A couple of issues and need some advice.

[SundanceKid]

Problem #1) Turning the idle screws in completely will not kill the engine and they have very little effect on the idle. The idle is very rich, and I "think" the primary throttle plates are open a bit more then they should be.

Problem #2) The tune overall seems rich, but is very responsive. Going down a jet size makes the engine lean and hesitate. The difference in one jet size seems rather drastic to me.

I'm currently running a 4 in.Hg power piston spring and the APT is screwed completely in. What spring would you suggest as a good starting point for my engine?

It runs well, but I know it isn't right. The plugs read rich and the exhaust smell is also rich.

Altitude: 4,500 ft
 
Vehicle: 68 Camaro

Engine: 468 Big block, 10.5:1 compression, Performer RPM intake and heads, Super Comp headers.

Drivetrain: TH400 with 3.50:1 gears

Camshaft: solid roller
lobe seperation: 110
Gross lift: .640 intake, .647 exhaust
Duration @ .015: 273 intake, 280 exhaust
Duration @ .050: 236 intake, 242 exhaust

Ignition: 18 degrees initial timing.

Vacuum at idle: 9-10 in.Hg

Carburator: 17059213
Idle discharge: .099
Idle bypass: .111
Lower idle air bleeds: .070
Upper idle air bleeds: .070
Idle channel restrictions: .059
Idle tubes: .040
Main air bleeds: .086
Main Jets: .076
Primary metering rods: 54M
Accelerating well holes: .040
Accelerating discharge holes: .055
Secondary metering rods: CE
Secondary Hanger: G


Thank you for any help you can provide.

[Cliff Ruggles]

Everything in the set-up looks fine, except for idle tube size.  It would probably be fine with slightly smaller idle tubes.  Too much idle fuel also effects the main system as it feeds the transfer slots and lower idle airbleeds....Cliff

[SundanceKid]

I switched to a .032 idle tube. I was going to use that as a starting point to enlarge.

The engine just almost dies with the idle screws turned all the way in. I found the best idle at three turns out. It seems to be happy, so I'm sticking with .032.

Much better,  thanks Cliff!

[SundanceKid]

My throttle plates were open a tad more then they should have been, so I opened the idle bypass to .130. This moved the vacuum up to 10 -11 in.Hg after adjusting the idle screws in to 2.5 turns out for my final setting. I'm happy with the idle mixture now.

The primary mixture is still too rich though. Using the idle tip in method at 2,200 RPM the engine slows. I'm using 76 jets with 54M rods. Like I said previously, going down a jet size makes the engine too lean particularly when making slow throttle movements. I'm not getting smoke or anything so it is close with the 76/54M.

The only thing I can think of that could cause the dramatic difference is the 75 jets are new where as the 76 jets are old and could be worn. I'd like to move up a rod and try that, but I don't believe they make a 56M?

Thoughts?

[Cliff Ruggles]

Always use new jets for tuning.  Might want to try a strong PP spring as well, to get the metering rods up quicker to improve throttle response.

That carb should not be lean at part throttle with 54M rods in it, even up to 77 main jets.  I would verify float level, might have too much fuel in the bowl.....Cliff