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

[uai]

I always had this a bit more or less. (In the beginning more and later I got less) But how I got rid of it was loooonger Trumpets.

Try to fit some 4“ or longer Trumpets and go for a test drive.

 

[sam1904]

Thanks for all of these thoughts, lots of strings for me to pull.
 

When I get back to the car later I will take some photos, check out what I have in the auxiliary venturis and also take it out for a drive to check not just the RPMs but if it is purely transition issue or if it is also at steady state at those RPMs.

 

Will report back later!

 

[sam1904]

OK, after some driving and wrenching to complete my homework assignments here is what I have:

 

Observations driving the car:

• The lean condition appears to only occur between 2000-4000 RPM (mostly 2000 – 3500). It does not occur during that RPM range every single time (or at least to the same extent) but is more often than not present.
• When accelerating (especially in 2^nd and 3^rd) it tends to be 2000-3000 RPM and as I apply throttle I move through it quickly
• Depending on load it can occur as a quick spike anywhere in that RPM range, or with slow throttle application in a high gear be present through much of that range
• It DOES seem to be clearly correlated to a specific throttle position and always seems to occur at around 20% throttle travel.
• If I pass through this throttle position it is a spike, if I hold the throttle there, or decelerate under a trailing throttle to that point the lean condition can be sustained for seconds at a time
• The lean condition does not seem to occur at any other throttle position under any RPM or load scenarios

 

Wrenching observations

• The car has 45 (4.5mm??) Aux Venturis. (See pic below)
• I have previously balanced the carbs, both between and within using both my squirrel cage and carbtune
  • I had to open the air bypass jet slightly on #4 to achieve this 
• Idle speed is around 900-1200.

 

Does this help?

Thanks

Sam

[sam1904]

[2002Scoob]

Very, VERY familiar with this lean spot. Have gone to the moon and back it feels trying to solve for it and haven't 100%. Its involved a ton of time, and a ton of money on little brass bits to get here. 

 

I was able to minimise it as best I could, followed by figuring out how to drive around it and considering it just part of the personality of the car. Meaning I need to roll into the throttle vs. mash it and expect it to perform like fuel injection. 

 

I can, however, tune it to where if I mash it it boogies with no stumble, but that requires ignoring that things are rich AF (and while I'm referencing 'air fuel' thats not what I mean) in a lot of other driving situations it really shouldn't be. 

 

What I should probably do (and you might consider) is taking it to an expert with a dyno that can set it up right. I'm sure if I added up all the money I spent on jets I probably coulda' afforded a few hours time with with a talented tuner.  But I'm fine with 'close enough' for now. 

 

Lots of really great helpful people to chime in, I'll try and digg up what I did to get to where I am. when I've got a bit more time if other attempts fail  

Edited May 8, 2021 by 2002Scoob

[Son of Marty]

In your shoes at this point, I'd get your tii distributor recurved to the ti spec, and replace the f11 emulsion tubes with f16's, it sounds like your getting close but the closer to perfection you get the more the minor details count.

[Chris_B]

That is a pretty broad range in idle speed. 1000 RPMs seems to be the sweet spot for these carbs. If you want to know where your tip in is happening, remove your main jet stacks and drive the car around. As you accelerate slowly, you will hit a brick wall. That is where the carbs are expecting the mains to tip in.

 

Many people are surprised about how much the idle circuit affects the transition. For a quick check,  you may want to try fattening up the idle mixture a bit- say to about 12:1- see how that affects your lean spot. I think your next step should be to permanently fatten up the idle circuit by going up a notch in the idle jets- to 55f9s.

[uai]

I have really tried a lot- all reasonable Jets and all Emulsion tubes including the "odd ones".

The only thing that REALLY helped were longer trumpets.
The step from 75 to 90 mm Trumpets did the trick for my car all lean hesitation is gone to a level that I can live with and only I feel it and not the passenger.

But I am consistent with the others you should first going for an F9 or F16 (or both) emulsion tube.

[Chris_B]

This is unlikely to be a tip-in problem.

[Chris_B]

4 hours ago, sam1904 said:

O

 

Wrenching observations

• The car has 45 (4.5mm??) Aux Venturis. (See pic below)
• I have previously balanced the carbs, both between and within using both my squirrel cage and carbtune
  • I had to open the air bypass jet slightly on #4 to achieve this 
• Idle speed is around 900-1200.

Have you checked your fuel levels?

[sam1904]

Thanks All,

One thing I should have shared is why I installed the carbs 5 months ago and am only seeking help now. There had always been a slight 'stutter' at this point but I just drove around it. It has now got (far) worse and is a definite 'stumble' that is hard to drive around. So, the key question is what changed......?

1. Weather got warmer.. (Can't see this as an issue as lower air density should have helped, not hurt this)
2. I changed from IE 90 Elbows and individual POD filters to Koogleworks cast 90Deg inlets and a single foam filter (See image below). This does not seem to have changed inlet length at all / much but I  assume this increased inlet flow as it appears to be a lower restriction (higher quality...) set up?

 

I will research and check fuel levels tomorrow and if that fails change emulsion tubes as suggested. Thanks again all,

Sam

Edited May 8, 2021 by sam1904

[sam1904]

1 hour ago, Chris_B said:

Have you checked your fuel levels?

 

I will read up how to do this with these carbs. Any set level you would propose or just whatever is 'standard' in the book?

Thanks
Sam

 

[Chris_B]

Most or all of what you are going to find in the Weber how-tos will suggest that you try and set the float level (which is a bit PITA, and really difficult to achieve any accuracy) and without much discussion of the actual fuel level. The alternate method is to measure the actual fuel level, which you can do from the top of the carbs. Pull the main jet stacks, start your motor and run it briefly, then stick a probe into the fuel wells (ideal probe, I dunno- a piece of cardboard or other paper, a wooden matchstick, etc.). The setting you want is 25mm from the carb deck (top of the carb, underneath the cover) which is visible with the jet cover off. This setting is a couple of mm lower than the port to the aux venturi.

Shine a small flashlight (LED is ideal) into the jet well, slowly move your probe toward the fuel in the well. When you hit the top of the fuel, you should be able to see the probe break the meniscus. I have used a micrometer in the past, which is helpful because when you see the end of the micrometer hit the fuel, you already have your measurement. 

[sam1904]

Quick question. After pulling the main jet and filling the bowl at idle can I shut off to measure or does it need to be running?

Thanks

Edited May 9, 2021 by sam1904

[Chris_B]

You can shut it off.