popping through the exhaust at 4500 rpm or so.. HELP!

[Captain Obvious]

So the wideband showed exactly what it sounded like. At the same RPM, you're 16 lean at part throttle, but flat 14 at WOT. At the same RPM... Does the same thing with the stock needles. Maybe even a little worse. That makes sense, but there's no way you should be lean with the SM's OR the stock needles.

It's not fuel delivery because you're clearly not running out of fuel.

It's not your "typical fixed" vacuum leak because if it was bad enough to cause that much of a problem at 5000RPM part throttle, then it would simply ruin your idle.

So... What can look like an RPM variable or throttle position variable air leak?

- I think I see a cap on the port vacuum signal on the front carb... If that cap or the nipple it is connected to is leaking, you wouldn't have a vacuum leak at idle, but you would have one as soon as you opened the throttle. That leak would be "fixed size", but wouldn't affect idle. Would make your WOT numbers higher as well, but the SM's might be making up for it due to the fact that the SM's are way richer at the tip.

Or, what could cause the suction piston to raise too much, too soon?

- Maybe some restriction on the atmospheric vent side of the suction piston? Something in the way of the vent holes to the air cleaner?

What are you doing for an air cleaner? I don't see anything in your pics. Also, I see that your bowl vents are not connected to anything. Is it always like that, or is that just for the pics?

What do you connect your cam cover vent to? Anything?

BTW - Have you talked to Z Therapy about this?

I love a good mystery, but it's got to have an good ending!

No part of the needle should show except for the needle - no shank should be seen.

All of the literature says that shoulder of the shank should be flush with the bottom face of the piston, not flush with the bottom of the venturi groove. That means you should be able to see like .035 worth of shank.

[madkaw]

Originally Posted by madkaw

No part of the needle should show except for the needle - no shank should be seen.

All of the literature says that shoulder of the shank should be flush with the bottom face of the piston, not flush with the bottom of the venturi groove. That means you should be able to see like .035 worth of shank.

I have never set the needles up like that and i am running mine flush with the "groove". I have been looking for "literature" to verify this either way, but I can't right now. I believe I saw this done on Z-therapys video. Like I said, my car runs great with the needle flush.

[beermanpete]

Here is the factory service manual page for setting the needle height.

Jet Needle Adjustment.pdf

It seems to me that if the needle setting is off a bit you could easily compensate for it with the jet adjustment. It will come out the same in the wash. Keep in mind that the jet height adjustment is primarily an idle mixture adjustment. Its affect on the mixture diminishes as the suction pistion is raised.

Perhaps you could shim the suction spring to richen the mixture without making the idle too rich.

[Zedyone_kenobi]

Okay,

Captain, you first...

I run a factory air cleaner with the valve cover and fuel bowl vent tubes connected to it normally. It is just so much easier to tune the SU's as much as I have with the air cleaner off.

Normally all is hooked up as the factory intended. The car acts the same either way.

I pulled a piston out and the amount of shank I have showing in my first carb is appropriate. It appears level with the bottom most surface of the piston, with a sliver of shoulder showing...Not the Venturi groove. The picture in the FSM provided above shows using a straight edge over the bottom of the piston, but it is hard to make out if the straight edge is over the bottom of the piston or over the bottom of the groove.

I think whether it is flush with the groove or flush with the bottom of the piston could be made up for with a turn or two of the mixture knob, as beer man pete suggests. I think the jet extension might be a red herring.

I have contacted Z therapy about this. I sent Bruce a long PM last week, but have not heard back from him. I was hoping he would find this thread!

I think I really need to start thinking about pulling off the intake and put a new gasket in. I MUST eliminate the 'smart' vacuum leak as a possibility or we will never make any progress. I am going to order two intake kits. One for this trouble shooting and ONE for the mikunis or webers that I am going to replace these with!!!!!!

Edited January 17, 2012 by Zedyone_kenobi

[madkaw]

Not sure what you make of that pic-I have the same manual and it doesn't even show the "groove". Granted, it isn't much of a difference, but there is a difference, and that difference would be the right direction for this issue.

Also it would manifest itself as more than just a idle adjustment. The mixture screws can affect performance all the way into cruise.

Edited January 17, 2012 by madkaw

[Zedyone_kenobi]

EDIT:

Captain, I will see what I can find out with a vacuum gage hooked up while running!

I am going to have gages wire tied all over the place.

The source of this is very tricky and nefarious. But I will find it! NO way this is going to beat me..

[Diseazd]

The bottom of the shank should be flush with the bottom of the piston....not the bottom of the groove.

[Captain Obvious]

Originally Posted by madkaw

I have never set the needles up like that and i am running mine flush with the "groove". I have been looking for "literature" to verify this either way, but I can't right now. I believe I saw this done on Z-therapys video. Like I said, my car runs great with the needle flush.

I'm not surprised that your car runs OK with the needles the way you've set them. It's not a huge difference, and as beermanpete says, you can compensate for it with nozzle adjustment. It's a little less than one turn on the nozzle knob to make it essentially the same as setting them at the bottom of the groove.

However, the correct way to set them is to put the shoulder flush with the bottom of the piston. That manual that Pete posted is pretty blurry, but a similar picture appears other literature as well. They're all set the same. I'll see what I can dig up.

Edited January 17, 2012 by Captain Obvious
Looking for other pics

[Captain Obvious]

So there's no spider webs in any of the holes on the intake face of the carbs? No gasket sealer goo smeared in the holes? When you're running with no air cleaner, the gaskets between the carbs and the air cleaner are gone too, right? Nothing on the intake face of the carbs except aluminum.

What about suction piston springs? Any chance you're forgetting to put them in? I know... I just gotta ask. Did the springs come with the carbs from ZT?

When you're popping at 5K, what happens if you just hold it there? If it just keeps popping and never smoothes out then we should be able to rule out damper issues (wrong oil, not enough oil, check valve not working)...

Really sounds like a lean midrange needle tuning issue, but there's just no way that should be the case with the SM's.

Have you considered switching to the flat tops? I'd be happy to guide you on a journey to the dark side.

[Zedyone_kenobi]

HAHA...I will pass on the flat tops, but hell, they may work better!

Correct just aluminum...

I have not held it there for a super long time, not sure. I seem to remember doing that test in 1st gear, and the problem went away if I held it steady, but I will have to double check.

[Bonzi Lon]

I'm still leaning to air getting in from the throttle shafts. I know the bearings & shafts are new, however some out of tolerance parts make it past QC. To me there is a spot at half way where air enters. Like it is oval in one spot, sealed at idle and wot, but lets air in at that spot. The grease trick should seal them enough to get a pull or two. Thick enough to hold a seal and high temp so it doesn't melt off. A minor mess.

Bonzi Lon

[Captain Obvious]

So what about the piston springs? The springs are the ones that came with the carbs from ZT and they look OK?

And when you get a chance, try that steady state test again and see what happens.The damper should only be a transitional thing. It's effects shouldn't last more than a second or two.

I think it's time to swap the carbs with a set that's known to work well on a different car. Shotgun if the problem is with the carbs, or something else on your car somewhere.

Mmmmmmm...... Flat tops......