Tuning 40 DCOE Weber 151 on an L24

[d240zx2]

4th gear? Don't you have a 5 speed yet?

[Zedyone_kenobi]

Your killing me Frank!

I just bought a new rebuilt type B 4 speed from zcarsource! The day I have a 5 speed will probably be the day I trade the numbers 2-4-0 with 9-1-1

[d240zx2]

Get a ZX 5-speed. You will NOT be disappointed, especially if you plug in an R200 3.90. I promise!

[LeonV]

Get a ZX 5-speed. You will NOT be disappointed, especially if you plug in an R200 3.90. I promise!

Agreed. It transforms the car!

[Zedyone_kenobi]

I drove the car again for some pleasure cruising, and I can say I have absolutely NO flat spot or hesitation. Fixing the float levels did more than anything else did. The car drives wonderfully. I still do not know if my AFR is spot on as I was enjoying the drive and feel and sound so much I did not even take the gage out of the glovebox. But final jetting still needs to be verified, but I am darn close now, I may just leave it alone until I get these on my L28.

[madkaw]

Time to go to the dyno with some extra jets in hand and get all you can out of the mighty L24.

[madkaw]

Better yet- do the STI. LSD diff for the real transformation

[240260280z]

Stephen, I followed in your footsteps the past few days! Thanks for your help. The car is coming along fine.

I figured I'd just append my data and observations to this post for future enthusiasts following in our footsteps.

Build:

motor: L24 0.75mm overbore

head: Maxima N47 with stock valves

headwork: medium porting/shaping intake and exhaust

cam and valve train: Delta 0.46' lift and 270/280ish

Carbs: 40DCOE 151

Intake: Cannon Long Runner

K&N filters

Stock 240z mechanical pump dead headed.

Carb Internals:

Idle Circuit:

Jet: 55F11

Main Circuit:

Jet: 125 F11 170

Main Venturi: 30

Aux Venturi: 4.5

Acceleration Circuit:

Spill valve: 50

Jet: 45

Starter Circuit:

Jet: 85 F9 150

Needle Valve: 225

All new gaskets

ZX distributor set 17degrees at 900rpm no vacuum advance

Notes:

- the long intake and big air filters barely fit....but they do! I have to pull the master brake cylinder out to measure the airflow into #5&6 when balancing with synchronometer

- As Stephen mentions, the fuel level is crucial for smooth idle-to-main transitions. My default fuel depth was 29mm down from the main jet well's top. I noticed a bit of a transition bog. When I set the fuel to 24.5mm/24.5mm/25.0mm for the three carbs, the bog disappeared...but I got fuel dripping between the throat and the main venturi on a couple of the intakes. This did not happen at 29mm fuel depth. I will re-check the fuel depth but I think it is a combination of Aux Venturi to body sealing, the stock fuel pump pressure and the too-big 225 needle valves that came with the rebuild kit. I will experiment on resolving and report back.

- initial setting of the carb's primary two interacting and adjustable parameters was my biggest hurdle (Thottle Valve opening and Idle Mixture Screw depth). From what I read, the importance of setting the "throttle valve as close to blocking the first progression hole" was the priority. I was able to view the "blocking" through the inspection port over the progression holes and easily make it the same for all 3 carbs . The problem is that I could easily block the first hole and have the left (with respect to (wrt) the car) edge of the throttle plate anywhere from the right side of the hole and beyond. I initially decided (incorrectly of course) that it was important to block as much of the first progression hole yet let a wee bit of air around so I set the throttle plate so that the left side of the plate was at the midpoint of the first progression hole... then the problem began... in order to get the car to run, I had to set the idle mixture ~3.5 turns out but it ran pig rich and it was hard to get the RPM's below 1100. It ran smooth and idled smooth and had lots of power but it was too rich and stinky and it would sometimes diesel after shutoff. It would not run at 2.5 turns out!!! After a lot of experiments I re-started from scratch, but this time, I set the idle Mixture Screws at 2 turns out then adjusted the Throttle Valve opening to get 900rpm idle. It worked much better. I am still playing around but it is how I would recommend others to do initial setup. I am now reading the plugs and not using the WB just like Stephen above with 2.25 turns out, the plugs are looking much better... a touch too lean but some brown on the porcelain. I have read that webers like richer and I experienced it so I will continue to experiment. The only "stumper" is that I did not expect the idle mixture screw and throttle valve to have such an effect on lean/rich... I thought it was the idle jet selection. I guess the pressure dynamics at the progression hole affects how much fuel comes out pre-idle mix screw. With more of the first progression hole exposed to manifold vacuum at idle, less fuel is needed from the mix-screw path...AND less fuel can reach it as it will leak out the exposed progression hole and not flow further to the mix screw.

- setting the throttle push rod arm lengths.... don't do it on the carbs using the turn buckles!!! All throttle arms should be the same length. Set them on a bench using vernier calipers. Simply stick the tangs of the calipers in the heim joints balls and set the length then set the locknuts. The way to adjust the cannon throttle push rods is to loosen where they bolt to the main rod. You can tighten with the car idling and easily hear if you push them down and increase the idle.

- I set the Air bypass Screws in all the way then adjusted as needed to get all of the carb throats to flow the same using the synchronometer. I think the value was 5.5 on the dial at 900 RPM.

- There is a stop bolt under the acceleration pedal on a 240z. I set it so that the pedal stops before it pushes the throttle valves past their stop... otherwise at WOT you can run the risk of twisting the throttle shaft in the carbs (not a good thing... I had to replace one already due to corrosion...it requires drilling the shaft so again..do take care)

- I had set the initial timing to 17degrees at 900rpm with vacuum advance disabled. I just set it to 15degrees as per Z garage's excellent weber advice. I will play more down the road. Going to 15 seemed to make no significant difference. http://datsunzgarage.com/weber/

Edited August 6, 2012 by Blue

[Zedyone_kenobi]

Awesome post.

I have to agree the marriage of the throttle plate and mixture screw is paramount and also convoluted. Both due the same thing. I was able to find a happy idle on my L28, but I want to get the mixture screw back to the magic 1 turn out from fully seated.

I am currently running 55F8 idle jets. Now it is important to know what those numbers mean. The 55 is the size of the jet, a bigger number is a larger fuel orifice. The last number is the air orifice. This lets in more air. Now this is where it gets tricky. The Fx numbers are not increasing as numbers go up.

In essence, the idle jet is like a miniature Main/emulsion tube/Air corrector. You have to find out where your engine is most happy, and unfortunately, it will be an expensive task.

Most expert tuners often fall back on the 'suck it and see' method. There are just too many variables in play to get it right on the first try. But having synchronizes and wide bands give you the information to give you more data with which to make decisions on.

A 55F9 will run richer than a 55F8. An F8 has a larger air opening than an F9. Go figure. I have the sizes in ascending order in one of my weber books for the DCOE jets. I will post those tonight. Currently I have the first progression hole totally blocked off, and I am about 2.5 turns out from fully seated on my mixture screws. So my idle needle is doing pretty much all the work, and any movement of my throttle plates will uncover the first progression hole.

If I put my mixture screws about 1 turn out , the car will not run, it dies, and I go crazy lean. According to all the literature I have read, I will need to up the idle circuit.

I have ordered 50F9 to see where that gets me. I am gambling a bit on reducing the size of my fuel orifice, but I think the smaller air orifice will drive my mixture to the rich side more. I will report back and find out. Getting the idle circuit spot on is so critical as that makes the transition to the mains easier to tune.

Also, remember to do all your weber tuning with the engine fully up to temp. You do not want to tune for start up. HA

[madkaw]

This is where I'm at on the Mikunis . Too small on the pilots make for a weak transition . If you richen up a small pilot to make transition better, you wind up with a rich idle.

Pilots are on order

[LeonV]

Awesome post.

I have to agree the marriage of the throttle plate and mixture screw is paramount and also convoluted. Both due the same thing. I was able to find a happy idle on my L28, but I want to get the mixture screw back to the magic 1 turn out from fully seated.

I am currently running 55F8 idle jets. Now it is important to know what those numbers mean. The 55 is the size of the jet, a bigger number is a larger fuel orifice. The last number is the air orifice. This lets in more air. Now this is where it gets tricky. The Fx numbers are not increasing as numbers go up.

In essence, the idle jet is like a miniature Main/emulsion tube/Air corrector. You have to find out where your engine is most happy, and unfortunately, it will be an expensive task.

Most expert tuners often fall back on the 'suck it and see' method. There are just too many variables in play to get it right on the first try. But having synchronizes and wide bands give you the information to give you more data with which to make decisions on.

A 55F9 will run richer than a 55F8. An F8 has a larger air opening than an F9. Go figure. I have the sizes in ascending order in one of my weber books for the DCOE jets. I will post those tonight. Currently I have the first progression hole totally blocked off, and I am about 2.5 turns out from fully seated on my mixture screws. So my idle needle is doing pretty much all the work, and any movement of my throttle plates will uncover the first progression hole.

If I put my mixture screws about 1 turn out , the car will not run, it dies, and I go crazy lean. According to all the literature I have read, I will need to up the idle circuit.

I have ordered 50F9 to see where that gets me. I am gambling a bit on reducing the size of my fuel orifice, but I think the smaller air orifice will drive my mixture to the rich side more. I will report back and find out. Getting the idle circuit spot on is so critical as that makes the transition to the mains easier to tune.

Also, remember to do all your weber tuning with the engine fully up to temp. You do not want to tune for start up. HA

Newer Webers (151) have idle mix screws with a finer thread pitch. One turn out on the old carb means more like 2 turns out on the new ones. FYI...

So being 2.5 turns out on the new ones is comparable to 1.25 turns on the old ones.

Edited August 6, 2012 by LeonV

[Zedyone_kenobi]

That is great information, I had no idea!

Back to the idle mixture jet information I promised...

"...In order of ascending size, the air bleed orifice diameters are F6, F13, F9, F8, F2. (F11 has the same size air bleed orifice as F8 but the axial bore is smaller)...

So an F6 would have the smallest air bleed orifice, and an F2 would have the largest air bleed orifice. Meaning if you keep the fuel orifice fixed(the XX in the XXFy idle jet number) , going from F6 to F2 would make you run much leaner.

Data taken from the book, "Weber Carburetors tuning tips and techniques", published by Brooklands Books, written by John Passini