SU Mixture Question

[Oiluj]

Background Info:

The engine idles reasonably well, (given the stage 3 cam), runs fine and temp is stable at 180F, even on hot days.

Mixture nuts are set at about 2-3/4 turns each. My colortune shows my mixture to be about right, measured at plugs 2 & 5. Timing is set at 7 BTDC & carbs are balanced.

Reading recent posts, I got "curious" and tried lifting the pins that raise the carb pistons. For both carbs, the engine stumbles badly... Not what it's supposed to do. Even when I increase mixture 1 full turn on the mixture nuts, it still stumbles.

I do have the larger jet needles to accomodate the bored & stroked 2.7L L24.

Question: So what do I believe? I'm inclined to think I should go with the color-tune results, but what do others think...

[beermanpete]

Are you lifting the vacuum piston by hand? If so, it will run poorly when you do this. You are essintially making it run on 3 cylinders. The idea is to see if the engine RPM drops the same amount for each carb. This is a way to check the idle mixture balance front to rear.

Your car has a big cam so the idle is not as good as a stock cam. It might not support idling on 3 cylinders well enough to allow this test.

[240260280z]

The "British Method" (picture below) is to lift each piston ~ 1/32"of an inch using the pin then see what happens to the idle.

I never had much luck with this on the six cars I tried it on as the front carb would seem to work fine but the rear carb never would, or there was very little discernible change. I now use a hybrid of Norm's method with an O2 wide band sensor "Canadian Method".

Norm's method "American Method". Is simply to disable one carb at a time by lifting the piston to disable it. In theory if the carbs are balanced, the engine should stumble the same when the front carb is disabled and when the rear carb is disabled.

Norm's method is interesting in that, by lifting a piston, you create a huge "vacuum leak". When you have a wideband in the exhaust, the value goes up around 27 to 29 (from my memory from last test in 2005). You can compare between the two cylinders at idle to balance the jet height. I also do this at 3000rpm to ensure the jets/needles match at higher RPM too. I can't recall what happens when you plug a carb's mouth at idle (stumble or stall) butin theory you can try that as well and the O2 sensor should read the same for both sides and be near Stoch.

[Oiluj]

In absence of an O2 meter, think I'll stick with the colortune for setting my mixture, especially since the car runs fine and gets decent gas mileage...

[LeonV]

Julio,

Put a WB02 in your exhaust and you won't have to guess anymore! The gauge and wiring can be easily hidden if you're worried about that.

[Captain Obvious]

Norm's method is interesting in that, by lifting a piston, you create a huge "vacuum leak". When you have a wideband in the exhaust, the value goes up around 27 to 29

I'm no expert, but I believe the above contains a little bit of a misnomer. If you create a "vacuum leak" you introduce additional unmetered unfueled air. Your traditional "vacuum leak" will reduce manifold vacuum because it goes around the throttle butterfly.

On the other hand, lifting the piston won't change manifold vacuum (except due to any changes in RPM because of rough running) and it won't introduce any additional unmetered unfueled air.

What you do when you lift a piston is decrease the venturi airspeed which renders the venturi ineffective and no fuel will be introduced by that carb. The amount of air flowing through that carb will not change, just the amount of fuel added to that air.

So yes, you will see a big change on your wideband because even though the amount of air hasn't changed (again except for any changes due to an RPM change), the amount of fuel has gone down.

I can't recall what happens when you plug a carb's mouth at idle (stumble or stall) butin theory you can try that as well and the O2 sensor should read the same for both sides and be near Stoch.

If you plug a carbs mouth at idle, it will create a huge "static" vacuum in the entire throat of the carb, including the venturi. That huge vacuum will pull raw fuel up and out of the nozzle. That's what "chokes" do.:classic:

Or... There's always the possibility that I have recto-cranial inversion. It's happened before!:laugh:

[Walter Moore]

Julio,

Put a WB02 in your exhaust and you won't have to guess anymore! The gauge and wiring can be easily hidden if you're worried about that.

So, how does the typical SU's mixture vary across engine speed? I have always wondered that since there is only one mixture knob... Is it relatively uniform across the RPM range, or are there rich spots and lean spots? Does it tend to lean out at high speeds because of the distance from the jet to the bottom of the piston that forms the top of the venturi?

[240260280z]

Lifting the piston adds more air as there is not enough venturi effect to pull significant fuel out of the jet.

Blocking a carb opening will cause manifold pressure to expand into blocked carb's closed-off area over the bridge however the piston will be down so it will limit fuel flow from reduced pressure above jet. A hose from the top of the fuel bowl to the carb body will also present the same manifold pressure above the fuel pool (equalized) so there will should be no push or pull on the fuel.

[240260280z]

So, how does the typical SU's mixture vary across engine speed? I have always wondered that since there is only one mixture knob... Is it relatively uniform across the RPM range, or are there rich spots and lean spots? Does it tend to lean out at high speeds because of the distance from the jet to the bottom of the piston that forms the top of the venturi?

For a stock system, the mixture knob should be set for idle a/f only and the needle's taper should be adjusted for a/f ratio at different rpms/load. (adjusting the needle means swapping it, sanding it, or turning a new one on a lathe). In a stock system, the needle taper should already be optimized for a balance of power, emissions and fuel efficiency as per the factory's best effort.

Edited September 1, 2011 by Blue

[Captain Obvious]

Lifting the piston adds more air as there is not enough venturi effect to pull significant fuel out of the jet.

No, you see... Lifting the suction piston does NOT add more air. The only thing that would add more air is opening the butterfly.

At idle, you've got a tiny crack past the two throttle plate butterflies. The engine is pulling as much air as it can past those two throttle plate restrictions. You lift the suction piston, and so what? The same pistons (crankcase pistons) pulling the same air past the same butterfly restrictions.

The total amount of air being pulled into the engine will not change. (Again, with the exception of the changes in RPM due to lifting the suction piston.)

Yes, you reduce the air velocity at the venturi which (will reduce the venturi vacuum and suck less fuel), but the total amout of air pulled past either butterfly will not change.

Edited September 1, 2011 by Captain Obvious
distinction between suction pistons and engine pistons.

[LeonV]

So, how does the typical SU's mixture vary across engine speed? I have always wondered that since there is only one mixture knob... Is it relatively uniform across the RPM range, or are there rich spots and lean spots? Does it tend to lean out at high speeds because of the distance from the jet to the bottom of the piston that forms the top of the venturi?

Not sure, but it depends on the carb settings (needle, spring, etc.). I have yet to do any testing on SUs.

[240260280z]

No, you see... Lifting the suction piston does NOT add more air. The only thing that would add more air is opening the butterfly.

I do not agree. The piston is an air restriction up stream of the throttle valve. In fact, there is a rubber bumper on the piston to prevent it from completely closing and blocking all air. As well, my O2 sensor goes very lean when I lift one piston so I am adding more air and reducing fuel at the same time. The air from the three non-firing cylinders dilutes the exhaust gas.