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Zedyone_kenobi

Tuning 40 DCOE Weber 151 on an L24

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Installed my 40 DCOE webers on the L24:

Engine specifications

2.4 liters original motor/head/cam.

Pertronix ignition

Pertronix 1.5 Ohm coil

MSA 6-2-1 headers

adjusted idle mixture screws 1/2 turn out

Adjusted all idle speed screws (without actuator rods hooked up) until they were about to uncover the first progression hole

I had at my disposal, an wideband O2 gage installed into my header (innovative LM1)

41X7BLLwbaL._SL500_AA300_.jpg

and a Air Sychronizer tool with weber air horn adapter

43.5712.0.JPG (note, air horn adapter not shown)

My factory out of the box settings for my 40 DCOE 151's were:

30mm venturi

130 main Fuel Jet

170 Air corrector

F11 Emulsion tube

55F9 idle/slow running mixture jet

Car would not start at all. It would run off of the gas put out by the accelerator pump, but would not stay running. I kept increasing the air fuel mixture (note NOT touching the idle speed screw at all) until the car would start and stay running. This ended up at 2.0 turns out from fully closed.

At this point, you MUST let the car warm up to operating temp. After the car warmed up, I was getting an AFR of 10.1 on the LM1. Too Rich. Mr. Franck's of side draft central's white paper points out we should always aspire to 12.5 with our DCOE's. Well I managed to get it to be there running about 1.5 turns out from full close don the air fuel mixture. Car ran smoother like this as well.

Initial drive was very promising. However, the dreaded flat spot did exist in the transition point between my third progression hole and when the main circuit came in.

I dove back into the side draft central white paper, and a way of getting the main circuit to come in faster is to reduce the size of the air corrector. You see the air corrector is like a vacuum bleed off. In order for the main circuit to work, you have to get the column of liquid up the emulsion tube and down to the venturi. This is done by way of sensed vacuum from the engine. See 3rd picture on the left.

dcoe_flow1.jpg

The larger the air corrector the more engine vacuum is bled away and it takes more vacuum to pull the fluid up the emulsion tube chamber. Capillary action only gets you so far, and often the stumbling effect is the engine vacuum is too low to effectively make the transition from progression circuit to main. This should happen at about 1400 rpm according to Mr. Franck.

Upon driving around I noticed that my part throttle, 4th gear, low rpm (2000 rpm) steady state cruise I was reading about 15.9:1. This is too lean, BUT I knew that running 130 main jets which are really a bit large for 400 cc of displacement I did not need to richen up the main jet. So instead I put in 160 Air Correctors,down from 170. I touched nothing else, and went for a drive. My flat spot on transition was greatly reduced and even eliminated in 1st gear. But was still present on all other gears. However the lean stumble was smaller and the main recovered quickly.

I bet your wondering, Hey Zedyone, what about your floats? Well I am not going to change anything else until I check those.

Let me make one thing clear, you should always check your floats BEFORE you do any tuning. The float level can drastically effect how and when your main circuit comes in, as a low float level will increase the distance the fuel has to travel up the emulsion tubes.

I purchased the Keith Franck float measurement tool off side draft central webshop. I will be trying to see where I am this week. It would be foolish to spend any more money until I check the float levels.

Also, not mentioned here, but I did check the air flow through the carbs for balance with my sync tool pictured above. Right off the bat with the engine warm, they were within 0.5 of each other on the tool. So I left them alone. If I wanted to change that, the only way to make fine tuning adjustments between the carbs is to adjust each idle speed screw located on the side of each carb. I did check balance on both barrels of each weber individually and the flow was identical on all three carbs. Weber does include a way to balance each barrel of one carb individually, but I figure that adjustment is probably to make up for some external imbalance, be it rings, seals, etc. If the butterflies on one carb are open the same, then the air flow should be the same. If they are different, there is an external reason. New webers the air flow should be identical.

When I check my floats, I will post pics and report back.

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The carburetors that keep on giving. I love them too. I don't use an AFR gauge, although I should. Here is what I discovered, the L loves to idle rich, and the popping in the exhaust during deceleration, if you have it, goes away completely, if you throw in a half turn more idle mixture, after you have found ideal idle. Counterintuitive...

Yes, fuel level is CRITICAL to the timing of the main circuits.

Edited by cygnusx1

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7137185439_c1bc3a1e4d_b.jpg

Behold the Keith Franck Weber float measuring device! This goes into where the emulsion tubes go. All you do to check the real float level, is to remove the round jet cover, and then unscrew one of your main jet/emulsion tubes. You run the car and in bright light, you can slowly stick the end down into the hole. IF you look carefully, you can see the fuel come into contact with the tube as you stare at the end of the tube. Keith says to do this is bright light and he was right.

I did this very quickly, and did not measure the distances, but my first carb (the one nearest the front of the car) showed the line on the tube several mm below the white sleeve meaning the fuel level was LOW. My second carb indicated contact with the fuel before the line ever came into view, indicating a HIGH fuel level. The third carb again indicated low, meaning the indicator line on the acrylic tube was below the white sleeve.

This goes to show you that the factory settings are merely 'in the ball park' of right. Not at all the ideal 25mm from the top surface.

Do not fret the double lines on the tube. The are just 25mm from either end, so it does matter which end you use.

I like this method, and I will adjust my floats when I have a couple of hours to dedicate to the job.

Great products here by Keith Franck.

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I've got the same tool, but haven't put it to use yet. It's very clever!

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Zedyone_kenobi

I think you are having waaay to much fun with this!! I'd also guess that by now you have spent at least as much on your Z as you originally paid for it. Another sign of serious addiction... LOL

Next step - - individual throttle bodies with F.I.... Yes, the path you have chosen always leads to a next step... LOL When you run out of things to do to your Z - - - - you'll be looking for a 911 next...

At any rate -keep up the good reports.

FWIW,

Carl B.

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I can attest to that next step! Unfortunately, I missed out on a set of HKS ITBs but I'm on the prowl!

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I think my addiction comes from my inherent need to improve things. I have not added all my receipts and I dare not attempt it. But yes I have spent a bit but not as much as some (denial is what stage of addiction, I forget)

I think my car and I are about average in the Z world.

I am having a blast. A z is such a beautifully simple thing. I love working on it.

I will try to get accurate measurements of my float levels tomorrow and report back.

Anybody know how many times you can reuse float bowl gaskets?

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Today I redid my cable linkage and set the floats to dead on 25 mm using my gage. Since I raised all of them (number 2 was actually low as well) I would imagine that my mains should come in sooner.

I will also predict that I will probably be far richer at cruise as well.

I will let you know how it is running Monday

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Initial test drive indicates my transition stumble has taken another progressive step to non existence. I drove it without even taking the AFR gage out of the glove box to see what I was running and I was very pleasantly surprised by the smoothness of the throttle. AFR results to come on Monday.

But setting the float level definitely helps in getting the mains to come on sooner. Very happy I did this. Everything you read about tuning webers says Do the float levels first. Now I see why, the difference from factory set to dead on is substantial. The transition from progression circuits to mains is almost imperceptible. I think if I were to go down on my air corrector again, and then see what my AFR are, I will be in business, but for now, I will leave all alone until I get it on the highway to see what my 4th gear cruise AFR is.

Edited by Zedyone_kenobi

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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

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

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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!

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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.

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

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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 by Blue

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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

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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

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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 by LeonV

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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

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Well my experience with triple weber 40DCOE151's so far is that I can make them run rich or lean by playing the mixture screw against the throttle plate opening.

I read the blurb "DCOE INITIAL SETUP & TUNING" in the DCOE chapter in "WEBER CARBURETOR REPAIR AND TUNING BOOK" by Pat Braden. He suggests:

1. Once engine is warmed up and idling, set all mixture screws at same position out then synchro-balance airflow into all carbs (using throttle valve opening screws and air bypass screws). All 6 should flow the same.

2. Install air filters and connect tach

3. Reduce RPM to ~ 900 with the three throttle valve opening screws (for triples).

4. Turn out all mixture screws to find the point of fastest/smoothest idle. (If no peak RPM is found, repeat step 3 but use 850 or 800 rpm if possible).

5. Once the maximum RPM point is reached reduce RPM to ~ 900 (or whatever idle RPM you desire) with the three throttle valve opening screws.

I sort of did this when I first set up the carbs but I was pig rich. Now that I have read a few articles, I will try again... when I get the car back from the "panel tweaker".

I will also plot idle screw, throttle plate variations, A/F with 1100 rpm constant "for fun"... maybe a graph will talk to me. Especially if I also substitute throttle plate turns in with first pilot hole exposure.

Edited by Blue

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