Understanding a Weber Side Draft Carburetor Through a Fictional Supposition.

[Duffman]

Blue, disregard, was able to confirm the level of fuel I needed.  Was told 3 mm below opening in side of tube wall by Pierce Manifold, that seems to make sense and I am still too low based on that description. 

[Chickenman]

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My specs: triple 40 DCOE carbs, 30 mm venturis, auxiliary 4.5, idle jets 50F9, main jet 125, air corrector 175, pump jet 45, pump exhaust valve 050, emulsion tube F11, float needle valve 1.75.  Idles like a champ at 750, hard acceleration runs 15.5 to 16.5 on AFM, flutters and farts on deceleration (super lean), typical transition miss from idle to mains. 

Once you have corrected float level. If you still find you are lean non Mid range and higher RPM's.

 

1: F11's have additional holes on the lower end of the emulsion tubes. This acts as an additional High speed air bleed. ( Fuel level drops as Booster signal ( RPM )  increases. ) Consider switching to an emulsion tube with fewer or no holes towards the bottom. An F16 E-Tube seems to be a good compromise. F9 should be even richer.

Here is a link to an Alfa Forum article that compares F11's to F16's and F9's.

http://www.alfabb.com/bb/forums/carburetors-fuel-injection-air-intake/102770-f16-vs-f9-emulsion-tubes.html#post601838

Here is another article mentioning a correlation of cylinder size to Emulsion tube selection. I would take this as a rough guide but it does illustrate why you could be so lean with the F11 Emulsion tubes. It's those extra bleed holes down low. Source Club Cobra. An L28 is 466.66 CC per cylinder.

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One of my Weber books lists these emulsion tubes versus cylinder capacity:

F11 - 250cc to 325cc
F15 - 275cc to 400cc
F16 - 350cc to 475cc
F2 - 450cc to 575cc
F8 - 550cc to 675cc
F7 - 675cc and above

2: 40 mm Webers with 30mm chokes are pretty danged small for an L28. That is going to create a LOT of booster signal and that strong booster signal will pull the fuel level down lower in the float bowl at high RPM's. F11's emulsion tubes have extra air holes down low, so this leans the mixture out too much.

IMHO, You'd be better off going with 32 - 33mm chokes in the 40 DCOE or a 34mm to 36mm chokes in a 45 DCOE, Choke size should ideally be  around 80 percent of bore volume for a mildly modified street engine or Autocross car. 85 percent for Race engines. Anything above 85% is strictly track use and kills Booster signal. .  40 DCOE's with 30mm chokes are very small for that cylinder displacement. Makes for good bottom end ... but starving engine for Air and Fuel above 6,000 RPM.

3: Main jet and emulsion tube size go hand in hand. Again, IMHO, 125 main jets are too small for your combination of parts. If you keep the F11's, you would have to go up quite a bit in size on the main jets. I'd start at a 135 main and you may have to go up to a 145 to get the WOT mixture correct. Your engine was way too lean on the top end.

Just as an example to show the correct combination of parts.. I ran twin 40 DCOE's on a street driven, but heavily modified, Datsun 1200. It was my daily driver to work. 32mm chokes with 115 mains, 155 airs and F15  tubes . That's 115 main jets on an engine with 300cc per cylinder. But note the correlation of Emulsion tube to cylinder size in the above chart. F15's are the correct size  for a  Datsun 1200. F11's on a  L28 are not. Mains are too small on that combo. The 1200 engine idled smoothly at 1,000 rpm. with a 280 degree camshaft ( Gotta luv IR manifolds ) was very easy to drive and would pull 32 mpg on the freeway. Dynoed at 120 hp at 7,200 rpm. No power below 4,000 rpm to speak of... because it was only 1171cc's... but engine drove like it had Fuel injection. No bucking, no snorting no flat spots. That is the magic of properly tuned Webers. You'd think it was a stock engine until it came on Cam... then all hell broke loose. Below 4,000 rpm it was a puzzy cat. ( I can't believe you can't type p****.... censored )

4: The stumble from idle to mains is not typical. Actually I think you are referring to a stumble from the low speed circuit to mains... not idle to mains. Weber's are a 3 circuit carb with separate idle, low speed and main fuel circuits. I think it's just a matter of getting the tuning right. Again I think it's a combination of wrong emulsion tubes with wrong jets.

My recommendations would be to start off with the basics. Change only ONE thing at a time and log results.

It just so happens that we have a local fellow with a very well tuned stock L28 running triple 40 DCOE's. I'm going to e-mail him and find out his specs if possible. Once you are in the Ball Park, it is all fine tuning.... but I think your combination is just a little off. I have an idea in my head what I would select, but I don't want to make too many suggestions at once.

 

Edited December 16, 2016 by Chickenman
Lost a sentence.

[Chickenman]

Just for giggles. I'm going to post up what I would run a stock L28 with triple 40DCOE's. Then I, going to E-mail Dean ( my buddy ) and see if he has the specs on his very well Tuned ( Specialty Engineering Tuners ) just to see how close I am bear in mind that you can juggle jets and Emulsion tubes and get nearly the same outcome. Juts some will be a little more " Perfect " than otheres.. Not saying this will be perfect... but just what I would choose as a starting point .

 

32 or 33 mm chokes. F16 or F9 emulsion tubes, 145 main jets, 175 to 185 air jets, 50F9 pump jets.  Final tuning on a Dyno to get a smooth AFR curve. Ignition timing. Important; Independent runner manifolds like a LOT of low end timing. Static timing of 15 degrees BTDC. Total mechanical timing of 32 to 35 degrees all in by 3,500RPM ( Depends on fuel quality and cylinder head ) . If you can, run a Vacuum advance but use an adjustable vacuum pot. And additional 10 degrees of Vacuum advance hooked up to MANIFOLD vacuum. really perks up an Ir engine. Especially if it has a larger Cam.  Just my .02c...

Edited December 16, 2016 by Chickenman

[Duffman]

Chickenman, your time and thoughts are greatly appreciated!  I realize that tuning triples is like putting together a massive puzzle, but with patience and input from people in the know, like yourself and Blue, it helps move towards an appropriate solution!

My stock L28 engine was rebuilt about 8-10 years ago (10,000 mi on it), still shows 190 compressions across all 6 cylinders, have ZX distributor for electronic ignition, recurved distributor (needs no vacuum advance) and 6 to 1 coated headers,  So, nothing too souped up, just a good running, stock Z!

Appreciate your thoughts on specs, will look at incremental changes and see if the results steer me towards your suggestions!

Blue, I apologize for taking over this wonderful post, don't want to take anything away from it!

[siteunseen]

I wanted to be able to find this again so I've added a post. 

Great information to know.  Thanks.

[240260280z]

Hi Duffman,

I'm not sure I understand the 16mm fuel level?  On 40DCOE the passage to the main circuit is 23mm down,  You will want to be ~6mm lower than this at 29mm down from the edge. At 16mm down you will be flooding the main circuit.

For Emulsion tubes, Chicken man knows his stuff but at this point I would not worry about changing them.  They basically are only in operation at WOT as you run up the rpms from 3000 to 7000 (unless your fuel is too high or main jet is too large).  They do have a bit of an impact at high load cruising but you can tune this with the main jet a little.  

 

One other thing I forgot to address are the "other" aspects that must be correct before tuning triples:

 

1. Valve Lash: Engine should be tuned nicely with correct lash set.

2. Intake Manifold air leaks: Absolutely none.  This will throw off any tuning.

3. Timing: DCOE's and L engines like lots of advance.  15 to 20 degrees at idle with full at 36 to 38 are typical ranges.  No vacuum advance is needed.

4. Linkage: Remove all slop and ensure all three carbs open at the same time and the same amount.

 

btw do you have a device for measuring air flow into each throat?  You will need this to tune once you have the fuel level sorted.

 

For your 40DCOE 151's you will find the Idle Enrichment (IE) screws will need to be ~ 1.75 to 2.75 turns out from full stop. These carbs have a narrow tapered needle and are more precise than early Webers that have a wider taper and seem to work best from 1/2 to 1 turn out.

 

Here is some data I collected when tuning and changing the idle jet number of turns on a set up much like yours:

 

Edited December 16, 2016 by Blue

[240260280z]

Opps.  I was off the mark.  Chickenman pointed to it being an L28.  I saw low vin 240z in your signature and assumed L24.

30mm chokes will give you lots of lower end torque (which most of us like) however it sacrifices WOT runs where more airflow trumps.

I recommend tuning the 30's now and go bigger if not satisfied,  Here is a great thread on a similar quest with an L28.  Torque won in the end.

http://datsunzgarage.com/weber/

Edited December 16, 2016 by Blue

[Duffman]

Thanks, Blue, again great stuff!  I agree, will try to best tune what I have and then change the chokes and emulsion tubes if necessary.  I am comfortable that I have followed your list of "other" aspects that need to be in place to best tune my triples,

Sorry, I was struggling with the 29 and 29.5 mm down the emulsion tube opening, my fuel level is 16 mm down from the top of the emulsion tube opening threads.  So, I called Stephen at Pierce Manifolds (he is very helpful) and asked him where the fuel level should be in the emulsion tube or, more importantly, what I should visually see down the tube.  He told me that the fuel should be 3 mm below the lower edge of the fuel opening on the side wall.  Based upon what I was seeing and after remeasuring the float level, this made sense.  So, I was more than 3 mm and I adjusted the float level accordingly,  This made a difference on my trial runs, providing smoother transitions from low to high end.

I love that DatsunZGarage article, have it saved.  Keep in mind, he has a cam upgrade, whereas mine does not.

Stephen

[240260280z]

3mm below the side wall is too high.  Bumps and turns will cause spill into the main passage.  29mm down is the place to be.

In practice, you can tune at any height given the adjustments available for every circuit however you will make some compromises in most cases.

My experiments with the fuel ridiculously high at 25mm down (as championed by an internet sidedraft "expert") made me waste a lot of time.  After I finally tried everything and  then questioned the 25mm fuel height, I discovered 29mm was a good place to be.  It was verified by expert Japanese tuners who made this fuel level tool that Grannyknot gave me:

 

 

 

 

 

And another reference to 29mm:
http://www.carry-back.com/CARRY BACK Carbu setting 3.htm

 

 

 

 

 

 

 

 

 

 

 

 

Edited December 16, 2016 by Blue

[Duffman]

I guess i am stumped then, Blue.  I measured 14 mm down from the lid with gasket to the top of the float and all carbs have the same fuel location at 16 mm down.  If I try to get to 29 mm down to the center of the fuel level, I am not sure I can bend the float tang enough to make that happen.

Love the measuring tool, by the way, would be great to have!

[240260280]

lol. look again at the tools. They measure to the top of the fuel surface  in the bowl.  The  lid of the carb and float are not involved in the measurement,

 

Here is another way:

 

[Duffman]

240260280, I am not measuring the fuel level from the lid, per your inference.  I am measuring the float level, per instructions, 14 mm down from the lid and gasket.  This is supposed to equate to an appropriate fuel level.

The key, I believe, is Blue is stating the fuel should be 6 mm below the aux venturi opening in the side wall of the emulsion tube, or 6.5 mm to the center of the fuel level.  I am measuring the fuel level from the top incorrectly. so I will focus on the distance below the aux venturi opening.

Edited December 17, 2016 by duffman