engine and drivetrain Weber 38/38 Tuning

[jireland2002]

For Weber down drafts>>  For modified engines or cars regularly going to the track.  You should change the needle and seat to the larger "250" size so the engine won't go lean at sustained high rpm.

[jdeitch]

For Weber down drafts>>  For modified engines or cars regularly going to the track.  You should change the needle and seat to the larger "250" size so the engine won't go lean at sustained high rpm.

Please explain. Seat removable?

[peterschop]

Please explain. Seat removable?

 

250 needle and seat, my new and old ones. It came in my weber rebuild kit.

 

BTW, the old needle and seat was a 200 but the new one is a 250

Edited October 30, 2014 by peterschop

[jdeitch]

Gotcha. Thanks. A pic is worth a thousand words. I didn't realize there was more than one size.

[TobyB]

I found that the transition ("idle") jet and main corresponded- as I went down in main, I had to go up in transition to

prevent lean- out in the transition phase.

 

For my car at the time, at 70mph on level ground, the darned thing was right smack in the middle of the transition

phase.  So I always had to compromise on one or the other depending on if it was track season

or commuter season.  I never really found one jetting that worked really well for everything...

 

t

[roadhog0]

Im fighting exactly what Toby is mentioning right now. I cruise around right in the transition spot and dont like the 15.5 AFR because when you tap it just a little it spikes way lean. Get on the mains and its good! I also hit a fat wall at like 5500 RPM in 3rd... So this winter is carb tuning time!

 

EDIT, ive got 50 idles and dont really like them, i want to go down a size. Also a 300 cam and 9.5 pistons with stahle header. I run a wideband.

Edited November 1, 2014 by roadhog0

[Tombonetoo]

One way that I found to improve the lean-out in the transition phase was to change from an F50 to F66 emulsion tubes.   I experimented with several emulsion tubes on my 38/38 (not a cheap proposition), and the F66 with its lower drilled holes eliminated the transition phase lean out as measured with an WBO2 sensor.   Final configuration:

 

Mains                               155  (stock 145)

Idles                                   55  (stock 45)

Air Correction                   165 (stock 185)

E-Tubes                            F66 (stock F50)

 

We are at 600 feet above sea level here in Fort Worth.    As you can see I took a different route in dialing this carb.   I should note that the 38/38 I am running has a 250 Needle Valve (not a 200 as noted by the ealier poster), 60 Pump Jet (slightly smaller than stock size) and 35 Pump Bleed (slightly larger size).  

 

The above combinition is on a relatively stock '76 2002 engine (Tii exhaust, electronic ignition) an gave no stumble, no hesitation and AFR readings of 13-14.5 over 900-5500 RPM in variousmloading conditions.   Driveability was as good as the 32/36, but pulled with more conviction and a breathed better at upper end.

[TobyB]

Interesting.  I always forget that the emulsion tubes

can 'encourage' the main jet to deliver fuel,

but they can, just as you describe.

 

DG emulsions are hard to find, aren't they?

 

t

[peterschop]

http://www.carburetion.com/Weber/weberparts2.asp

 

I got mt F7 emulsion tubes for my 32/36 at CarbsUnlimited. They seem to have quite a selection. 

 

 

[Zorac]

Im fighting exactly what Toby is mentioning right now. I cruise around right in the transition spot and dont like the 15.5 AFR because when you tap it just a little it spikes way lean.

 

i have the exact same problem!  ive done some weird jetting to get rid of it, but can never get rid of it completely.  Tombonetoo makes an interestting suggestion, changing the e-tubes is about the only thing i havent tried.

 

The only thing that really came close to working was undersized idle jets with oversized main jets and massively oversized air correction jets, this seemed to get me on the main jets a little earlier, but the problem was still there, just not as bad, and it was doing some funny things when on the idle circuit.

[TobyB]

Wow- there are a lot of 'em, aren't there?

 

Now, if there was some way of QUANTIFYING the emulsion tubes

so you had any idea of what was drilled where...

 

t

Edited November 2, 2014 by TobyB

[Tombonetoo]

This web page from the Carburatori Weber site  provides some information I found useful in setting up the carb.   Read the stuff about the role of the emulsion tubes and how it describes their selection.   I also read the Weber Tuning manual when I went to tuning the DCOEs.

 

http://www.webercarburatori.com/?p=handbook&s=2

 

"For mixture enrichment at low rpm or during slight acceleration use tubes without orifices at top (note that the F50 emulsion tubes have larger orifices at the top portion).   For mixture weakening at low rpm or during slight accelerations use tubes with orifices on top.   When mixture enrichment for slight accelerations is needed, the fuel reserve in the emulsion well must be increased - this is obtained by fitting a tube having a small outside diameter, orificates located predominately in the lower portion of the tube and larger size air bleed to prevent excessive mixture richness at high rpm." 

 

As you will see on the web page under part group 61440, the F66 emulsion tube have three sets of 4 holes of larger diameter, located lower on the tube when compared to the F50 emulsion tubes.   I also tried the F80s, but they provided too much fuel at the transition point.

 

Pierce Manifolds also has a wide selection of emulstion tubes.

Edited November 2, 2014 by Tombonetoo

[peterschop]

I switched from F50 to F7 after reading online Carl Nelson's recommended jetting for a 32/ 36.  Pat Braden's book, Weber Carburetors also has a lot of good information on emulsion tubes.

 

[TobyB]

Oh, heck, Tom, that breakdown is fantastic!

 

I've seen it for DCOE emulsions, but not for the DG's.

 

Thank you!

 

t

[Tombonetoo]

Thanks Toby. Per Peterschop's note, I would agree that the F7 emulsion tube would be a very good choice as well - that one should kill a transition stumble for sure (you can't argue with a Carl Nelson recommendation).