C3Paul

Hi,

Despite of my hardest attempts to try and tune my 383 I am still running little rich on idle.

My Holley 770 SA is running main jet size #72 and secondary #75

If i was going to change my main jet to #70 and the secondary to #72 will i lean out my idle circuit, or do i need to start thinking how to drill the butterflies?

cv67

Dont drill the butterflies!
The jets dont affect the idle sounds like you may need work on the idle cirucuit.
Thought my 750 was rich at idle cause the eyes burned turned out it was caused by the car being too lean instead. going down in numerical size idle air bleeds cured the problem instantly

MelWff

Not sure if it's a valid comparison for your engine but the Holley 780 used on the 1970 LT1 used 70 primary and 76 secondary. You dont mention what power valves you are using, are they low enough numerically to be closed at idle? Is your secondary throttle stop correctly adjusted?

Barry's70LT1

If your idle is high, your butterflies may be open enough to expose the transfer slots, if so you cannot "lean" it enough with the mixture screws.

DC3

Get in touch with Lars and get his paper on the Street Avenger setup. I'm running the 670 SA on my 350. I set mine up on the bench according to his instructions and it runs fantastic. I'm running 68s in the primary and 76s in the secondary with the tall yellow spring in the secondary diaphragm.

DC

lars

Most common issue causing your problem is excessive primary transition slot exposure: Do you have more than .020 transition slot exposure on the primary side? If so, balance out your idle airflow by getting your secondary idle speed correctly set, thus decreasing your primary transition slot exposure. This will give you full control over idle mixture.

Changing main metering jets will have little, if any, effect on idle mixture - the carb runs on the idle and transition circuits at idle - not on the main metering circuit.

Lars

C3Paul

All has been reset now however,

Throtle blades showing perfect square on the transition slots and secondary are mirroring primary blades.

Idle mixture screws are 1 turn out.

once the engine is fully wormed the car is idling at 1200 rpm and my O2 sensor is reading 22.1 (too lean)

Manifold vacuum is 15Hg

when i try to open the idle mixture to richen the mixture the engine just picks up revs.

when i try to close the the throtle plates to lower my idle to around 850RPM, the engine dies as the plates are almost fully closed

what should i do, please?

scottyp99

Are you adjusting the idle speed with the front throttle plates, or the back ones?

Scott

C3Paul

Both equally.

chevymans 77

the secondaries don't need to be adjusted the same as the primaries they don't have transition slots.

set the primaries as Lars indicated then use the timing and secondaries to set idle.

Neal

scottyp99

You set the primaries so that they have a good relationship with the transition slot, and then don't touch them again. Use the secondaries to adjust idle speed. If you start to pull fuel through the secondary boosters, you've opened them up to far, don't worry, it probably won't be an issue. Use the idle mixture screws to get the highest vacuum reading you can, then adjust idle speed. Keep going back and forth until it's stabilized. Make sure your timing is where you want it first, or you'll be doing the whole thing all over again. Probably will anyway, you can tune an engine forever, if you're fussy enough......Remember the movie Two-Lane Blacktop? Everytime they stopped, the mechanic wanted to re-jet the carbs.

Scott

C3Paul

Ok, I understand however, in the Lars paper he says I should adjust the curb speed screws equally??

For the sake of my sanity.

My primary throttle plates are set to see the square shape of the transition slots.

Secondary plates are open the same.

Idle mixture screws are 1 turn out.

Basic timing is 16* with 12* of Manifold vacuum advance.

At idle I am 28* advance on my 383ci roller

What should I do next???

chevymans 77

At 1000 to 1200 rpm you may be seeing some timing advance and when slowing to 850 the combination of the timing dialing back and the lowering the rpm may cause the issue you are seeing.

Maybe you can watch the timing with a light when lowering the idle to see if it is also dropping.

try installing rubber bands on the advance weights and see if when slowing the idle down it doesn't die out.

Neal

lars

Yes, they do. And you never use timing adjustments to set idle speed.

Paul -
I'm not understanding the problem... You're saying that you have no control over idle speed, and if you try to idle less than 850 rpm that the engine dies..? Adjusting both idle speeds equally, you should have infinite control over idle speed, and if your engine dies at 850, you have other problems...

Lars

C3Paul

I think, I am sorted!

Idle at 850-880RPM
1.1/4 turns out on idle mixture screws
Vacuum 15Hg
O2 reading 14.1

All seem pretty good.

Thank you All, for your help

Paul

chevymans 77

glad you got it worked out Paul,

Neal