Baseline Jetting - Triples on a 280Z?

[cygnusx1]

Does anyone have a baseline recommendation for triple 40DCOE's on a stock 280Z (L28)? The 240Z is well documented. Not the 280Z.

Feel free to throw some suggestions or combinations that work, or just keep an eye on my notes as I set it up by ear. I have performed all the basics: throttle plate angles, idle speed, best lean idle +1/4 turn rich, synch, float levels....

Here are my notes so far:

240Z Running with stock L28E and Triple Weber DCOE40-151 carburetors.

Date-4/10/10

125/F11/170

55F9

28mm choke

4.5mm aux choke

Comments: Starts, Idles well when warm, runs smoothly, low on power above 3000rpms but runs great, stalls on quick application of heavy throttle below 2700rpm.

Suspected: Chokes too small, running a tad lean, accel circuit to be tuned later.

Date-4/24/10

Drilled Mains, Installed Chokes

150/F11/170

55F9

32mm Choke

4.5mm aux choke

Comments: Starts, Idles well when warm, will not take any heavy throttle below 3000, drives ok with light throttles below 3000rpm, will take full throttle at 3000+, decent power up top.

Suspected: Mains coming on too late, measure fuel levels in bowl, open air corrector to 200.

Edited April 25, 2010 by cygnusx1

[inZane240]

There's some baselines towards the beginning of this thread that might prove useful:

http://www.classiczcars.com/forums/showthread.php?t=37116

[cygnusx1]

Thanks, I am getting closer. Soldering and drilling to save money takes time. The rainy weather delays my test drives. By this weekend, I should have it fairly well dialed in.

[cygnusx1]

Here is an updated version of my tuning log:

A (original setup)

Date-4/10/10

125/F11/170

55F9

28mm choke

4.5mm aux choke

55 accel bleed

Comments: Starts, Idles well when warm, runs smoothly, low on power above 3000rpms but runs great, stalls on quick application of heavy throttle below 2700rpm.

Suspected: Chokes too small, running a tad lean, accel circuit to bee tuned later.

B

Date-4/24/10

Drilled Mains, Installed Chokes

150/F11/170

55F9

32mm Choke

4.5mm aux choke

55 accel bleed

Comments: Starts, Idles well when warm, will not take any heavy throttle below 3000, drives ok with light throttles below 3000rpm, will take full throttle at 3000+, decent power up top.

Suspected: Mains coming on too late, measure fuel levels in bowl, open air corrector to 200.

C

Date-4/25/10

Raised fuel level to about 26mm from top of bodies, soldered accel pump bleed holes.

150/F11/170

55F9

32mm

4.5mm

0 accel Bleed.

Comments: Idles well, good throttle response in neutral, much less lag when rolling on throttle from 2000-3000, very strong from 3000+, a little black smoke around 4500 wot,

feels nice overall but still has some lag and bumpiness around 2000-2700 with more than half throttle. reponse to snap open throttle iffy below 3000.

D

Date-4/25/10

Checked fuel level again > Need 2mm more in #1 and #2, 1mm more in #3. Drilled Air Correctors to 195. Confirmed position of throttles to progression ports perfect.

150/F11/195

55F9

32mm

4.5mm

0 accel Bleed.

Comments: Idles well, good throttle response in neutral, much less lag when medium rolling on throttle from 2000-3000, very strong from 3000+, power seems to flatten out past 4000, no black smoke.

feels nice overall but still has some lag and bumpiness around 2500 with more than half throttle. Snap throttle 2000-2500 results in almost complete stall.

E

Date-4/25/10

Turned idle mix screws out 1/4 turn each to fatten idle to maybe help progression.

150/F11/195

55F9

32mm

4.5mm

0 accel Bleed.

Comments: No test drive yet.

Edited April 26, 2010 by cygnusx1

[cygnusx1]

E

Date-4/25/10

Turned idle mix screws out 1/4 turn each to fatten idle to maybe help progression.

150/F11/195

55F9

32mm

4.5mm

0 accel Bleed.

Comments: No test drive yet.

F

Discovered plastic BB's on top of accel pump weights and removed them. Found faulty accel pump check valve and repaired. Redrilled .055 bleed holes into acell bleeds.

Raised fuel in all bowls to nearly 25mm exact spec, measured down from carb body top. The floats are at least 7mm higher than spec to achieve proper bowl height!

Date-4/27/10

150/F11/195

55F9

32mm

4.5mm

55 accel bleed

Comments: Responds great to heel and toe, idles great, revs nice in neutral, pulls hard at all RPM's maybe falling off slightly around 5000 or so. Best setup yet. Going to try leaner mains and

smaller air corr to try for more upper end power. In 4th gear at 2000rpms you can roll on slow throttle, to full throttle, and get zero bog with smooth pulling. If you move the pedal too fast, you get bog.

This tells me that the accell circuit is loading up the motor with too much fuel. I will drill larger bleeds in the accell circuit next time.

[cygnusx1]

The car is running great right now. Here are the rules/knowledge that I used to set up the Webers by ear, so to speak.

My Weber Tuning Digest

I assume you have the correct carburetor size for your engine, and a fully functioning, non binding, linkage.

Tuning a DCOE is as follows:

KNOW THE BEAST

Write down all the specs of your carbs.

Model

Choke size

Aux venturi

Idle Jets

Main Jet

Emulsion Tube

Air Corrector

Pump Jet

Pump bleed jet

FUEL PRESSURE - 3psi < I filled this one in for you. Make it that.

Needle valves

THROTTLE PLATE POSITION

Mechanically establish the throttle plates, all identically, just downstream of the progression ports. You can do this by looking through the progression holes with a light. (future angle adjustments should be limited to +/- 1/4 turn on the stop screws from here on and in the future for idle tuning)

WATER IN THE POOL

Float levels (CRITICAL) measure the fuel level in the bowls. Take out a main jet stack and measure the fuel level. Should be 25mm from the top of the main jet hole. This affects transition greatly! higher levels = earlier mains.

MIXTURE SCREWS?

Turn the idle mixture volume screws out to the spec for your carb model. These screws don't mix fuel and air. They are controlling how much premix of air fuel is going to the engine at idle.

BEST LEAN IDLE

Establish idle speed by iterating adjustments in the following; Mixture screws/+- 1/4 turn on the throttle stop screw/ignition advance-retard. Repeat over and over to get best lean idle (I do it by ear turn each mix screw in SLOWLY until the motor roughens and come back out SMOOTHLY until it smooths out again). Then turn all the mix screws out(rich) 1/4 turn if you get popping on decel later. (helps reduce lean pop during decel) Stay within the specified TURNS for your carb. If you need to open them up too much, you need a larger idle jet. This is critical to understand because the idle jet is also feeding the non-adjustable PROGRESSION PORTS. If the idle jets are too small or too large, you will get bad progression off idle. REMEMBER ONLY!!> +- 1/4 turn on the throttle stop screw. If you can't get it in that range, you have other issues like vacuum leaks and/or too much timing. If your timing is too retarded the motor want to idle slower, forcing you to open the throttle stops. This exposes the progression ports and renders them useless for progression. Fell free to use more ignition timing to keep your progression ports COVERED!

TEST DRIVING

Establish proper main circuit to get good AFR's and drive-ability from 2500rpm and up. This is where a wideband is priceless. Experiment with slow throttle movements and focus on steady state and power smoothness above 2700 rpms.

Choke Size x 4 = main jet

main jet + 50 = air corrector

performance engines can have mains and air correctors closer in size.

Generally, use smaller auxilliary venturi with larger chokes.

Main Jet affect mix from about 2500 and up depends on choke size, and air correctors, fuel levels.

Larger Air corrector leans out the mix towards higher revs and can bring the main circuits in sooner.

Emulsion tubes are generally "fuel" curve tweakers. Use them later after everything is working, IF you have too.

MECHANICAL FUEL INJECTION

Accelerator Pump tuning.... I really don't have much advice here except that you can adjust the DURATION of the fuel squirt, and the VOLUME of the fuel squirt independently. These are the variables: pump stroke, jet size, and bleed size. The little jet at the bottom of the bowls is a check valve that feeds the accell circuit called the Accelerator Pump Discharge Valve. It also has a bleed back hole. You can change the size of the bleed back hole, and the pump jets. Generally leave the pump stroke alone. SMALLER bleed back hole and LARGER pump jet will make more fuel go to the engine. Add the two hole sizes together to find your DURATION index. (I invented this. It could be trash theory If the sum of the two is large, duration SHORTENS, if the sum is smaller, duration LENGTHENS. The bleed hole size mainly determines the sensitivity to throttle position changes. Example: A large bleed back will squirt very little fuel to the accel jet, during slow throttle movements. When moving the throttle slowly, the fuel will simply bleed back into the bowl instead of going to the engine.

ANALOGY: Fill a cup with water, drill two holes in the bottom sides. One hole feed the engine and the other hole drips back into the sink. Those are your variable. Size of the cup, and sizes of the holes.

OK I am sure I missed stuff and there is more to it than that but it's my mental digest. I didn't talk about synchronizing because that is not really a carburetor tuning thing. It's more of "put on clean underwear thing."