SU Mixture Question

[Captain Obvious]

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.

Yes, you're right. The venturi itself by nature has to be a restriction to some degree or it is not a venturi at all. But the amount of restriction the venturi provides pales in comparison to the restriction provided by the throttle plate.

The suction piston cannot block all the air.

Ever!

Even if that little phenolic bumper did not exist at all!

The slot cut into the bottom of the piston is still available as an air path even when the piston is slammed all the way down home. At idle, that slot concentrates the majority of the airflow into the center of the piston and directs it right over the nozzle. Concentrated air flow, more velocity, less pressure at the nozzle, and fuel gets sucked up past the needle.

Lift the piston, even a little, and you disable that slot. Non-concentrated air flow, less velocity, more pressure at the nozzle, and no fuel gets sucked up past the needle.

The cross sectional area of the venturi at idle is about .012 square inches. I propose to you that the distributed area of the crack around the almost completely closed throttle plate is smaller than that and the majority of the pressure drop occurs at the throttle plate, not the venturi.

Edited September 1, 2011 by Captain Obvious

[240260280z]

Internet arguments (the friendly Ancient Greek way like this) are not productive when we are both partially correct and both partially incorrect... I concede and we owe each other beers

It has motivated me however:

I just pulled out my USB AtoD developer kit, some Motorola air pressure sensors , some VB books and loaded VB5 on my laptop.... my flow bench project that was stopped many years ago when my friends/co-workers back stabbed me and I momentarily lost my job and career is now back in business I hope to have a calibrated system running soon for measuring this stuff.

[Oiluj]

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?

Both before and after changing needles in my SU's, I checked A/F with using my colortune (per the instructions) at various engine RPM's. I did notice that the mixture varies a bit across the rpm's at steady state, (idle, 200 RPM & 3000 RPM), but not too much. However, it varies alot during "changes" in throttle. Of course, this is all done at no-load...

[Captain Obvious]

I concede and we owe each other beers

Haha! Done!!

Let me know what you find with the flow bench stuff. Sounds like a concise way to investigate this stuff.

I've spent some time analyzing the SU design, but I'm certainly no carb expert. I am, however, completely capable of having absolutely no idea what I'm talking about! I've got a degree in that.:stupid:

[Captain Obvious]

it varies alot during "changes" in throttle. Of course, this is all done at no-load...

Which direction does it flash when you change the throttle position? Rich or lean?

[Gary in NJ]

Lifting the piston leans the mixture that then passes the throttle plate.

[Oiluj]

If I recall correctly, changing from idle to 2000 RPM, A/F "flashed" rich. Changing from 2000 to 3000, it goes slightly lean, but almost immediately and catches-up and is in the correct range.

This was observed with fairly quick throttle changes. The A/F doesn't vary much with a slow, steady change in the trottle.

[Captain Obvious]

If I recall correctly, changing from idle to 2000 RPM, A/F "flashed" rich. Changing from 2000 to 3000, it goes slightly lean, but almost immediately and catches-up and is in the correct range.

This was observed with fairly quick throttle changes. The A/F doesn't vary much with a slow, steady change in the trottle.

Assuming it doesn't take like ten seconds to come back to steady state, flashing rich blipping off idle is a good thing. It means that your piston damper is working.

As for anything else except for off idle, I think there's a lot less to be learned under no-load. You're only using a fraction of the needle. The second part about the A/F not changing much with a slow steady change in throttle is an indication that the first few stations on your needle are well sized, but in order to test the rest of the needle, you need a load.

Back to your original question, I'm not surprised at all that your car stumbles badly when you lift one of the pistons. You're effectively killing those three cylinders. The only fuel they will get is what happens to head their way through the balance tube. Even with both nuts one extra turn down, I'm not surprised. No number of turns down on the other carb will get fuel to those three dead cylinders. The only thing you can do is make sure the OTHER three cylinders are working as well as possible. Then swap and do the other three.

Here's a different way to think about it... When you lift the piston on the rear carb, you aren't testing the rear three cylinders. You are testing the FRONT three. Does that help?

Here's where I remind you that I'm no expert though... YMMV!!:classic:

Blue? You with me on this?

[Oiluj]

I'm with you on that... I was only lifting the pin very little, but perhaps a bit more than 1/32", probably twice that which would account for the stumbling.

Far as it goes, I was mostly just curious. Reality is that I'm quite happy with the way she runs at both idle and on-the-road. These cars really are such a pleasure to drive...

[Captain Obvious]

I've heard two versions of what is "supposed to happen" when you lift the pistons. First is that it will simply stumble badly (which is what yours does), and the second is that the RPM's will go up for a brief instant and THEN stumble badly.

I've also read where people were trying to achieve that "RPM rise before stumble" nirvana and it was elusive.

I say that if the colortune looks good at idle, flashes rich off idle idle, engine runs well and doesn't bog when you punch it, and the plugs look good, then you're way ahead of a lot of people messing with their carbs!

[240260280z]

Innovate LM-1 In the cabin:

Innovate LM-1 crudely attached to the exhaust using a broken pipe and steel wire:

Correct pickup:

The 260z:

[Captain Obvious]

Innovate LM-1 In the cabin:

That's a neat setup. And I assume a lot easier to use than a colortune for when you're driving around.

What's the response time like? Do you think it would pick up Oiluj's transient states?