Weber selection and initial jet tuning

[blodi]

I am going to be ordering a triple weber setup for my L28. 

First, my engine basic stats:

L28 F54/Flattops, .040 bore,  E31 Head which is ported/etc, 500/300 cam, 6-1 header, pertronix igntition, etc.  My whole engine install thread here:

 

I've been running the round top SU's with SM needles.  They go lean up top despite the needles and repeatedly checking the floats/fuel delivery/etc. 

Looking to go triples. The good news is my engine was built as a backup for a race car and it came in the crate with jet sizing written on the valve cover. So I have a good starting point. Just hoping you guys can help fill in the blanks. I am learning here. 

image2 (1) by blodi, on Flickr

So what else is missing from those settings?  Based on those and what I've learned in some helpful searching it seems I need to go for the 45's and use a 38mm choke? 

Do the rest of those settings make sense? 

Thanks!

 

[Zedyone_kenobi]

I have a mildly built Datsun spirit L28 and I run DCOE 40's with 32mm chokes.  My jetting is based on a 40mm carb, so I cannot comment on how the 45's would take to my setup.

Your settings are the critical ones for getting the car running for sure.  Idle jet, air corrector, main jet and Emulsion tube. Those the largest tuning knobs outside of just picking a carb (40 or 45 DCOE) and picking a choke size (30, 32, 34, 36, 38, etc)

However, lets us take a step back.  You own a street car I assume and this is a back up for a race engine, which means those settings may be great for a race engine but not for a street engine.  The difference is actually quite important.  A street engine spends a lot of time at idle and even more time in the midrange were you want a good amount of torque and response.  Off idle response and the transistion from idle jets to the mains is a vitally important for a street car.   Now for a race, car what matters is peak HP/torque and quite a lot of time is spent Wide Open Throttle (WOT). Idle quality, efficiency, smooth transition are not nearly as important. A weber is a wonderful thing. It can be tuned to hit many critical areas. But it usually has to compromise somewhere.  Be mindful of the difference between a street engine and a race engine.  A race engine will not be happy living in daily traffic.  I would think that those settings will make lots of power, but may not produce a happy street car. 

 

 

[blodi]

Thanks for the reply! Fast forwarding here..I have installed a set of 45's and installed the jets as seen above. I just got the car up and running this last weekend and have driven it a couple times.

Chokes are 36's

Idle is great...900rpm...mixture screw is 2 3/4 turns out which seems good for the later 152 45's.  I synced the carbs as well. 

The car right off idle and low throttle driving around is great...great response off the line. 

As you roll into it or if you stomp on it a lower RPMs...there is a big stumble or even complete lack of power. So I assume this is the "transition" phase and there are like 4 things impact that. 

High RPMs...holy crap! It's a completely different beast now than it was with the SU's...1st is all wheelspin and so is most of 2nd.  

So where do I start with tackling the transition flat spot? 

I have a tailpipe O2 sensor I can plug in and see if it's going lean or rich where the stumble is. 

The engine seems good with running in traffic and such. I put 2K miles on it last summer with the SU's.  The idle is certainly a bit lumpy, but sounds good and holds...doesn't overheat. I kept the stock flywheel on it so I think that helps it out at idle and such. 

 

 

[siteunseen]

I think you've seen this but I'll put it up anyway, maybe it will be helpful somehow.

 

[blodi]

I hadn't seen that thread yet, thanks!  The choke X 4 theory means I need to go back to the 145 mains that came with my carbs and can keep my 185 air corrector most likely.  For all I know the settings he had above  were for 38 chokes since it was for a race car. Gives me a good spot to start!

[blodi]

So my "flat spot" is a lean spot in the transition as I found with my wideband.  I double checked the float heights, also tried putting in the 170 air correctors I had on hand to try to richen it up a bit.

So...where do I start?  I hate to just buy a bunch of jets randomly.  Richer Idle jets? Richer Mains? Accelerator pump jet? The accelerator discharge valve I have is the one with no hole in it....not sure if that helps or not. 

The flat stop isn't there when on the highway and the revs are up...probably because it's already running on the main circuit by that time. At lower speeds if I stomp on it or roll into it, the engine just loses all power until I let back off the throttle.  

 

Edited April 24, 2017 by blodi

[Chickenman]

Have the Carbs been rebuilt after you bought them?

Did you check for Accelerator pump operation with the carbs off the car. I put some solvent in the bowls and then give them a quick test off the car... in a safe area of course and with a Fire Extinguisher handy.

Often the accelerator pump passage gets gummed up with Varnish or crystals. 

Excellent video on how Weber pump circuits work and shows all the components. Component check is good, as sometimes Race Carbs have some strange Modifications made to them. 

https://www.youtube.com/watch?v=VLct6_XWOQE

To fix a flat spot in transition, that is due to a calibration error ( Not plugged passages etc ) you want to lean towards a larger pump jet and a stiffer spring. This gives a sharp hard shot of fuel needed for street use. The spring tension plays a bigger part than most people realise. What number are your accelerator pump discharge jets and are they clean? That tiny aluminium washer on the bottom is important. 

[Chickenman]

Independant runner carbs also require a Unique distributor advance curve. They need a lot of static advance at idle. 15 to 20 degrees depending on Camshaft is normal. Total mechanical advance is the same as the SU's, so you usually have to Weld-up the advance slots or limit total mechanical advance somehow. 

 

Vacuum advance really improves cruise milage, but it's hard to run a Vacuum advance with Triples on the street due to the strong Independant runner pulsations. Some manifolds have a balance tube built in... much like the balance tube on the SU's. The purpose is the same. to act as an accumulator and dampen the Intake pulses ( or " Combine " them ) to get a usable Vacuum signal for the Vacuum advance. These manifolds are rare, because most of the Triple or Twin ( 4 cylinder ) IR runner manifolds were meant for racing.

However a modern solution to that problem is the 123 Ignition with programmable Ignition curves. that can be used to emulate a Vacuum advance at cruise RPM with IR manifolds IR manifold pulsations are wicked at idle, but they smooth out at cruise RPM So you can get a stable MAP reading. That enables you to run extra advance ( Vacuum advance ) via the built-in MAP sensor of the 123 ignition. It's not cheap though...

http://www.123ignition.nl/product.phtml?id=221

[blodi]

The carbs are brand new. So everything is good to go. The pump jets are 45's. 

[Chickenman]

Here is the MAP programming image from 123. You can see where you can turn off Vacuum advance at low RPM's where you have large pulsations in the Intake runner. Then switch it on at a higher rpm once things have smoothed out. ( 2,300 to 2,500 plus  rpm is usually good, ) . Note. Mechanical advance curve shown is just for viewing. It's not a very good curve and is wasting set points.  I would  make a Triple carb tune as follows.something like:

Static timing of 15 degrees

Additive RPM timing: All degrees are in Crank degrees All values are automatically interpolated on a curve. 

1: 650 RPM  =  +2 Degrees ( This is an Anti-Stall feature ) 

2: 1,000 RPM = +0 Degrees Crank ) 

3: 1,400  RPM = +5 Degrees

4: 2,300 RPM = +10 Degrees 

5: 3,500 RPM = +21 Degrees ( 15 static  + 20 dizzy = 36 Total Crank. High Torque range  ) 

6: 6,000 RPM = +19 Degrees (  15 static + 19 dizzy = 34 High RPM retard )

 

Pictures of 123 Demo Ignition curves:

 

 

Edited April 24, 2017 by Chickenman

[Chickenman]

15 minutes ago, blodi said:

The carbs are brand new. So everything is good to go. The pump jets are 45's. 

Oh... I figured they came with the Race Engine due to the first picture of carb jetting on the VC.

Accelerator pump jets and the pump spring tension may still have to be changed to match particular engine builds. New does not necessarily mean " Good to go ". Carbs come with " Generic" settings.  Tweaking after install,  to suite your particular engine characteristics is often required. You may need to go to the stiffer pump springs and 55 pump jets. That will give a sharp hard shot to fill the low speed lean spot.

And do physically check pump operation. Sometimes machining swarf will block passages. happens more these days with " Global " economy.

Another question. New carbs typically come jetted for street use and the chokes installed from the factory. Did you change the factory jets and chokes from what they came with?  The picture of the Race engine carb specs and previous replies seem to imply that??? What were all of the original jettings and specs, and what, if anything did you change ? 

( I haven't read your complete build thread as I've been up since 4:00 AM with insomnia. I don't know your engine specs. Fading now,,, going back to bed... ) 

BTW, do adjust timing as suggested. It doe make a big difference to IR carbs. If you have a larger camshaft you can even start off with a Static timing of 17 to 20 degrees.  

Edited April 24, 2017 by Chickenman

[blodi]

I did see the pump working when I was looking at it last night. At least on the rear carb...I'll check the other two.

I did change the jets to what is in the first picture. Currently that is what I am running but switched back to the 170 air correctors that came with the carbs to see if that fixed the lean spot...which it didn't. 

Chokes are 36's that came with the carbs. 

Timing is set to about 18-20. Vacuum advance is disconnected. 

Thanks for the help!

So to recap, right now my setup is

36mm chokes,

155 mains,

170 air, (have 185's as mentioned above)

F16 emulsion,  (should I go to F11? Seems to be richer?)

60F9 idle

45 pump,

00 exhaust

So....55 pump jets? Where do I get the stiffer springs?  Or should I do a F11 E Tube?