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...

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.

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...

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:

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?

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

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.

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?

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...

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!

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:

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?

It has a DC output for logging. I am not sure of the sampling rate, hysteresis or DtoA details but can find out.

I am playing with this AtoD to make a flow bench. We can talk about it on Monday

http://www.vellemanusa.com/us/enu/product/view/?id=500349