Cliff

Bailey28:
I believe that the Chevy 350 crate engine p/n 10067353 is very similar to the L48 Corvette engines of the 70's. 

The 6202 is on the car right now as it is much cleaner than the other carb. 

I will open up the down channel restrictions to .055".   I have rebushed the throttle shaft bores on both carburetors and blocked the choke passage.   

I have no off idle flat spots, surging or popping on decel. 

Right now I am using the thick open manifold gasket.  I will test a 4 hole gasket as well in the future. 

I am using a new AC Delco  CV774C pcv valve, not a parts store replacement. 

Can't wait to try your .044" rods too. 

Bailey28:
Cliff, I just ordered some parts from your store including the 44 rods. 

The website is easy to use and the pricing and shipping is very reasonable.

Thanks for all of the help, you really make it easy to get these carburetors dialed in the right way.   

Frank400:
Hey Bailey,

   I've worked on a few 17059217 in the past, and have a few cores here still.  That particular carb DOES NOT HAVE a ported vacuum source.  In fact, what seems like a ported vacuum source (baseplate, front passenger side) is a ported source with a bleed hole that gives a very weak signal (used for egr).  You have to block that little hole to get the full vacuum signal.  If you look carefully, the 17056202 has three of those kill bleeds in the base plate. 

  On another note, all the 17059217 I've measured had .069"-.070" idle air byass, and the only 17056202 I've worked on had .110" idle air bypass.  You probably already know the 17059217 is a 750 cfm and the 17056202 is a 800 cfm.
 
   Frank.

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