Tuning 40 DCOE Weber 151 on an L24

[Zedyone_kenobi]

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)

and a Air Sychronizer tool with weber air horn adapter

(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.

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.

[d240zx2]

:-) :-)

[cygnusx1]

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 April 30, 2012 by cygnusx1

[Zedyone_kenobi]

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.

[LeonV]

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

[Carl Beck]

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...

Next step - - individual throttle bodies with F.I.... Yes, the path you have chosen always leads to a next step... 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.

[LeonV]

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

[Zedyone_kenobi]

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?

[d240zx2]

At least once.

[Jason240z]

until they start leaking.

[Zedyone_kenobi]

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

[Zedyone_kenobi]

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 May 6, 2012 by Zedyone_kenobi