Tuning 40 DCOE Weber 151 on an L24

[milgeo]

goodmorning just waking up for work

well they are 45s dcoe i dont know the variation have to check venturi is 32 the carbs idle just fine they work fine up top in between they spit and hessitate if i pull the air they are fine no spitting but they wont idle does that mean that can be jetted out by changing the idle jet to richer?

[conedodger]

Stephan, just back from the first Triple Trouble Carb Clinic. Big news for you, Keith will work with you until you get it right. You can just pay for shipping stuff back to exchange the Hypojets you don't need. Second bit of big news is he will be looking into making Hypojets for Mikunis.

I can't believe the difference in my carbs! Not much difference in the top end, but wow do I have a crisp and quick responding bottom end and midrange! Highly recommend Hypojets!

[Zedyone_kenobi]

I was looking into those, but the prices get big quick.

Rob, can you tell us what you were running prior to the hypojets?

[240260280z]

Also, listing which hypojets and hypo etubes will be an initial starting point for:

Stock 240z 40DCOE 28mm choke

Stock 240z 40DCOE 30mm choke

Stock 240z 40DCOE 32mm choke

Stock 280z 40DCOE 30mm choke

Stock 280z 40DCOE 32mm choke

Stock 280z 40DCOE 34mm choke

Stock 280z 45DCOE 34mm choke

Stock 280z 45DCOE 36mm choke

Stock 280z 45DCOE 38mm choke

3.1 litre Stroker

Would be nice to see!

[LeonV]

My setup:

• L24/E88 with stock cam
  
• 40DCOE18 Webers with 30mm chokes
  
• Nissan Motorsport header with dual 2" pipes all the way back
  

Started with:

• 50F9 idle jets (too rich)
  
• F11 e-tube (junk)
  
• 170 air corrector
  
• 125 mains (bit rich)
  

*** I need to track down accel pump specs

Performance resulted in really soggy idle and cruise mixture and some bogging on aggressive tip in at lower rpm. WOT above 3500rpm was good, no matter the gear or how aggressive I was with the throttle.

After tuning day:

• H223 hypojets (.022" orifice, 3rd air hole)
  
• O3 e-tube
  
• Custom air corrector for O3 tube
  
• 125 mains
  

Idle is way better, less soggy but no flat spot. Cruise mixture is better but still rich. I plan on going to a smaller hole on the hypojets, which would enrich the idle but lean out cruise. I think the H222 or H221 setting will get me close but we'll wait and see.

At WOT, my mixture still tends to go richer as rpms climb. I'll talk to Keith and see what he thinks. I'm thinking this may require a different e-tube, perhaps an O2, as it has less holes to lift the fuel up. However, low and midrange is fantastic. The mixture is spot on with much less jumping around as shown by my wideband.

I still have an initial hesitation issue when quickly going WOT. It'll hesitate and then pick up, so this points to an accel pump issue. I need to investigate this further, as I don't know how much bleed-back I have and don't remember the size of my pump jet (40 or 45 I think).

One other thing I discovered a few days ago is that my internal return springs need replacement. I was doing idle-sync and decided to see how well each carb returns. After disconnecting the linkage, I blipped each carb independently to see if it had the same airflow before and after. Each carb was flowing more after I blipped the throttle, indicating that it is not fully returning. Pierce Manifolds has the right ones for a good price.

[conedodger]

I was looking into those, but the prices get big quick.

Rob, can you tell us what you were running prior to the hypojets?

Stephan,

I already did. Go back a few pages and you will see my initial post in this thread. I called Peirce Manifold and did some changes and those are listed too.

[madkaw]

Okay you Weber guys, besides the specifics slated for your carbs, was there some general wisdoms discussed for triple carb operators??

Would have been there but couldn't make the 16 hour drive!

Things of interest to me would be Venturi sizing in relation to engine size, intake length?

Edited October 15, 2012 by madkaw

[LeonV]

Okay you Weber guys, besides the specifics slated for your carbs, was there some general wisdoms discussed for triple carb operators??

Would have been there but couldn't make the 16 hour drive!

Things of interest to me would be Venturi sizing in relation to engine size, intake length?

Oh man, this discussion could go on for a loooong time!

A good start would probably be buying Obert's book.

But to sum it up, of course there is the precautionary warning of making sure all other systems on the car are functional, ignition, charging, etc. A dysfunctional engine will make it impossible to tune and will frustrate you to no end.

Once you're ready to tackle the carbs, the fuel levels must be set accordingly. Keith has found that to be 2mm below the brim of the passageway to the aux venturi. Then make sure your carbs are in order: no leaks, barrels flow equally, throttle returns correctly, linkage set up properly. Once that is set, you're ready to "tune". Ensure throttles are positioned correctly to the progression holes, sync at idle and high rpm, play with the jetting...

Keith is working on a paper to detail all of this, hopefully he'll have it up soon. We had some very interesting discussions on the physics of what is really going on inside those carbs, and I'm sure he will be covering that in the paper as well.

As far as venturi-to-engine size question, that is a good one and not simple to answer. Venturi size would be best determined empirically. The goal would be to go as big as you can without sacrificing driveability, and this depends on parts availability and the skill (moreso patience) of the tuner.

Intake length sizing is its own discussion, and I've made some fairly detailed posts on it on HybridZ. In a nutshell:

1. Use air horns, or at least have some sort of radiused opening at the inlet of the carb
   
2. Longer intake valve-to-carb inlet length is more efficient at lower rpm
   
3. Shorter intake valve-to-carb inlet length is more efficient at higher rpm
   
4. Have as few bends as possible and try to make it a straight shot into the cylinder
   

I've designed a spreadsheet that is supposed to predict intake and exhaust lengths according to RPM but I've yet to test it.

I'm sure Obert's book has the details on it. Internal Combustion Engine Fundamentals by Heywood has some very good sections on intake and exhaust design. I've recently bought Obert's book, and in some respects it's outdated (which makes it cheap ), but most if not all of the information will be very relevant to the L-series engine.

[conedodger]

Here is one of the things I took from the discussion;

Most problems in cars that run ok, are covered up by running the carbs rich. When you back off pig-rich, you may uncover some other problems you need to take care of.

When you drive with these carbs on the street, you are likely running on your idle jets. Even possibly the highway.

Leon got the tuning sequence according to Franck down in his post above. I think Leon's car came out better than mine yesterday because he has a rock-solid ignition system and mine is 40 years old but in good repair. His O2 readings were rock solid. I also think Leon wants to get his so close to perfect that he will be the guy to watch with his next moves... 8)

[LeonV]

Here is one of the things I took from the discussion;

Most problems in cars that run ok, are covered up by running the carbs rich. When you back off pig-rich, you may uncover some other problems you need to take care of.

When you drive with these carbs on the street, you are likely running on your idle jets. Even possibly the highway.

Leon got the tuning sequence according to Franck down in his post above. I think Leon's car came out better than mine yesterday because he has a rock-solid ignition system and mine is 40 years old but in good repair. His O2 readings were rock solid. I also think Leon wants to get his so close to perfect that he will be the guy to watch with his next moves... 8)

I definitely agree with the rich-running sentiments. Getting a carb to run an engine is one thing, getting it optimized is the challenge. You're definitely on the idles at low-speed cruise and I remember Keith saying that you're about 50/50 on the idles and mains at high-speed cruise, say 75mph.

Mine came out running a lot better than it was but there is definitely work to be done! I'll be getting some 110 mains and a set of internal throttle return springs (mine are dead) soon. Keith's Elan (400cc cylinders, 30mm chokes, same as my Z) runs the O3 tubes and only needs 110 mains.

Besides Rob, my Z hasn't seen a track yet. My goal is for someone else to be able to drive the car and not tell it's carburetted. Your Z is almost exclusively a track car, so as long as tip-in and WOT are set, you're ready to roll! Of course getting better mpg on the way to the track is always a good thing as well. I need to get my Z out there with you guys.

[Zedyone_kenobi]

Very interesting thread for sure! I am very tempted to try the hypojets, but the cost is so scary. Seems Keith still uses the weber idle jet for the most part. I have had great luck with the 55F9 idle jet. But my mileage is not stellar. But here is the kicker. I pulled my plugs after a long drive on the free way and I am running a beautiful shade of light brown. I mean spot on perfect. It would seem his fancy etube is something to strongly consider. Just the price of entry is daunting especially since I probably have 300 dollars in jets as is.

[LeonV]

Very interesting thread for sure! I am very tempted to try the hypojets, but the cost is so scary. Seems Keith still uses the weber idle jet for the most part. I have had great luck with the 55F9 idle jet. But my mileage is not stellar. But here is the kicker. I pulled my plugs after a long drive on the free way and I am running a beautiful shade of light brown. I mean spot on perfect. It would seem his fancy etube is something to strongly consider. Just the price of entry is daunting especially since I probably have 300 dollars in jets as is.

I'll report back with my findings once I get things dialed in. You can see first hand whether it's worth it or not!