HELP please! DCOE’s stumbling (leaning out) at ‘tip in’...

[sam1904]

Hi all,

I have a mostly stock 76’ motor (Tii Dist, unknown cam) which I installed 40 DCOE’s on around Xmas and they have put a smile on my face every drive. At a recent check valves and timing were good.

 

My current jetting is shown below and the car runs really well….mostly. I also have an A/F meter in the car which has been a huge help and provides the A/F ratios below.

 

The car starts and idles well at around 13:1 and with any meaningful throttle gets slightly richer through a WOT reading of around 12.5 – 13:1 (I was originally running closer to stochiometric but found enriching slightly made a great difference.)

The only issue is that at tip in / transition from a trailing or idle throttle the car leans out to 15-18:1 ish for a brief moment, hiccups and then as throttle travel increases transitions quickly and moves to the high 12’s. If I balance the throttle in that position I can hold it in the 15-16:1 range.

Reading the books I am thinking that as my idle A/F ratio and WOT ratio seems good, and it is just the transition where I have an issue (E.g. low RPM Main jet zone) I need to decrease the size of the air correction jets a little (from 200 to 175?) and if that then makes me too rich at higher RPM's I would then decrease main jet size a little (I still have the 120’s that came from IE) (See what I did there, I preempted the “buy the book” comments ?)

 

For the Weber gurus amongst you how does my logic sound?

 

Many thanks as always!

Sam

[Son of Marty]

Before doing anything what size are your aux venturis? these are one of the controls the determine where the main circuit tip in. 

[sam1904]

Good question. Let me Google “what and where are my aux venturis’, go look and get back to you tomorrow!

[sam1904]

Will pull the filter and elbows tomorrow and check...

Edited May 8, 2021 by sam1904

[sam1904]

I am curious though. It is not that when I transition to the mains it leans out and then gradually enriches as throttle progresses, it seems to be just a short spike at a specific position and then it ‘switches’ back to 13-12.5:1 and stays there... ?

[Son of Marty]

A lot of jetted webers over the last couple of years have  been fitted with 5.0 aux's way to big for a stock 02 your stuck between 4.5 and 4.0 but the 4.0 are hard to come by these days.

[Chris_B]

4.5 aux venturies are typical for this application.  At what RPM range are you experiencing this lean patch?

[Chris_B]

And a few other questions (sorry...): at what RPMs are you running your idle? Have you checked the fuel level in your carbs? Have you balanced the carbs, barrel to barrel and carb to carb? And, are these new carbs?

[TobyB]

While you're googling, look up the Weber idle jet F number progression.

The F part of the idle jet indicates the air correction drilled into the base of the jet.

 

It sounds to me like your idles are too small, since they feed the transition circuit as well.

 

Or that the throttle plates are closed a little too far, and are not being uncovered soon enough,

leaving a gap between the idle and progression circuits.  

That's easily fixed on the new carbs by closing the idle air bypasses a bit and bumping up

the idle screw. 

 

The main airs don't come into play until you're well up on the mains (say, 4500) at full throttle.

 

Here's some light reading

 

https://www.lainefamily.com/images/WeberTuningManual.pdf

 

t

 

 

[Son of Marty]

To add to what Toby said f16 tubes are better for a stock or mildly warmed 02 the f11 are better for modified m10 they both work but cover different ends of the range.

[gracer]

The information that I have shows the following:  Main Venturi=32, Aux Venturi=4.5, Main Jet=120, Emulsion Tube=F16, 

Air Correction Jet= 190, Idle Jet=50F8' Accelerator Pump Jet= 40, Pump Ex. Valve= 55.

The Emulsion tubes that work well for a BMW are the F16, and the F9.  You can get more HP by going to 32 Main venturies.

I always dealt with Pierce Manifolds  408 842 6667.    G

[Son of Marty]

Gracer, your right f9 or f16 tubes for a m10, f11's are for air cooled VW's I'm old and easily influenced and also on my 3rd beer. 

[Chris_B]

1 hour ago, TobyB said:

The main airs don't come into play until you're well up on the mains (say, 4500) at full throttle.

Toby, I have to say this is entirely contextual. Given that calibrated parts of these carbs are so adjustable, it all depends.

[Mike Self]

Perhaps too simplistic, and I may be exhibiting my lack of knowledge on sidedraft Webers, but don't they have an accelerator pump?  If there's a hesitation going from idle circuit to mains circuit as Sam describes and the ratio drops to a very lean situation, could there be a bad accelerator pump diaphragm or clogged pump jet?  The point of an accelerator pump is to provide an extra squirt of fuel under sudden-ish acceleration; what if it's not?

 

mike

[Chris_B]

Mike, yes DCOEs have acell pumps (but no diaphragms), but this lean spot Sam is describing is very typical of DCOEs on M10 motors, typically caused by poorly tuned idle circuits or carb balancing.