Nozzle Drip not enough fuel, not enough air or Both?

[deric83616]

Finally have most of the bugs worked out of my Edelbrock 1901 except for nozzle drip.
Several years ago I purchased Cliff's book and Overhaul kit and went through the carb trying to follow one of Cliff's recipes. This week I installed one of Cliff external ATP screws after finding the power piston stop was way out of adjustment. A Change in jetting and the car runs better than it ever has.
Here's what I have:
1971 Corvette LS5 454
Bored 0.060 
CR increase to a Calculated 9.5
Stock Intake
Stock Exhaust
Comp Cams High Energy 268H 218/218 @ 0.050 110 LS   0.485/0.485 Lift
Carb EDL1901
My Edelbrock 1901   
Idle Tube   0.036
Idle Down Channel   0.052
Upper Idle Bleed    0.070
Lower Idle Bleed           0.070
Idle bypass air           0.081
Holes under Mixture   0.090
Holes in Throttle Plate   N/A
Jets                           73
Main Rods                   41
Secondary Rods           DA

When doing the tip in test as described I get no RPM increase as the choke is gradually closed and no RPM change when pulling a vacuum line.

Engine will Idle down to 500 RPM, starts to run a little rough when set a 600 or below. At 750RPM idle is smooth and engine is pulling 16 inches of Vacuum. I have very little with the needle valves but they need to be out about 3 turns to maintain smooth idle and 16 inches of vacuum. I can remove the needle valves completely with out much change.

As happy as I am with the way the car run I'd sure like to get rid of the Nozzle drip. But don't want to start increasing sizes in the idle circuit without some idea of where to start. I initial thought would be to increase the idle tube from .036 to .037 or .038 and maybe the bypass air. But I don't want to screw up a good Carb

Any thoughts would be appreciated.

Thanks
Dave

[Cliff Ruggles]

What size are the Main Air Bleeds?

Did you do a "tip-in" test at fast idle around 2000 rpms or so?

At a minimum I'd add some idle bypass air to make sure it's on the idle system. 

I'd also go up to .038" idle tubes and .055" DCR's.  This will put more fuel at the mixture screws and get rid of the nozzle drip.....Cliff

[deric83616]

Darn. I used the wrong terminology.
Upper main air bleeds in the air horn .070
Lower main air bleeds in the  body .070

I did the tip-in test at 2200rpm

I'll increase the idle tubes and DCR's as recommended.

Does increasing idle bypass air from .081 to .090 seem reasonable.

Thanks
Dave

[Cliff Ruggles]

The tip in test shows that you need to install a larger metering rod as you get no change in rpm's.

If you still have nozzle drip and minimal control with the mixture screws it will need more bypass air.

I can't image that it needs a lot of idle fuel, that is a tiny cam for a 454 cid engine at that compression ratio.  The stock cam had nearly 30 degrees more seat timing and on a much wider LSA......Cliff

[deric83616]

Thanks for all the help.
I've made the following changes as you suggested:
Opened up the Idle down tubes to 0.038
Opened up the DCRs to 0.055
Opened up bypass air to 0.090.
Changed to needles to 44's Jets remain at 73

I have limited control with mixture needles. I removed the springs to insure I could get the needles to seat. started at 3 turns out Idle speed increased as the needles were turned in. I can seat the needles idle speed remains the same for the last 1 1/2 turns in.

Opening a 3/16 intake manifold vacuum port increases idle rpm 150 to 200 rpm. With the vacuum port open and mixture needles bottomed the engines will idle at 700 rpm with 15 inches of vacuum. Can't see any nozzle drip but suspect I have some as the throttle valves appear wet. closing the port cause the RPM to drop.

I believe I need more bypass air. But I don't want to get in to a cycle of too much fuel not enough air, too much air not enough fuel and so on.

First, Do you think I'm on the right track by adding more bypass air
Second, if I'm on the right track how much do I add, 10%, 20% or ??

Minor point of clarification my 71 Corvette 454 is an LS5.
Factory LS5 cam specs are:
   214/218 @0.050 115 LSA  0.461/0.480 Lift   
my Comp Cams 268H Specs are:
   218/218 @ 0.050 110 LS   0.485/0.485 Lift
close to factory specs
Wished pick a little more cam but because in my youth I over camed and carbed everything, perhaps I went a little too conservative.
 
Thanks in advance
Dave

[Cliff Ruggles]

First thing is NEVER compare cams at .050" specs and different LSA.

The Comp XE268 cam is a tiny cam in a 454 engine and should be treated as such.  The 110LSA also put the intake valve closing point early, and 110LSA increases overlap considerably. 

It is NOT a good cam choice for that big engine at 9.5 to 1 compression, and the short seat timing event, super quick ramps and tight LSA can create issues at idle speed, and issues at high rpm's due to instability in the valve train.  We call this deal "lifter crash" and it's common with XE camshafts.  I've tested several of them on the dyno early on and they not only came up WAY short on power production, but experienced complete loss of power around 4900-5000rpm's.

IF you are completely on the idle system no nozzle drip will be present.  If some nozzle drip exists there will not be full control of A/F with the mixture screws.  You MUST make sure that there is no nozzle drip present before proceeding with carb modifications.

.038" idle tubes and .055" DCR should be PLENTY of idle fuel for that combination.  Initial timing plays a huge role here as well, and I would NOT run a lot of it with that engine combination, it should be fine with about 10-12 degrees timing and no additional timing from the vacuum advance......Cliff