Lean at Idle

[71vette]

I finally got my carb built (used a 1979 carb) following all the procedures in the book.  I bought all the parts from Cliff.  It was a ton of fun and it turned out great on the bench.  I installed it this afternoon but it isn't idling well at all.  It seems to be lean at idle because when you put your hand over the airhorn, it speeds up and seems to run better.  I of course also have the nozzle drip because the throttle plates are open too much just to keep the car running.

I checked everything for a vacuum leak and found none.  I kept the original carb untouched as it runs pretty good with that carb so whatever problem I have is with the new carb.

Can anyone give me some tips or things to check? 

1971 Corvette, 454 manual, 9.5 CR with 180 lbs static compression, very mild cam with 218 duration on intake, 228 duration on exhaust with .510 lift.   

[Cliff Ruggles]

Did you measure the idle system specs?

Lower idle airbleed, upper idle airbleed, idle tubes, idle downchannel restrictions and idle bypass air.

It needs more idle fuel, which is relatively easy to accomplish by looking at how the carb is set-up, then using one of the "recipes" from our book.....Cliff

[71vette]

Hi Cliff: Yes, I used recipe #1 from the book exactly.  You and I had a few emails back and forth. I am the guy that asked about driving in the air bleeds vs. drilling the holes to the brass air bleed outside diameter. 

Anyway, I have the main bleeds at .070, the idle tubes at .036, the idle down channels were already at the proper diameter .046, I drilled out the mixture screw holes to .090, set the float at .250.  I also blocked off the choke vacuum hole by tapping and threading a screw coated with epoxy.  It has#71 jets with your #44 primary rods.  I used the light blue power piston spring.  I left the by pass air stock because I think it measured .080.

I took a lot of time and care with the measurements including double checking the drill sizes with a digital micrometer. 

Once I get beyond the idle system with the throttle, it seems to run fine.  I can't find an external vacuum leak.  I have adjusted the mixture screws to about 4 turns which seems like a lot to me especially with the increased hole size. 

Since this is a newer carb on a 1971 factory intake, is there any mating issue I should look for?

I'm sure it is something I've done wrong but I don't know what it is. 

[Cliff Ruggles]

Nothing done wrong, the engine simply requires more idle fuel than you've allowed for it.

Also measure the idle airbleeds in conjunction with the "recipe".  A larger upper of lower idle airbleeds requires sizing it down or compensating with a larger idle tube.....Cliff

[71vette]

I backed out the idle screws another couple of turns (4 turns out) and the engine idled better but it still didn't idle well.  I started looking for a vacuum leak using brake cleaner.  I know I may be fooled but there is a big pickup in idle speed when I spray brake cleaners on the secondary throttle shaft on both sides of the carb.  I have read where Cliff doesn't put bushings in but there is a pretty good amount of up and down and side to side movement of the shaft from both sides of the carb.  Additionally, when I opened them up a bit and let them settle back, it idles better for a few seconds and then starts to get rough. 

I realize that I might not have the secondary plates lined up but I was very careful when installing them and held the base plate up to light to make sure they were sealing perfectly.  I checked my original carb and the secondary shaft is not nearly as loose as the carb I rebuilt. 

I'm wondering if I've found the issue or if I am just being fooled?

[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