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

[Zedyone_kenobi]

Actually the WOT plot is very flat.

Does the car have a smog pump or cat upstream from the O2 sensor?

NO sir.

Completely smog free

[240260280z]

NO sir.

Completely smog free

Perfect. That means you either have a manifold leak or carb issue.

Did you try spraying a solvent at all margins and hoses/caps of the manifold to see if idle rpm's increase? This is a quick way to check for a leak.

[30 Ounce]

Interesting, I've rebuilt 4 L series motors and never noticed orange paint on the cam. None were L24 though. 1 L26 & 3 L28.

Edited January 16, 2012 by 30 Ounce
Additional info

[madkaw]

Did you ever think this thread would go 10 pages strong!

Another hypothesis- z therapy carbs use the gross jets- correct? Is there a different interaction between these jets and stock needles? Still leaning( no pun intended) towards a low float level which is causing an insufficient amount of fuel at stock settings. It seems that it has been determined that the engine is running lean and his vacuum is strong at idle. If richining the mixtures at the knob doesn't improve anything does NOT eliminate a bad float setting. The two must work together-IMHO

[30 Ounce]

I think ZTherapy stopped using Gross jets plus I think they were for the fuel inlet to the float bowls.

How is your fuel pressure & volume? If the fuel level drops as you accelerate it will lean out at the top end like that.

[madkaw]

My hypothesis still remains the same - the level of fuel has to be right for everything to work together. Maybe even more so if a different needle and seat is used for setting the level- other than stock.

[doradox]

Don't assume that the idle "circuit" is correct and that leaning out is the problem. Maybe the idle "circuit" is too rich. When set for a good idle mixture the carbs go "lean" at cruise. Look for something that makes idle rich and goes away otherwise as well as the converse.

Steve

[motorman7]

I agree these afr readings look very lean for sm needles. I'd crank the adjusters down until you get the idle in the mid 11's and go for a drive. This might be a bit rich for best fuel milage but it should not be so rich it stumbles.

Or maybe you get a set of the KD needles which would be better at the high end. I used to have a pair, but I think I may have put them in the '73 that I recently sold.

[Zedyone_kenobi]

Well the thing is that no stock 240Z should not be getting enough fuel with SM needles installed. (Unless they are installed wrong,which by all accounts they should not be)

I checked my fuel bowl level using the clear fuel tubing trick, and the fuel level is coming up to the right height as measured from the fuel bowl sealing surface on both carbs...

I am interested in what Blue was saying about scooting the needles up a bit.

The unmachined portion of my needles, if you look in the picture above this post, where the letters KD and SM are stamped sticks out quite a bit from the bottom of my piston. I used the trick in the SU video to install them , but maybe they need to be up more into the bore. That would make me run lean. But before I do any of that, I am going to do a few things.

First, since the valve cover is already off, I am going to check the valve clearance against the cold valve specs...

Second, I am going to install a new valve cover

Third, I am going to get the car running and spray around all the intake gaskets to see if there is a leak

fourth, I am going to richen up the mixture either by the nut, or by sinking the needles into my pistons more...

Did my timing chain look lose to anybody ?

[motorman7]

The unmachined portion of my needles, if you look in the picture above this post, where the letters KD and SM are stamped sticks out quite a bit from the bottom of my piston. I used the trick in the SU video to install them , but maybe they need to be up more into the bore. That would make me run lean. But before I do any of that, I am going to do a few things.

I have always installed the SU needles with the base of the unmachined portion flush with the base of the piston.

[240260280z]

A quick and dirty to make sure you get more than enough fuel:

Do another WOT run and measure A/F

Drop the jets two full turns then repeat the WOT run.... hopefully the A/F will go below 14.

You can also idle and measure the A/F then pull the choke and see if the a/f drops

[madkaw]

Okay- I'm not seeing this so called pic of the SU piston, but I'm using a I phone. I hope that the needle is seated flush against the bottom of te piston. No part of the needle should show except for the needle - no shank should be seen. If the needle is showing part of the shank, then I guarantee that's your problem