two schools of thought: big air jets and small air jets...48IDA

[Bruce]

Jim do you know which emulsion tubes were used in the 904?

F14

[Jim Ratto]

40mm vent, 160 main, F2 emulsion tube, 130 air, 65x120 idle/air, idle-mix screws out just past 3/4 turn

car runs clean to 6800

no timed runs back to back with large air yet for comparison

[Jim Ratto]

here is some jetting from 587/2 Carrera 4 cylinder 4-cam 1966cc motor, as in 356 or Abarth, running sports-style "stinger" exhaust

40mm vents
60 idle
165 main (175 for Le Mans)
F11 emulsion (F20 with standard two-outlet muffler)
240 air!!
3.00 neddle valve

I have 1 F20 tube around my garage somewhere....

here are F20, F4, F11...left to right
F20 and F4 share same 7mm diamter but F20 has 4 fewer holes (at bottom), top holes are same
on the right is the F11 "IDF" tube 8mm diameter and much fewer holes than F4 or F20
I didn't pull the F2's from my car for the post or picture.
who is good at deciphering diameter and hole locations?

[lawrence]

Interesting reading so far. Wish I could scan some emulsion tube pages from my weber manual. Can anyone tell me if the idle jet holder meters fuel or air?

F4 and F20 are practically identical. The f4 provides a slightly richer mixture during slight acceleration or low rpms because of the 4 more holes( .100mm) above the angled holes (.250mm).

The f11 has many holes towards the top, which makes it ideal for mixture weakening at low rpms and slight accelerations.

[Jim Ratto]

Interesting reading so far. Wish I could scan some emulsion tube pages from my weber manual. Can anyone tell me if the idle jet holder meters fuel or air?

F4 and F20 are practically identical. The f4 provides a slightly richer mixture during slight acceleration or low rpms because of the 4 more holes( .100mm) above the angled holes (.250mm).

The f11 has many holes towards the top, which makes it ideal for mixture weakening at low rpms and slight accelerations.

thanks Lawrence. I get that smaller diameter tube = richer
larger = leaner

hole amount? Location? and the "step" comes into play too.

[Jim Ratto]

update:

spent some good quality time with car and box of jets, tubes etc.

started with: 40mm vent, 65 idle, 120 air, 3/4 turn out on mixture screw, 160 main, F2 tube, 130 air
ran well except for loading up @ idle and faint trace of richness (surging) at 2500-3500. Turning mixtures in after load up cleaned and raised idle, but after driving it would get rough and pop in muffler.

next step was 60 idle, but initially to get smooth idle, took 1 full turn out.
Car had lean hiccup at top of idle/progression range, still with 160 main, F2 , 130air

next step was increase main to 170, everyhting else same. Still had hiccup lean spot, and midrange got soft. Top end same

next step was to ditch F2 tubes, go back to 160 main, but this time with F4 tubes.  So the combination now was 60 idle, 120 air,
1 turn out on mixtures, 160 main, F4, 130 air
No more lean hiccup in at top end of idle/progression, no rich surge at steady rate cruise, no hesitation in accleration, still very strong top end. I drove car for about an hour, all kinds of rpm and conditions, and brought car back home, and tried mixtures once more, and found, now just a bit past 3/4 out smoothed idle even further and after more driving, stayed more consistent.

I wish I owned one of the high dollar AFR meters to test my impressions or had access to my friend's dyno up north. I can say though, after the time spent, the car has a new sound and response to it.

[lawrence]

Cool stuff, Jim. Did you happen to see what the plugs look like through out the process?  I need to play with my jetting a bit. My car has the same symptoms as yours. My first step will be smaller air corrector(currently 180) thinking about a 165-170. Ill see what happens then go from there.

[Jim Ratto]

no I have not pulled plugs yet, as I was all over loads, rpms, idling.... and letting it idle and then pulling plugs would have thrown the plug reading out the window.
Maybe this weekend I will go make some country road passes and clean cut and look @ plugs

[lawrence]

Finally got time this afternoon to tinker with the IDAs. The motor would stumble and the carbs would pop when the throttle was barely open, so basically steady cruise around town and sometimes on the highway. Thought it was the airs but that does not jive with the symptoms.

I remembered that my 120 idle jet holders were enlarged when I had the motor dynoed. I was running a 60 idle jet at dyno time and currently have a 50, so I assume the guy drilled the holders out to lean the mixture. Not the best move. I borrowed unmodified idle holders that my friend has and put them into my carbs. The motor is much more civil now. Im stoked!!

[Jim Ratto]

Reading and re-reading the post with the old Dean Lowry and Motion Minicar stuff brought me back to this old post.
If you scan the material posted from the 1977 book, you'll see that the jetting was much like we discussed here (i.e. large fuel jets and small (er) air correctors.

Did we ever determine why the trend from this jetting evolved away from the small air corrector? I bet somebody reading this knows... 

I'm kind of obsessed about getting this answered. Put me out of my misery...

[Cheesepanzer]

Jim,
I got my "new in the box" Italian IDA's back in 1982 and they came with 37's and F7's.  Like you, I switched out the 37's for some 42's before ever bolting them on.      Somewhere along the line I bought a set of F2's, but wound up always running F7's.

Not trying to go off topic, but what's the general "rule of thumb" thinking regarding emulsion tubes?  As mentioned, I've run F7's forever, and was recently told on another forum to toss them in the garbage can.  In their place, I needed to get some F11's.

What's the number system F2 to F11 (and higher) mean?  Lean to rich?
 

[Bruce]

The emulsion tube number doesn't mean anything.  The numbers were applied in order of when that particular tube was designed.

[deano]

I think it would be a hot idea is to make a chart or spread sheet, list the engine size(s), compression, cam, valve size, headers, heads, etc, and then the jetting combination that is being used.

[John Rayburn]

Get to work on it , Dean.

[drgouk]

Who cares how you get there, the goal is to get the A/f or lambda line as straight and as flat as possible throughout the rev range. To me jetting charts are only a rough starting ponit.

[Cheesepanzer]

The emulsion tube number doesn't mean anything.  The numbers were applied in order of when that particular tube was designed.

Huh?

[cameron shorey]

The emulsion tube number doesn't mean anything.  The numbers were applied in order of when that particular tube was designed.

The numbers were assigned at random. Looks at the scans, to see what Bruce means;




Edited to make the pictures a bit easier to read.

[Jim Ratto]

Who cares how you get there, the goal is to get the A/f or lambda line as straight and as flat as possible throughout the rev range. To me jetting charts are only a rough starting ponit.

Well, I don't think an engine wants a "flat" a/f curve through the rev range. An engine runs at a different a/f ratio @ idle than it does @ light cruise or under full throttle full power


don't you guys wonder what happened (it almost seems like some guy discovered Weber made air corrector jets larger than 155).... how can a jetting pack at two extreme ends of the spectrum (talking air jets here) be "ideal"? Or maybe we can deduce that since some guys found ultimate jetting to be 150m x 130a and other guys found that ideal was 165m x 210a (in same application, same vent diam), that maybe there really are TWO ways to feed a motor the ultimate in mixture? I doubt it though
Where's Roger Crawford? I'm sure he could answer this for all of us. 

[Cheesepanzer]

OK, then I'm missing something then...  What's the difference between a F2 vs. F7 vs. F11, etc...  If the numbers don't mean anything, then why run one over the other?

[lawrence]

David, the emulsion tubes differ in diameter and orafice orientation. I posted some info a little further up the page and Jim posted a picture, which shows the differences. I would post more data, but don't have my weber manual here at work

[Jim Ratto]

I think guys run what they know worked for other guys, hence the F7 tube. I have tried them in my car with various engines and they did not respond as well as F2's, but I also only changed tubes. I didn't change jetting, nor somethign else that works hand-in-hand with emulsion tubes, the float levels.
About 8-9 years ago a friend and I were dyno-testing a 2276 IDA motor @ Heads Up in Fullerton. It was a street motor for a manx-style buggy, and we initially ran it with F7's, and made 207hp. We were told by Roger to change to F11's, not to touch timing or jets, and try again. We went to 213hp (best pull of the day). Knowing this, I tried F11's in my car last year, just for a spin around town, and it had a hiccup that I was too lazy to chase, so I just put F2's back in. But very interesting that the F11's woke the 2276 buggy motor up. I wonder if VW guys had access to more than the F7, F2, F11.... what developments would be found?

[Ohio Tom (DdK)]

Good topic Jim....

I have found that for most street/strip motors with IDA's in the 40-45mm vent range alway seem to make more HP with smaller than recommended air correctors.
Most applications use the F7 tube, but I have a set of F11's in my 2276 race motor (with 45vents) that I get to play with alot.

I have a couple sets of 190 and 200 air correctors in my tuning kit.
I have never ended up on a combination that worked best with them for any motor/application that I have had the honor of tuning on.
(I even took out my rod bearings once trying to run a 200 air combination at the track once.)
I always seem to land in the 160-175 range for most motors (for both air and Main jets). (square jetting, or there abouts)..

I have a buddy who runs F7's with 180 mains and 210 airs.. and it rips...
Go figure...

[Jim Ratto]

Good topic Jim....

I have found that for most street/strip motors with IDA's in the 40-45mm vent range alway seem to make more HP with smaller than recommended air correctors.
Most applications use the F7 tube, but I have a set of F11's in my 2276 race motor (with 45vents) that I get to play with alot.

I have a couple sets of 190 and 200 air correctors in my tuning kit.
I have never ended up on a combination that worked best with them for any motor/application that I have had the honor of tuning on.
(I even took out my rod bearings once trying to run a 200 air combination at the track once.)
I always seem to land in the 160-175 range for most motors (for both air and Main jets). (square jetting, or there abouts)..

I have a buddy who runs F7's with 180 mains and 210 airs.. and it rips...
Go figure...

Hi Tom (get Mike's oil pump?)
I have a close friend up in San Francisco/Oakland area (Pinole actually) that does race/hot-street 911 stuff, and he also swears by the air corrector being smaller than main on the 46IDA3C stuff he does for the hot Porsche stuff. He just dynoed a 3.4L motor with 50mm PMO carbs (Weber IDA3C copies) with 45mm vents and found best hp and a/f ratio running 185 main x 130 airs! Not sure which tube. We've gone around and around discussing "why" this works and we can't seem to understand "why"
He keeps offering for me to drag my Bug up to him and we'll spin it up on the dyno and see just what my thinking does or doesn't do.

My ex boss told me you want to lean out the top end fuel curve, and that is why you'll see some guys running 230+ a/c jets.

Still losing sleep...

[drgouk]

Who cares how you get there, the goal is to get the A/f or lambda line as straight and as flat as possible throughout the rev range. To me jetting charts are only a rough starting ponit.

Well, I don't think an engine wants a "flat" a/f curve through the rev range. An engine runs at a different a/f ratio @ idle than it does @ light cruise or under full throttle full power


don't you guys wonder what happened (it almost seems like some guy discovered Weber made air corrector jets larger than 155).... how can a jetting pack at two extreme ends of the spectrum (talking air jets here) be "ideal"? Or maybe we can deduce that since some guys found ultimate jetting to be 150m x 130a and other guys found that ideal was 165m x 210a (in same application, same vent diam), that maybe there really are TWO ways to feed a motor the ultimate in mixture? I doubt it though
Where's Roger Crawford? I'm sure he could answer this for all of us. 

To further explain, I was talking about jetting an engine on a Engine Dyno, such as a superflow 902 or a DTS like Rogers, where you are able to accelerate the engine under full load , from anywhere from 50rpm per second to 1000 rpm per second. We normally test at 200rpm per sec to simulate a 4th gear pull.
Why would you not want a flat lambda line? Isnt that why fuel injection is superior? I know you will never get it completly flat, due to the limitations of the adjustments you can make, mains, airs, emulsions etc, But it still remains the aim of the game. I will scan a lambda line off a formula ford engine i dyno'ed last week to show you what i mean. We even turn down emulsion tubes to adjust the fueling at a particular rpm.

[Jim Ratto]

Who cares how you get there, the goal is to get the A/f or lambda line as straight and as flat as possible throughout the rev range. To me jetting charts are only a rough starting ponit.

Well, I don't think an engine wants a "flat" a/f curve through the rev range. An engine runs at a different a/f ratio @ idle than it does @ light cruise or under full throttle full power


don't you guys wonder what happened (it almost seems like some guy discovered Weber made air corrector jets larger than 155).... how can a jetting pack at two extreme ends of the spectrum (talking air jets here) be "ideal"? Or maybe we can deduce that since some guys found ultimate jetting to be 150m x 130a and other guys found that ideal was 165m x 210a (in same application, same vent diam), that maybe there really are TWO ways to feed a motor the ultimate in mixture? I doubt it though
Where's Roger Crawford? I'm sure he could answer this for all of us. 

To further explain, I was talking about jetting an engine on a Engine Dyno, such as a superflow 902 or a DTS like Rogers, where you are able to accelerate the engine under full load , from anywhere from 50rpm per second to 1000 rpm per second. We normally test at 200rpm per sec to simulate a 4th gear pull.
Why would you not want a flat lambda line? Isnt that why fuel injection is superior? I know you will never get it completly flat, due to the limitations of the adjustments you can make, mains, airs, emulsions etc, But it still remains the aim of the game. I will scan a lambda line off a formula ford engine i dyno'ed last week to show you what i mean. We even turn down emulsion tubes to adjust the fueling at a particular rpm.

Dyno testing as such allows you to check actual throttle response under load, opposed to just a flat "pull" at peak hp rpm right?
Throttle response was something i was thinking about last night after re-reading this post.

Is jetting for max hp in a drag race situation different than setting up IDA's (jetting) for throttle response?

I'd guess the answer is "yes." What do you see on the dyno, as far as improvements in throttle response, by changing air correctors?

I see three "ways" to get an engine to see a "too rich" curve, aside from too much fuel psi or incorrect float levels:
1. go too large on idle and main fuel jets (obvious)
2. go too small on Venturi Diam (increasing signal too far to jet circuits)
3. go too small on air corrector jet, in turn, swaying the ratio of air/gas ratio in aux vents to a overly rich condition.

Most people know ideal a/f ratio for hp is 12.5-12.8, but does an air cooled motor live (at full power/load) at this a/f-r? Being the air cooled motor relies so much on it's "rich" fuel curve for cooling, I think we (air cooled guys) need to think in different terms than the GT40 or Cobra or DCOE (Alfa, Lotus, MG's, etc) guys.

Overall, aside from worn linkage eyelets on my Berg linkage, my car runs fairly well as a 95% daily driver, 272 @.050" cam, 9.7:1, large oval port heads, 42mm vents 170m x 175a IDAs, but I wonder where something is being left on the table. 

[Mike Lawless]

Most people know ideal a/f ratio for hp is 12.5-12.8, but does an air cooled motor live (at full power/load) at this a/f-r? Being the air cooled motor relies so much on it's "rich" fuel curve for cooling, I think we (air cooled guys) need to think in different terms than the GT40 or Cobra or DCOE (Alfa, Lotus, MG's, etc) guys.

Overall, aside from worn linkage eyelets on my Berg linkage, my car runs fairly well as a 95% daily driver, 272 @.050" cam, 9.7:1, large oval port heads, 42mm vents 170m x 175a IDAs, but I wonder where something is being left on the table. 

Having tried to apply all the stuff I learned in my time racing V8s, I can say for certain that the VW defies common race motor logic in almost every way. Most drag race V8s at the sportsman level use ROLLER cams shorter in duration to your street motor Jim, and smaller primary tube headers than most VWs use.

Do you thing the air corrector might be related to the intake tract pulse?


Anyway, I've found that it's best to just not to argue with the motor about what it wants!

[Jim Ratto]

Most people know ideal a/f ratio for hp is 12.5-12.8, but does an air cooled motor live (at full power/load) at this a/f-r? Being the air cooled motor relies so much on it's "rich" fuel curve for cooling, I think we (air cooled guys) need to think in different terms than the GT40 or Cobra or DCOE (Alfa, Lotus, MG's, etc) guys.

Overall, aside from worn linkage eyelets on my Berg linkage, my car runs fairly well as a 95% daily driver, 272 @.050" cam, 9.7:1, large oval port heads, 42mm vents 170m x 175a IDAs, but I wonder where something is being left on the table. 

Having tried to apply all the stuff I learned in my time racing V8s, I can say for certain that the VW defies common race motor logic in almost every way. Most drag race V8s at the sportsman level use ROLLER cams shorter in duration to your street motor Jim, and smaller primary tube headers than most VWs use.

Do you thing the air corrector might be related to the intake tract pulse?


Anyway, I've found that it's best to just not to argue with the motor about what it wants!

I'm sure the intake pulses/reverson affects which emulsion tube and air corrector is optimum.

Something else I thought about this afternoon, though I really wonder how valid this thought is (as far as the change in jeeting theory in last 30yr), manifolds for IDA's, generally, have gotten longer than they were in the past, haven't they? Most of the 70's IDA motors we see pics of have the short style manifolds with the carbs reversed from the Skat Trak tall style we all know today.
Plus...
Look at some of those cam specs shown in that old 1970's book that was posted, some of those cams are advertised with 342 degrees duration... much wilder than the K8, FK87, stuff that is run today (and usually on much bigger displacement motors today than yesteryear).
I'd love to know what was done to get the over 200hp from Bad Company's 1800cc motor (carburetor wise).

[Ohio Tom (DdK)]

Hey Jim. Yes I got Mike's oil pump. Thanks...

His 2110cc motor is gonna be super nice.
The rotating assembly is at the balance shop currently. Case work is done, just wating for the rotating assebly to come back so I can do a final clearance check.
K-Roc does real nice work...
Thing should ripp...

It never gets old building hot VW motors.. I love it...

[Lanny Hussey]

My best run to date (108.9 MPH) was with a 70/110-170/F7-180 stack. A tad fat but runs smooth and cool. The LM1 w. data shows all the info pretty clearly, addictive really. IIRC Alan Uyeno started running 170/170 years ago with excellent results.

[Sarge]

When we were selling new IDA's at Auto Haus in the 70's, they came with 37mm vents and 70/120 Idle, 130/F7/120.  Most of us new nothing of jetting back then and the engines these carbs wound up on were for the most part 1600-1700cc, Engle 110, 1 3/8" exhaust combos.  Seems like we found that 135 main 130 air worked well but the idle was always real fat and tended to load up easily in traffic.  Eventually, someone discovered 65 Idle fuel jets and that helped a bunch.  My 1835 with 125 cam and 1 1/2" exhaust ran a 145/F7/140 stack with 37mm vents as I recall and ran well.  The 2074 in my first rail ran 37mm vents with a 145/F7/170 stack and made 155 hp on Vittone's dyno at 5500 rpm.