General Category > Quadrajet Carb Talk and Tips

Lean at Idle

<< < (2/2)

Cliff Ruggles:
The amount of side play doesn't matter.  The secondary throttle plates must fully seat each time they close. 

I spend a LOT of time with this part of the build, until the are well centered, and fully seated each time they close. 

Next time it's up and running, gently place your hand over the choke housing and see if it responds favorably.  Sounds to me like it just needs more fuel to the mixture screws.....Cliff

71vette:
I did some more checking and I think Cliff is right about the rear throttle plates not being the issue.  The carb that I am not messing with is the correct replacement carb 17054917 for a 1971 Corvette with manual transmission.  The measurements from this carb are:  Upper idle air bleed .067, lower idle air bleed .075, idle tube .039, idle down channel .047, by pass air .139, idle mixture screw holes .077 and it has the large .119 main air bleeds with #77 jets and #49 primary rods.  With this carb, it idles pretty well but i have the nozzle drip out out of the left side. 
So, when I used the #1 recipe from the book when building the 1979 carb, the idle tube, idle down channel, and by pass air specs are much smaller than the original carb.  With the #71 jets, 44 rods, and .070 main air bleed sizes, I assume they are providing enough fuel to the mixture screws.  Since the car is lean at idle, I'd say that I have to open up the idle tubes and by pass air etc. to at least what the original carb is in order to provide enough fuel and air to get it to idle well.
Does this make sense?

Cliff Ruggles:
In any and all cases, the idle fuel delivery has to be sufficient for the engine combination.  Although dubbed a "mild" camshaft, the cam you are using is probably reducing signal, or vacuum to the carb, at least in comparison to the stock cam. .

"Recipe" #1 in our book is for stock, or near stock applications, where they have very good vacuum at idle speed. 

Several other things come into play here.  Cam timing (where the intake valve closes), ignition timing, and the true static compression ratio.

The compression ratio is a BIG player here, as it offsets late intake closing points, overlap, and conservative initial timing.

I'd open up the idle tubes, and possibly idle down channel, leave the bypass air alone, at least for the first test.  Also check to see if the shaft is twisted slightly, as you mentioned only having nozzle drip from one side.....Cliff

71vette:
Ok, will open up the idle tube and idle down channel.  The engine has 14" at 700 and closer to 15" at 800 rpm.  The intake valve @ .050 opens at 1 degree btdc and closes at 35 degrees abdc.  Exhaust opens @ 51 degrees bbdc and closes @ 3 degrees btdc. 

The compression ratio of 9.5 is accurate as I cc'd the cylinders and heads when I built the engine.  Ignition timing is set at 10 degrees initial and I am using manifold vacuum to the vacuum advance to help with being able to close the throttle plates as much as possible.

I used recipe #1 because of the engine vacuum reading.  Regardless, we can always open things up more. 

Thanks Cliff for all of your help to everyone.

Cliff Ruggles:
I'm not a big fan of using manifold vacuum to the advance on mild applications.  By tuning the carbs idle system, I canget a smooth/stable idle in and out of gear without a lot of initial timing.  With some set-ups, adding a lot of timing makes them difficult to tune at idle, as the throttle plates are nearly closed, and the idle speed still too high.....Cliff

Navigation

[0] Message Index

[*] Previous page