Need Carb Jet Guidance, Weber 40 IDFs Options

[nditiz1]

Just ordered 2 of those colortune's last week. Hopefully they will be here soon. @infraredcalvin how was it tuning the banks with it?

[rfinegan]

Did you mention what your base timing is set at? And how much total advance @3200- 3500 rpm? This will effect your AF numbers too

[bbrock]

You can always open the .45 to .475 if they are too lean. Or if that works out to be what you need I think I have a set of .475...let me know (here) and I will dig them out for you...
Robert

Thanks! I'll keep you posted. I wondered about drilling but figure the bits would cost more than a set of jets. If I end up needing .475 and you have them, I can trade my unused jets for them.

Did you mention what your base timing is set at? And how much total advance @3200- 3500 rpm? This will effect your AF numbers too

I just set to stock specs of 27 deg BTDC@3500 rpm. I've never measured base advance since I just have a basic antique timing light. I probably should do that.

[930cabman]

I have been following this to a certain degree as I am a carb guy and currently wrestling with a hex (hexed) bar linkage setup.

Couple things:

It's possible the technology of the Webers will never provide the information with regards to A/F numbers

Did you mention the car is running good? Maybe leave well enough alone until you can convert to an FI system

From what I know the idle speed screw needs to be about 3/4 turn max from initial contact. Also the mixture screws should be around 1 1/2" turns our from the seated position.

[nditiz1]

With the numbers you provided on the AFR were your cruising speeds under light throttle? I ask because you might actually be ok. If you are driving around 50 mph on flat ground you are most likely in your idle circuit still. It isn't until you open the butterfly more to drop the vacuum that you actually get into the main circuit.

[bbrock]

With the numbers you provided on the AFR were your cruising speeds under light throttle? I ask because you might actually be ok. If you are driving around 50 mph on flat ground you are most likely in your idle circuit still. It isn't until you open the butterfly more to drop the vacuum that you actually get into the main circuit.

Yep, light throttle. The transition seems to occur right around 50 mph on level ground as the AFR is pretty rock solid around 11.0 until 50mph/3200 rpm and then climbs to 12-13 territory. Even at 70 I'm under light throttle on the level but just above that magic 3200 rpm. Of course, level road is the exception rather than norm around here which is why my first set of measurements were all over the place what with accelerating, decelerating, and accelerator pump squirting every quarter mile.

I think the main circuit is in good shape, it's just the idle that is running rich. I also think the accelerator pumps were wasting a lot of fuel. I don't remember where I got the bench setting for them, but seemed adjusted way too aggressive. I have them disabled for now.

[bbrock]

Did you mention the car is running good? Maybe leave well enough alone until you can convert to an FI system

Depends on how we define "running good." If it means being able to drive like a bat out of hell and feel a strong pull all the way to 90, then it is dialed in to the nines. If we add fuel economy to the equation, it is not running good at all. The mileage sucks and I'm lucky to get 200 miles out of a tank of gas. I got closer to 400 miles when it ran on the long in the tooth DJet. I have a long trip I'd like to take it on the end of this month, but won't if I have to stop every 200 miles to fill it with liquid gold.

New jets are already paid for and on their way so should be able to do something to improve economy. Honestly, I never expected out of the box stock jetting to be ideal at 6,000 ft. so none of this surprises me.

[bbrock]

BTW, the 3200 rpm transition squares with my brief time running with the main stacks pulled yesterday. As soon as I hit 3200, the engine leaned out and went dead until I brought the revs back down.

[infraredcalvin]

Just ordered 2 of those colortune's last week. Hopefully they will be here soon. @infraredcalvin how was it tuning the banks with it?

Totally worth the cost, the piece of mind is great. It’s a bit awkward the first time, but when you get them all dialed in you notice things are better balanced.

@nditiz1 There seems to be a pretty good range of “blue” light as you’re tuning, i set all mine just as it goes fat (orange) to blue, my wide band reads in the 11s, when I take it to the point just before it leans out (white), it reads 13/14s on the afr. Since I track my car, I’ve left it on the fat side for now.

[bbrock]

I might have made some progress today. The new jets arrived this afternoon early enough to have a little time fiddling. I started by installing the .55 idle jets. At first I thought I was going to have to eat crow because when I started the car, it idled with AFR of around 12.8 which seemed like a very good sign. But after I had dialed in the idle mixture screws, the AFR was down in the low 10s. Wrong direction.

I swapped those jets for the .45s and went through a few rounds of balancing and adjusting the mixture. After adjustment, the car idles at around 12.3. I took it for a very short spin at low speeds and the AFR seems to run in mid-12s and 13s up to about 3000 rpm and then leans out pretty much the same as if the main stacks were pulled before.

I had to call it quits for the night after that but am hopeful I'm headed in the right direction and the larger air correction jets smooth the transition between idle and main circuits. I'm only guessing of course, but that's where I'll start tomorrow.

[930cabman]

Where are your mixture screws with the 45's, 50's, 55's? From what I have read/seen the mixture screws should be in the 1 - 1 1/2 turn our from lightly seated.

[rhodyguy]

I don't understand why you're trying to achieve a 800RPM idle. Shoot for the same idle as a FIed engine. How ancient is the carb linkage? Open or close the bypass. I start at closed. Their purpose is to fine tune the flow to a common reading on all 4 barrels. The actual number value is somewhat insignificant as long as they read the same.

[bbrock]

Getting dialed in a bit more. Swapped out the 200 air jets for 210s this morning and took it out to warm up and test. Way too lean on the idle circuit. Was running high 14s to high 15 most of the time below 3000. Transition to mains wasn't smooth but once I was in the main circuit, got the reasonable mid 12s to mid 13s as before. Brought it home and put the original .50 idle jets in but leaving the larger air jet. I have it idling at stop at 13.0 AFR which is almost dead on with the target ACN suggests. One difference is that I started adjusting the idle mix screws to best lean idle and no more. ACN suggests adding a half turn richer from best lean idle, but that seems to be contributing to the overly rich idle condition. Haven't actually driven it yet as I'm helping the wife with some chores, but will do so shortly.

Where are your mixture screws with the 45's, 50's, 55's? From what I have read/seen the mixture screws should be in the 1 - 1 1/2 turn our from lightly seated.

As expected, the 55s were the least (around a turn out) and 45s were the most (not quite 2 turns including ACN's half turn richer). CB says more than 2 turns indicates the jets are too lean and I've seen the 1-1/2 turn rule too. In the end, the wideband is what tells the story and it says 55 is too rich and 45 is too lean. I think either the 50 I already had or possibly .475 is the right number. Still more testing to do.

I don't understand why you're trying to achieve a 800RPM idle. Shoot for the same idle as a FIed engine. How ancient is the carb linkage? Open or close the bypass. I start at closed. Their purpose is to fine tune the flow to a common reading on all 4 barrels. The actual number value is somewhat insignificant as long as they read the same.

The carbs are perfectly balanced. That is not a problem. As I mentioned before, there is disagreement among the experts on how to use the bypass screws, but I went back and closed all of them and then opened up screws only as much as was needed to balance the two throats. They are dead on.

The linkage is a new (well, 3,000 miles on it) CSP linkage. No issues there. In fact, for those who don't know, the whole car is basically new. Just finished with a full bare metal restoration and every nut and bolt has been replaced or rebuilt. Only thing I haven't touched is the transmission.

Not necessarily chasing 800 rpm idle but ACN says this in their tuning guide:

"In order to properly adjust your engine, set the ignition timing to idle around 8 ° before top dead center. Then set the idle speed to 800 RPM, synchronize the two carburetors, adjust the idle speed (refer to the paragraph on adjusting the idle richness)."

And Tomlinsons says the following, "Don't settle for an idle speed of over 900 RPM, even if you're running a 290 degree cam. Most street engines can be tamed to idle at around 750 to 850 RPM once you show those dual carbs who wields the uni-syn [yeah, that dates it]."

The stock FI spec is 900 +/- 50 RPM. IMO, the best idle is the lowest speed the engine will idle smooth and not stutter on acceleration. I don't see much purpose in a higher idle. Just more noise and gas. This engine does that just fine at ~750 RPM - or at least it did on the pig rich tuning I was running on. We'll see where it ends up, but I'm guess in the ballpark of 800.

[rhodyguy]

you only measure the flow on the 2 front Venturi for final tune? I doubt the 100-150 increase in idle speed will have much impact on gas mileage. A higher idle smooths out the jumpy needle syndrome on the snail. I have no personal experience with the CSP linkage. A Triad linkage works swell for me.

Backing one of the idle speed screws off the stop, I use the pass side backed just off, lets you raise and lower the idle easily with no drama. This works swell for setting the timing. One person job.

[bbrock]

you only measure the flow on the 2 front Venturi for final tune? I doubt the 100-150 increase in idle speed will have much impact on gas mileage. A higher idle smooths out the jumpy needle syndrome on the snail. I have no personal experience with the CSP linkage. A Triad linkage works swell for me.

Backing one of the idle speed screws off the stop, I use the pass side backed just off, lets you raise and lower the idle easily with no drama. This works swell for setting the timing. One person job.

I use the 2 rear venturis for final tune. I read you should use whichever side the linkage is connected to. Wouldn't think it would matter much but the CSP linkage (center pivot bell crank) connects to the rear. I use the bypass screws to get the 2 throats within a carb perfectly balanced with each other then balance the two carbs using the rear throats.

I don't get any needle bounce on the snail even as low as 700 RPM. It's always rock steady and easy to read.

[bbrock]

Okay, it might be time to eat crow. I managed to get some testing and tuning in yesterday afternoon and think I found the winning combo. I am surprised but shouldn't be because my jetting now matches what @NARP74 posted on the first page of this thread and @rfinegan and @r_towle also called it. Maybe some others too.

I took it for a drive with the .50 idles and 2.10 air jets and found the idle circuit running just as rich as before. There was a big dead hole transitioning to the mains and then the mixture lean out to mid 13s as before. I was thinking that if .50 was too rich and .45 was too lean, then .475 might be what I needed and still kind of think that, but I realized I hadn't tried the .55 idles with the larger air jets so pulled over to the side of the road and swapped them out and readjusted.

To my surprise, the idle leaned out. I don't understand how, but suspect sorcery (IMG:style_emoticons/default/smile.gif) Still a little rich IMO, but an improvement. It idles right at 13.0 - 13.2 which is exactly where ACN says to shoot for. Immediately off idle, it drops to about 11 and then climbs back up. I suspect that is from the accelerator pump rather than the jets. I had to crank the pump in enough to get the car to cold start and think I may have overdone it and also didn't get them even. However, once it gets to 30 mph, the mix leans to mid 12s to mid 13s and pretty much stays there except a dip into low 12s in the transition zone. Also, the mix leans to 15s to low 16s on decel/overrun which is more what I expected. WOT under load is in the mid 12 range. I drove 30 miles and was generally happy. Also, the oil temps were running a little warmer than usual which is actually a good thing as I think the engine has been running too cold (rarely gets more than a 1/4" off the bottom even under hard driving). It is still running cool, just a little warmer.

It isn't perfect though. Cruising at 80 mph I noticed a bit of roughness and the AFR fluctuated going rich and then leaning out again. Also, driving the gravel road back home @20mph, the AFR fluctuated a lot and would run in the 13s for awhile, then drop to low 11s. I suspected my nearly new plugs were fouled from the overly rich mix and sure enough, when I pulled them, they were covered in dry brown soot. I cleaned them the best I could but will buy a new set if the weather clears.

I consider all of this preliminary, but promising.

Something still puzzling is that Tomlinsons and ACN both suggest backing the idle mix screws out 1/2 turn beyond best lean idle. They claim it is more "stable" whatever that means. When I do that with this jetting, the idle mix is way too rich, but adjust just to best lean idle and no more lands me right about 13.0:1 which is where I have it set for now.

[930cabman]

Something still puzzling is that Tomlinsons and ACN both suggest backing the idle mix screws out 1/2 turn beyond best lean idle. They claim it is more "stable" whatever that means. When I do that with this jetting, the idle mix is way too rich, but adjust just to best lean idle and no more lands me right about 13.0:1 which is where I have it set for now.


I would be curious if either was using any sort of A/F measuring device? or their ears or some other magic

[IronHillRestorations]

Sounds like you are making progress. You might want to try different emulsion tubes. I don’t have my Weber book, so I can’t make suggestions. I know I’ve used F3, 7, and 11. FWIW the different numbers are relational to nothing, that’s why you have to look at the chart.

[Superhawk996]

To my surprise, the idle leaned out. I don't understand how, but suspect sorcery (IMG:style_emoticons/default/smile.gif)

Think of it as like a straw. You pull liquid up the straw by putting a partial vacuum above the fluid.

Small straw - very little air need to be evacuated from the straw to pull the fluid up say 1" in the straw. Now think 4" PVC pipe -- same amount of vacuum will provide same fluid lift but you'll need a good set of lungs to move enough air to establish that partial vacuum and to to lift fluid an inch in that PVC pipe that would be no problem in a small straw.

Now think of your jets. Bigger fuel jet and bigger air correction jet are more like the 4" PVC pipe. Your engine can only displace so much air per revolution -- so you get less lift of the fuel when flowing the same amount of air.

The "trick" here is that both jet and the air correction jet need to change (get larger) together for that analogy to hold. Otherwise, you're back to a singe variable and the way you're thinking of it (i.e larger fuel jet = richer & larger air correction jet = leaner). The two of them at the same time yields the 4" PVC analogy.

The main problem with carb tuning is that there are so many variables and they aren't all related in a linear manner and they don't always abide by the Superpostion Principle.

[bbrock]

I would be curious if either was using any sort of A/F measuring device? or their ears or some other magic

Tomlinsons definitely was not. The book was published in 1993 and I don't think there is mention of wide band in the book. The ACN advice comes from their instructions from How To Use a Wideband To Tune Your Carburetor(s) On The VW Flat-4 Engine linked earlier so he definitely was. Here is what he says about it:

Note we do NOT want to set idle mixture for highest RPMs. Chemically the highest RPMs will be at “stoic”, but this is also unstable. 1/2 turn richer will be stable and still pretty lean. Setting idle mixture to stoic (highest RPMs) will result in an unstable engine, the engine idle speed can drop if you just turn the headlights on, or the idle speed will change as the temperature does. It will drive you crazy. On an engine with no choke, if you simply tune to 1/2 turn richer than “stoic” (fastest idle) the engine will be much happier and more predictable. A properly set idle mixture will likely pop and fart a little bit when dead cold, this goes away after 30 seconds or so of running. We want to get a steady idle speed in all conditions (hot / cold, dry / wet) and to achieve this, we simply adjust the idle mixture a little richer than “ideal”.

Sounds like you are making progress. You might want to try different emulsion tubes. I don’t have my Weber book, so I can’t make suggestions. I know I’ve used F3, 7, and 11. FWIW the different numbers are relational to nothing, that’s why you have to look at the chart.

I thought about that, but since I will hopefully be ditching the carbs for EFI over the winter, I think I'll call it good if I can get the mix in the 12-14 range. If I were going to keep the carbs permanently, I'd for sure be chasing that rabbit.