Rich at high vacuum

[Seff]

380 Olds stroker, 218/221@.050" roller cam. 77 Olds carb modded to third recipe.

Idles at 14 hg at 650 rpm.

Cruises at 10-11,5 hg at 50 to 80 mph (both in 4th and 5th gear): AFR 14.5-14.8
Cruises at 16-18 hg at 20 to mph (both in 3rd and 4th gear): AFR 12.8-13.6


Coasting: AFR 15.5-17.9


As soon as I touch the throttle after coasting (the lightest touch, to maintain speed), it jumps (and stays) to 12.8 AFR, and doesn't lean out until vacuum nears 10-11 hg.


Am I correct in assuming that my main air bleeds are not proportional to my main jets, or proportional to my vacuum/air flow? It's like they meter too much fuel at low throttle opening/high vacuum (not a lot of air passing through the throttle blades), and meters the right amount at higher load (lower vacuum).


Thanks.

[Cliff Ruggles]

High vacuum pulls fuel from the transfer slots in that range, so most likely it will not lean back up until the vehicle slows and some load is put back on it.

The best way to tune with an A/F meter is to put a piece of black tape over it.

Then tune for best results in all areas.

I like to tune WOT at the drag strip with ET/MPH.

Idle is easy, best idle quality at the leanest settings you can manage with the mixture screws.  This is done by turning them in until the engine just slows ever so slightly, then back 1/4 to 1/2 a turn till it just smooths back out/highest RPM.

Light part throttle done with the APT to provide best power in the "normal" driving range and best fuel economy.  That deal can be difficult if you are a "lead foot" like me, because with over 550hp on tap it's IMPOSSIBLE to run thru a tank of fuel without quite a few full throttle blasts!.

Heavy part throttle is done with jet size only.

You can also tune light part throttle with the vacuum advance, so don't forget that it is a BIG player in engine efficiency in the "normal" driving range.

When you get the carb all dialed in, remove the black tape, you now have the base settings that your engine likes/wants and is happy with.

I've learned over the years that trying to go lean with the APT your primary jet/rod combo's typically doesn't work well as lean settings require increased throttle angle to achieve/maintain vehicle speed so the carb automatically adds fuel to compensate.

It's also difficult, actually damned near impossible to get past the basic laws of physics with these things.....it's going to take "X" amount of BTU's to move a specific MASS from point A to point B......hope this helps some.......Cliff

[Seff]

Very helpful, thank you!

Regarding your last paragraph, I think that's the primary issue - I'm expecting too much of a mechanical metering device. 

[Cliff Ruggles]

Not really.  It's a variable A/F metering device and very efficient.  Folks get all caught up in this deal thinking with a "static" mind set.

The carburetor by design will add fuel with increased throttle angle and pressure differential above/below the throttle plates.  It is NOT a fixed system. 

You can actually remove the metering rods and power piston and run them straight off smaller jets with near perfect results.  The changes in metering area simply help the carb be better at what it already does well.

Folks also don't add in timing changes via the vacuum advance when tuning.  It is a BIG player and a load sensing device.  Adding timing at light throttle starts the burn time earlier and allows us to run a much leaner mixture at light engine load.

So if you are making carb changes and not adding or removing some VA timing you are not effectively tuning that range.

Most engines similar to yours will need about 10-15 degrees from the vacuum unit at part throttle. It can also be added at idle (manifold vacuum to the advance) if the engine has a weak enough signal to the carb and idle fuel is deficient at the mixture screws.

I typically use ported vacuum to the VA for most well thought out set-ups.  They will typically not like, want or need a butt-ton of timing at idle speed.

Some engines enjoy a lot of timing at idle and it may help to bring some in with the VA instead of trying to run too much initial timing where you start to run into difficult hot re-start issues.

Also keep in mind when tuning that right off idle and at very light throttle openings a good portion of the fuel comes from the off idle transfer slots.  This is why carburetors provide very smooth off idle transition and it's a wet-flow system so they can be made to work as well as or even better than most TB type "fuel injection" systems.  Just simply laws of physics and there is never any lag in fuel flow to the engine and no TPS involved with very slight throttle movements.

In any case when we build "custom" engines they require custom tuning.  Your engine is not like any other, so if you want the most out of it continue to tune for best results in all areas.  That just the way it is, and has been since I got into this hobby nearly 50 years ago........Cliff