Tips on how to fix that dreaded Weber Flat Spot / Lean Hole?

[thatmacguy]

Yes, I'm still here. Been busy moving, but my Beetle is happily relaxing in my new garage and I really, really, hope to see a few of you guys out at an Autocross event this season!

Anyway, my problem today is the Flat Spot / Lean Hole that seems to be a common problem with webers on transition from idles to mains. What I did was install a nice set of Dual 40 IDFs from Aircooled.net on my Porsche (remember, the one with the Type 4 VW motor, basically). I've learned a LOT about jetting, and I have a digital AF meter in the car.

The problem is this: Most people "cure" the lean-hole by simply jetting too rich. The lean-hole then doesn't cause a problem because even at the leanest point you aren't too lean. However, I'm shooting for drivability and economy, and when I "fix" the lean-hole on transition this way, no matter how rich I go, I can still see the lean-hole on the AF meter, and I have to run WAY too rich. Anytime I get the rest of the jetting correct I end up with a pretty bad lean-hole.

The emulsion tubes I have are F15s and F11s, both behave the same way (the F11s seem to have a tiny-bit smaller lean-hole). I have a TON of jets to play around with, but as I mentioned, the lean hole is always there, all I can do is mask the problem.

How do I fix this? Is there any way to force the IDFs to keep the idles on a bit longer or bring the mains in sooner? It's truly driving me nuts because with the AF meter in there tuning the circuits is really pretty easy... Nobody has been of any help so far, and then I realized that I bet a lot of you guys have experience with dual IDFs (this is my first set).

Thanks so much for any help you can give me.

(PS, I did try to search, but the search function on this new forum unfortunately does not function for me.)

[vw1s]

Make sure your throttle plates are almost closed completely at idle. Idle should be adjusted using the bypass screw on the side of the carb, not using the stop screw. I adjust my stop screw until it just touches then about 1/8 turn. Ensure your venturies are sized properly for your engine, most people run venturies that are too big. Big venturies make for lower air speed through the carbs. Lots of good tuning info on the samba, just have to sift through a bit.

[MadMike]

I've never had a flat spot with IDF's?? Most problems are with the Weber Progressives'(POS)& 009 distrib's What Vents? Dizzy? you might want to try F2 tubes or give the acc pumps a bigger squirt,since u got them idf's from John @ aircooled he is very helpfull or quit looking at the A/F gage and drive it!!! Type 4 in a bug?u ain't gonna get the greatest mileage anyhow,LOL

[clintonddk]

What jets have you change? What size idle jets? Most of your driving is done on the idle circuit. Is your linkage in good shape? Is your linkage in sink (opening both carbs evenly)?

[thatmacguy]

OK, to answer some of the questions:

1) The carbs are perfectly in sync. I check them every single time I change jets, just to be sure.
2) Linkage is new and seems to be fine (CB Performance set from John and Aircooled.net)
3) I have a TON of jets, the problem just doesn't seem to be jetting, because they ALL do it. I can run say 60/130/215 and it "masks" the problem because then the lean-hole is still only around 14 on the AF meter - car runs fine, but gets like 17MPG and runs RICH across the board (this was a 30+mpg car with the stock FI system, unfortunately parts availability on the FI system is ZERO, hence the move to carbs). So I need a way to fix this outside of just "jetting". Maybe a different emulsion tube? Maybe emulsion tube modification?
4) Hi Patrick, nice to hear from you. I would like to hear a bit more on your advice about using the air-bypass screws to adjust idle. That runs counter to pretty much everything else I've read, but I've seen your beetle run so I'm willing to give it a try if you can very briefly tell me the why & how of it! My Venturi are stock for a 40 IDF, so I think that's 28? I'll have to look it up.

I have an aircooled.net ACN SVDA distributor, could a flat-spot be caused by a timing issue?

Here is a quick off-the-top-of-my-head list of jets and emulsion tubes I have (but see note above, any combination of jetting displays this problem for me!)

emulsion tubes: F11, F15

Idles: 45, 47, 50, 55, 60

Mains: 100, 110, 115, 120, 130, 140

Air: 140, 160, 180, 190, 205, 215

Fuel pressure is 2 - 2.5 pounds.

[MadMike]

Bump fuel pressure up to 3lbs. 60 idles are why ur getting bad mileage ,try 55's and some 200 airs and keep the 130's ,since it does this with all the combos of jetting u tried I'm thinking it's the Timing/Dizzy!!! swap it for another!!

[clintonddk]

Carbs might be in sink but is the linkage opening them evenly? Ckeck them at just off the idle stops and at full throttle.

[81pumagtc]

I am kind of agreeing with both Patrick and MadMike...what are the odds of that happening?

I have seen numerous descriptions of Patrick's setting system, but you have to have a car set up correctly, and then you monkey with the bypass screws to get it correctly dialed in. Adjusting the idle using throttle stops is a lot easier, but it is really not the best way to set it. Idle simply does not draw enough air...that me be why they call it idle.

As to your mileage, well, you have moved from fuel injection to a carburetor....and carbs are not sophisticated at all. I had a friend who worked on cool old Italian car (read VERY expensive) and he always said that side-draft Webbers only truly work at idle, and full throttle. Everything else is just moving problems around so an RPM range where they are least objectionable. I have talked to a number of folks who use the SVDA dizzy, and like them a lot, but maybe yours might be troublesome.

I agree with MadMike, and wonder if you might have some timing issues. I have a real Bosch chrome 009 from Brazil, and it will occasionally idle at a different RPM when you come off the highway or high RPM for a length of time. I have traced it to the spring rates being slightly different at different temperatures. Made me worry, but in reality, not important. I have an 010 and Pertronix if I can ever shake free enough time to finish and install them...

I do have another thing you may want to play around with, but it is really only helpful for a car that is running pretty well. I took me three attempts get my velocity stacks right. I'll figure out what I am using. I run the velocity stacks inside the K&N's. This really won't help anything other than throttle feel, but I would run stacks if you are not. The air flows a great deal better.

Good luck!


Dave

[thatmacguy]

Thanks for the input, I did a bunch more testing today and will report back on that tomorrow After I try one more combination. I do have a set of velocity stacks inside my air filters. Out of necessity they are pretty short, only about two or 2 1/2 inches tall, but they do make a slight difference.

[thatmacguy]

Okay, I too am starting to believe this is a timing issue rather than a Carburetor issue. Although I guess I don't understand the relationship between timing and enrichment enough to understand how it would cause this Specific problem. Using John Connolly's thread at the samba I was able to get the mains to come in just a tad sooner by using an extremely large air corrector. In fact I'm using no air corrector at all which according to John is the equivalent of approximately 300. That's counterintuitive, but a larger air corrector jet does indeed bring the mains on just slightly sooner. The Difference is small, but in my case it seems to have work. However, that solution causes me to have to use a relatively large main at 130. The car Runs better with smaller mains, having a much better throttle response.

I'm going to try a 120 Main jet soon. But here is what I'm currently running, and the car runs well:

F11/47/130/300

[81pumagtc]

One other thing to check out.

If you are using anything other than a real K&N filter, I would change them.

Mine had Bugpack filters on it, and they are pretty terrible. If you put them in a bright light, you could see bright spots of light were there were no filtering gauze in the filter. Not good, but probably not really important in the real world. I did, however, notice a difference in airflow after they were cleaned, dried, and re-oiled with K&N oil. I suspect they were from different batches, or or something. It was noticeable.

Glad you are getting a handle on it.

Timing issues really suck...



Dave

[thatmacguy]

OK, the 120 was too lean. My idle circuit up through progression is still on the rich side, so I'm going to try my 45 idles as soon as I get a chance. In the current configuration I'm still running on the "rich side" across the board, so I'll never get great economy. Ideally I would want to manage to get things running so that I'm pretty lean at cruise (when there is little load or stress on the motor) and then get rich as soon as I'm at more than around 1/4 or 1/3 throttle. If it wasn't for this flat-spot problem I would be there already. But at least I now have a default setup I can go back to and drive and enjoy it. I never thought to try removing the air-correctors completely as it seems like my WOT above 4000rpm would get insanely lean, but that doesn't happen with the 130s. Overall, however, the car really likes 115/180 or even 110/160 better - it runs crisp and nice with those, but HUGE lean hole. And to be clear, this is not an accelerator-pump issue, this is a lean hole that I can drive into at part-throttle and literally keep the motor there because the mains just aren't coming in soon enough. I also discovered in my testing over the past couple days that the idles make essentially no difference in this issue. I can run a stupid-big idle (like my 60s), and it doesn't fix the lean-hole at all, the only thing that's worked so far is using the air-corrector trick to bring the mains in just that tiny-bit sooner. Unfortunately, with the 130 mains my gas-mileage is hilarious, so this can't be a permanent solution.

I'll also try Patrick's recommended idle-tuning method once I decide which set of idles jets I'll be using.

Does anyone have any experience modifying emulsion tubes? I understand that if I plug-up the topmost holes in the emulsion tubes I can make the mains come in a bit sooner that way. I've been afraid to do that for fear of ruining the tubes and/or having something come loose and get into the carbs. Ideally someone would sell a flat fuel-safe o-ring that could be used to cover various holes in the tubes.

I'll come back and discuss timing again too, but right now my brain hurts...

[MadMike]

Did you check the float settings? and bump up the fuel pressure to 3 or 3 1/2 lbs max.? I never trust factory settings,most of the time they are off and clean both carbs too with compressed air,I'm not 'up' on type IV motors but what size is this thing? 1800cc+? might try some 30-32mm vents,did you try a differant dizzy yet? Wish u were closer to me ,I have a cpl.set of 40's and a set of 44's u could try,madmike

[thatmacguy]

The float settings are set to weber's specifications (although they did NOT come this way from aircooled.net, they came set to something like 6 & 18, which caused weeks of headaches and problems since nobody thought to check them as I had paid for John's carburator setup service. He says he checks float setting and that they probably got messed up during shipping, but as they were both set incorrectly and the same, my guess is that he simply forgot to set them.) The fuel-pressure makes no difference at all between say 1.5 and 3.5 lbs max. Unfortunately the "correct" pressure for Weber IDFs is essentially impossible to find because every expert in the business tells you something different. I've heard everywhere from 1 to 3.5 lbs.

Here is a question for you regarding timing though: Where should I be taking vacuum off these carbs? Does the stock vacuum fitting location work properly on these carbs or should I have done something custom? Should I take vacuum from a single point or from 2 points and run a "T"? I've read both ways and tried both ways, and it doesnt' seem to make any difference, but I might as well ask so that I get it "right".

The motor is 2067. I wouldn't mind changing out the vents, but John Connoly seems to think they are probably the correct size, which is whatever the stock size for these carbs is and I can't remember it off the top of my head....

I don't have a different dizzy, and this darn thing is basically brand new (got around 2000 miles on it now, nearly all of which is test-driving to try and get it running right). I'm going to sit down this weekend I hope, with the timing light, and see exactly how the timing & vacuum are working. I'm wondering if the vacuum advance isn't working correctly, as when I tested it last it didn't seem to make as much of a difference as it should.

[thatmacguy]

Wow, Well, maybe this is the problem: I just took another look at the Weber instructions and diagrams, and I'm using the port labeled "Vacuum Advance" for my vacuum advance. Well, obviously. However, this appears to be ABOVE the throttle-plate. There are two other "Vacuum" ports that appear to be BELOW the throttle plates. Don't I want to use those? Above the throttle-plate gives me nearly zero vacuum and thus zero (additioinal) advance at idle, right? This explains the results I see on the timing-light last time I checked. Plus, if I recall, a retarded timing at idle can cause really poor fuel-economy.

Should I move the location from which I take the vacuum?