[kwilford]

Your main jets mite be better at 135s then your current 130s, considering your engine is going leaner and leaner as rpms rise. That's my thinking anyways

After your Main jets are set, then you do your Idles jets / screw adjustments , and only after that would you start messing around with your air correctors.

I believe that's how Bob does it.....

I've sent Bob a request to chime in, so hopefully he does so.

I am considering increasing my mains to 135 from 130 (started at 140 and was too rich), but I think the next step is to try decreasing the air corrector sizes. Smaller air correctors DECREASE air to the emulsion tubes, so that will richen the mixture at mid and high rpm (air correctors apparently have little effect at idle and low rpm) .

And, again from what I've read, the order is:

1) main jets
2) air correctors
3) idle jets (based on mixture screw results)

RallyBob actually suggests leaving changing air correctors until AFTER the idle jets (and mixture screw setting) is on target. I think that might depend on how close the idle jet setting is after making the main jet change, and the profile of AFR's vs rpm.

Not sure when different auxiliary venturis and pump jets should be tried, but I assume early in the process.

Remember that idle jets affect everything a little. If you are too rich or too lean even up in the main circuit changing idle jets will often change your AFR by .5 or 1 unit on a DGAS.

I like my Idle at 11.8 AFR, and everything else to be no leaner than 13.5 unless I’m cruising steady at highway speed.

Those AFR's readings are VERY helpful Kyle, thanks!

Perhaps DCOE's function differently than downdraft Webers, but the guidance I have received so far is to always change idle jetting last. The idle jets are fed by the main jets, so the main jets should be settled on BEFORE attempting idle jet changes

 

[Knorm65]

I've sent Bob a request to chime in, so hopefully he does so.

I am considering increasing my mains to 135 from 130 (started at 140 and was too rich), but I think the next step is to try decreasing the air corrector sizes. Smaller air correctors DECREASE air to the emulsion tubes, so that will richen the mixture at mid and high rpm (air correctors apparently have little effect at idle and low rpm) .

And, again from what I've read, the order is:

1) main jets
2) air correctors
3) idle jets (based on mixture screw results)

Not sure when different auxiliary venturis and pump jets should be tried, but I assume early in the process.




Those AFR's readings are VERY helpful Kyle, thanks!

Perhaps DCOE's function differently than downdraft Webers, but the guidance I have received so far is to always change idle jetting last. The idle jets are fed by the main jets, so the main jets should be settled on BEFORE attempting idle jet changes

That’s true I forgot that the down drafts idles are fed from the mains. Lame! All that cool testing isn’t possible then. But sounds like you are on the right track. I used 135s btw
Have fun!

 

[kwilford]

Today I replaced the 170 air correctors with 160's. RallyBob's earlier post suggested that a given change in an air corrector's size is equivalent to a one-third change in a main jet . So a 10 decrease in the air correctors size is like a 3 or 4 increase in the main jets, and will mainly affect the mixture at mid to high rpm, which is the desired direction.

While it is best to make one change at a time when re-jetting, I also changed the pump jet from the 55 to a 60. A jet pump change shouldn't affect steady-state running, just "tip-in", and there was a bit of a bog when stepping on the gas.

I considered also changing the auxiliary venturis from the 3.5's to 4.5's, but the ones I had ended up being narrower, so they didn't fit.

I happened to have a different set of auxiliary venturis that were the same 3.5 size, but had a slightly different design, with a brass tube that went across the venturi and a small slot in the bottom. I don't know that they are actually better, but I simply like the brass tube design. I've attached a couple photos in case someone knows any details.

Next post is the result of these changes.

 

[kwilford]

Decreasing the air corrector sizes from 170 to 160 worked well, but didn't get the full change desired. Here are the Air/Fuel Ratios (AFR's) before and after the air corrector change:

Main jets: 130
Air correctors: 170
Idle jets: 45, best at 1 1/2 to 2 turns

Idle 1000 rpm: 12.7:1
2000 rpm: 13.7:1
3000 rpm: 14.5:1
4000 rpm: 15.3:1
So pretty good AFR at idle, but somewhat too lean at mid to high rpm.

Air correctors reduced to 160, aux venturis changed to brass tube style, same size; pump jet changed to 60 from 55:

Idle 1000 rpm: 11.4:1
2000 rpm: 12.0:1
3000 rpm: 12.6:1
4000 rpm: 13.8:1
4500 rpm: 15.0:1

Best idle (highest vacuum) at 1 3/4 to 2 turns

Aside from the smaller air correctors richening AFR's, the throttle response was much crisper, the tip-in throttle response is very smooth, and the AFR's are much more stable (less fluctuation) throughout the rpm range.

I would like a slightly leaner idle (say 12.5:1 vs 11.4:1), and a bit richer at high rpm (~13.5:1 vs 15.0:1 at 4500 rpm). I think I'll start by increasing the mains from 130 to 135, adjust the air correctors to shape the mid-rpm AFR, then decrease the idle jets from 45's to 40's.

These changes are all predicated on assuming that the steady rpm/no load AFR's translate to driveability and power. We're a few weeks away from dry warm roads in Calgary, so final jetting will require significant real-world driving.

Please stand by...

 

[RallyBob]

Keith, under load the power valve will come into play and richen the mixture.

This is something that can’t be simulated sitting in a driveway.

 

[Vincent]

Keith, under load the power valve will come into play and richen the mixture.

This is something that can’t be simulated sitting in a driveway.

X2

Tuning in a driveway doesn't work.
My first ride on the road was a complete wake up call....

 

[kwilford]

Keith, under load the power valve will come into play and richen the mixture.

This is something that can’t be simulated sitting in a driveway.

Thanks Bob, good to have your eyes on this thread.

Yea, I knew that running the engine while sitting on the driveway isn't even close to a "real world" test. Or, in this case, in my heated garage with exhaust hoses sticking out under the door, 'cause it's still bloody unseasonably winter-like in Calgary for the first week in April. Not that I can go anywhere, what with the COVID-19 lock-downs. Even Banff National Park is closed...

So what might I do in the meantime? I think the idle is still a bit rich, and the mixture adjustment isn't providing much joy in reducing that. Setting the screw leaner causes the engine to stumble at idle, and in this case, using a vacuum gauge to fine-tune the idle mixture seems to work very well. Perhaps just leave it be at 11.4:1 until I get some miles on it?

That is a very good point about the power valve, so let's discuss power valves for a moment.

I assume that the power valve only comes into play (or should only) when the throttle is nearly wide open, such that port (and manifold?) vacuum is very low. That reduces the vacuum pull on the diaphragm (in the underside of the cover), which allows the spring to push the rod down, which opens the power valve in the float bowl base. If I read the Weber manuals correctly, that essentially bypasses fuel around the main jets, through a "metered tube" (which seems to be fixed, as in not adjustable or replaceable), significantly increasing fuel to the aux venturis and into the intake.

Mine is set to close at a touch under 12" Hg (one spring loop cut off, as it was at 14" Hg), and the idle vacuum is a touch over 15" Hg. I noticed that once the spring is retracted (ie, power valve is closed), then slowly releasing vacuum from the port, the spring doesn't bounce back to "valve open" until vacuum is reduced to about 6" Hg. Perhaps a bit of hysteresis, and it is repeatable. That indicates that the power valve won't open until the carb port vacuum is significantly reduced, such as at WOT. I suppose the accelerator pump circuit kicks in first, but if I get a stumble at full acceleration, might be worth looking at the power valve again.

For clarity, unless "cruising" entails a very wide throttle setting (like high, high speed!), causing low carb port vacuum, the power valve isn't (or shouldn't) be open.

Does that all sound correct and proper?

In the meantime, I am going to make a few additional (small) jetting changes, mainly to determine the effect, but to also get a feel on engine running at various AFR's...

 

[Vincent]

@kwilford

One of the most educational tests I did is run my car with a DISABLED power valve . It REALLY opens your eyes on how the power valves effects the overall carburetor functions across the rpm range under various loads!

What I found is my Mains jets simply couldn't react fast enough to the requirements of moderate to heavy acceleration. Unless you'd accelerate smoothly and gradually the mixture would always go lean initialy, and then catch up shortly after.

Long story short, power valves do a great job of smoothing out those low vacuum transitions!

 

[Vincent]

Drove almost 100 kms today, and I can say I have no real need to fool around with my Weber 38 dges carb. Car runs fantastic, start good hot or cold.

It can pull cleanly from 2000 rpms in 4 th gear!!!!!

-Idles very well at 1100 rpms and 12.5 fuel air ratio.
-Cruise down the highway I'm at 14-15 .
-Accelerating I'm at 12 -12.5
-Floored 4000 to 6000 rpms 12-12.5
-Cruising at 30 mph 3 rd gear 13-13.5


I'm a happy camper!

 

[Vincent]

Mains 145
Idles 50
Screws turn out about 1 1/4 to 1 1/2
Air correctors 170
Emulation tubes F66
Idle vacuum 11"hg
powervalve opens @ 8"hg

Ignition adv.
15 deg @1100 rpms
36 Deg @ 3300 rpms

Elevation plays between 500 to 1000' above sea level.

 

[Vincent]

@kwilford..... Any updates on your end?

 

[kwilford]

@kwilford..... Any updates on your end?

Patience, Grasshopper....

Yes, re-jetting a Weber is like a marshal art. And I missed all those Kung Fu classes.

On top of re-jetting the Weber 38DGES, I have also been making adjustments to the distributor mechanical advance. To recap, it is a '75 dizzy, has a locked vacuum plate (no retard pot, never had an advance that year) and has adjustable mechanical advance using set screws. Pertronix II Ignitor and FlameThrower coil (but the 3 ohm unit, as I started with a Pertronix I that Sterling needed more than me). The advance target is 12 to 15 degrees BTDC at 1000 rpm idle, maxing at 36 degrees at ~3500 rpm.

I am "pretty" happy with the current tune, so I took my GT out on its very first long'ish road trip yesterday, totaling 180 km (112 miles). It was a good mix of highway and city driving, from my house in NW Calgary, down to John Warga's acreage (the Calgary Opel Parts Co-Op storage) south of the city, to drop off and pick up some co-op parts. Then to Tom Bradshaw's (not sure of his user name here) to see his progress in dismantling his teal-green GT (some photos attached) and drop some co-op parts off, then all the way Vince Tucker's place to see his GT re-assembly (forgot to take photos) and back home. Odometer was accurate to within 1 km of 180 as tracked by my Garmin GPS, but my NOS zero km Euro speedometer reads about 9% high. Might prevent a few speeding tickets. Maximum speed attained was 128 km/hr (~80 mph) so I still need to watch my right foot. Car ran very well, except for a slightly leaky heater valve ( a fancy thermo-controlled NOS unit, but hey, likely 30 years old) which will be replaced with an OGTS valve loaned to me by Vince.

Here is a copy and paste from the notes I have been making along the path to Weber and distributor advance enlightenment. Its a bit wordy, but what you guys might expect from me.

Revised jets: 2020-04-05 a)
Venturi's: 3.5 (w/ brass cross tubes)
Pump jet: 60 (double)
Emulsion tubes: F66 (both sides)
Main jets: 135 (both sides)
Air correctors: 160 (both sides)
Idle jets: 45 (from 40's, both sides) best at 1 3/4 turns inboard, 2 3/4 turns outboard (adjusted w/ vacuum gauge)

Idle 1000 rpm: 12.3:1
2000 rpm: 13.7:1
3000 rpm: 15.3:1
4000 rpm: 15.4:1
4500 rpm: 15.2:1
Too lean thu-out, stumble off idle, idle uneven
Tried 50 i/b idle jet, best at 2 turns, idle AFR improved to 12.1:1 but still too lean throughout

Revised jets: 2020-04-05 b)
Main jets: 140 (from 135, both sides)
Air correctors: still 160 (both sides)
Idle jets: same 45 o/b, 50 i/b, best at 2 turns inboard, 3 turns outboard (adjusted w/ vacuum gauge)
Idle 1000 rpm: 12.6:1
2000 rpm: 13.9:1
3000 rpm: 14.8:1
4000 rpm: 13.9:1
4500 rpm: 13.9:1

Road test 2020-04-30:
1000 rpm 17 deg, 12.0:1
2000 rpm 27 deg, 13.5:1
3000 rpm 35 deg, 14.0:1 & too lean
3500 rpm 36 deg, 14.5.1 & too lean
Stumble off-idle tip in
13.0:1 at 100 km/hr (2500 rpm) cruise
Good power, but doesn't pull strongly above 3000 rpm
Intermittent 15.0:1 lean when accelerating

Revised jets: 2020-04-30
Main jets: 145 (from 140, both sides)
Air correctors: 170 (from 160, both sides)
Idle jets: 50 o/b, 50 i/b, best at 2 1/2 turns inboard, 2 1/2 turns outboard (adjusted w/ vacuum gauge)

Idle 1000 rpm: 12.5 to 13.2:1
2000 rpm: 14.6:1
3000 rpm: 15.1:1
4000 rpm: 16.0:1 and lean
4500 rpm: 13.5:1
Road test:
Idles better but not great after turning screws out, AFR still too lean; fair to good power, AFR 14.5:1 at 100 km/hr (2500 rpm) cruise, richer under acceleration, too lean at mid-range

2020-05-02
Re-locked vacuum plate at advanced position (was retarded)
900 rpm: 12 deg, 12.0-13.0:1, 11"Hg
1000 rpm: 20 deg, 11.5-12.5:1, 13" Hg
2000 rpm: 23 deg, 12.2-13.1:1, 18" Hg
3000 rpm: 35 deg, 13.3-13.6:1, 21" Hg
3500 rpm: 36 deg, 13.5-14.1:1, 20" Hg

Main jets: 150 IB/155 OB (from 145 both sides)
Air correctors: 180/180 (from 170's)
Idles: still 50/50, best at 2 turns IB, 2 1/4 turns OB
Installed Bosch Platinum WR7BP plugs
1000 rpm: 12.6-12.8:1, 16 deg.
2000 rpm: 13.1-13.5:1, 22 deg
3000 rpm: 14.0 to TOO LEAN, 35 deg
3500 rpm: 13.5-14.2:1, 35 deg
Intermittent stumble on tip in, good power thru band, lean at 3500 rpm under power

2020-05-05
Main jets: 155/155 (was 150/155)
Air correctors: 175 IB, 180 OB (was 180/180)
Idles: still 50/50, best at X turns IB, X turns OB
1000 rpm: 11.8-12.5:1
2000 rpm: 12.9-13.1:1
3000 rpm: 14.0-TOO LEAN
3500 rpm: 13.2-13.8:1
4000 rpm: 13.0-13.2:1
4500 rpm: 12.4-12.8:1
Pulls strong from 3000 to 5000 rpm, still leans out at 3000 to 3500 rpm under load, but no stumble
2500 rpm, 5th gear, 110 km/hr, 13.5-15.0:1
3200 rpm, 4th gear, 110 km/hr, 13.4-14.5:1
4100 rpm, 3rd gear, 100 km/hr, 14.5- 15.0:1

It surprises me that I am running such large main jets, at 155 both sides. Bob's suggestion is to run the air correctors 30 to 40 larger than the mains, less at higher altitude (I live at 3000 ft ASL), but I am running only 20 and 25 respectively to make the mid and high rpm AFR rich enough without over-fueling.

I'll pull the spark plugs (Bosch Platinum) tomorrow to see what they tell me, but the Champion plugs that came out a week ago looked almost perfect, with a couple hundred km on them.

Likely not the absolute final set up, but good enough for now.

HTH

 

[Vincent]

Car looks good! I'm also surprised you need such large mains. Heck your motor is running stock valves!!!????... Whatever works I guess. Crazy how different our engines carb's settings are.

Thanks for sharing all your info.

 

[Vincent]

@kwilford

School me on air correctors.... The larger the number the leaner or richer the jetting effect? Do they mainly take effect at higher rpms?

 

[Commodåren]

The larger the number, the leaner the effect. And yes, they effect at higher RPMs.

 

[kwilford]

@kwilford

School me on air correctors.... The larger the number the leaner or richer the jetting effect? Do they mainly take effect at higher rpms?

The following is my general understanding of Weber downdraft carbs. The truth might be better found by reading an authority on Weber carb function. Or reading what some of our more knowledgeable members have to say

Air correctors mainly affect mid to high rpm operation. Main jets in downdraft Webers also affect the flow of fuel to the idle jets, which is why main jets are set first, BEFORE idle jets. Might need to be done in steps, but idles AFTER mains. THEN air correctors. Emulsion tubes ALWAYS first, before all else. Air correctors are for fine tuning AFR's at mid to high rpm.

The air correctors control the amount of air entering the top of the emulsion tubes. The fuel enters the emulsion tubes at the bottom and sides of the tube, via the main jets, or via the power valve which bypasses the main jets when it is open (at low port vacuum). Air from the air correctors combine with fuel from the bowl in the tubes, and emulsify (makes a mixture of liquid fuel and air) that is then conveyed to the auxiliary venturis (the thing thru the middle of the main venturis) that delivers fuel (actually a fuel/air emulsion) to the engine. For clarity, this emulsion is mainly liquid, and not atomized. That happens in the main venturi.

The emulsion tubes have holes and internal orifices that control the amount of fuel (and hence emulsion) according to "draw", i.e. carb throat air flow and vacuum, that in turn lowers the fuel level in the bowl at more draw. That provides a specific profile of fuel/emulsion according to throttle position and air flow. I believe the emulsion tube designs (eg, F50 vs F66) "customize" the transition of fuel to throttle and air flow. The recommendation of F66 emulsion tubes comes from RallyBob, who basically determined that by trial and error.

Larger air correctors (bigger number) allow more air into the emulsion tubes. Smaller air correctors (smaller number) allow less air into the emulsion tubes. The amount of air in turn changes the rate of fuel ingested to the emulsion tubes. More air, less relative fuel. Less air, more relative fuel.

That is a long way of saying that a larger air corrector leans out the mid-to-high rpm air/fuel mixture. Smaller air correctors richen it up.

According to our esteemed RallyBob (my paraphrase):

"Higher altitude, leaner (smaller) jets and/or larger air correctors
Set air correctors to be about "30" to "45" higher than main jets. If you have a 140 main jet, you'll probably need somewhere between a 170 to 185 air corrector. The exception is with a hot cam, it'll need about "20" to "30" higher, this will allow less air to emulsify at higher rpms, making the mixture richer at those rpms.
The air corrector primarily affects the mid-upper rpm range, not so much the lower-middle range. Three changes in air corrector equivalent per one change in main"

In other words, a numerical change in air corrector size is equivalent to one-third (1/3) that of a main jet change.

HTH (and hopefully Bob or Kyler will correct any errors in my understanding)

 

[The Cub]

The following is my general understanding of Weber downdraft carbs. The truth might be better found by reading an authority on Weber carb function. Main jets in downdraft Webers also affect the flow of fuel to the idle jets, which is why main jets are set first, BEFORE idle jets. Might need to be done in steps, but idles AFTER mains.

I thought this was worth mentioning. When I was beginning to learn about jetting I spoke with Mike (the guy who made the Weber videos) at Pierce and told him the same thing, I repeated this to him fully believing that the mains dictate the how the idle circuit gets jetted. He said, in a rather loud voice I might add that the idle circuit(s) are totally unrelated to the main(s) and I could remove the main jets & air corrector jets completely and get no change on the idle circuit.

 

[kwilford]

I thought this was worth mentioning. When I was beginning to learn about jetting I spoke with Mike (the guy who made the Weber videos) at Pierce and told him the same thing, I repeated this to him fully believing that the mains dictate the how the idle circuit gets jetted. He said, in a rather loud voice I might add that the idle circuit(s) are totally unrelated to the main(s) and I could remove the main jets & air corrector jets completely and get no change on the idle circuit.

I have three books on Weber tuning and jetting. The downdraft Webers DEFINITELY flow fuel from the main jets to the idle jet circuits, and so changing the main jets DEFINITELY affect the idle mixtures.

Not so on the side draft Webers.

Mike might have been referring to side draft Webers...

 

[Autoholic]

I keep going in circles on does the main jet impact the idle circuit. Ideally, I think the two should have been separated. The main jet will always flow more fuel than the idle jet during transition. [removed chunk that didn't make sense now ]

 

[Autoholic]

Looking at the below image, which was found in this thread...

While the idle & transition circuit is connected to the main jet, it's the idle jet that is regulating how much fuel from the main jet gets into the idle & transition circuit. This pathway is would influence how much fuel gets to the emulsion tubes at higher RPMs. So, the main jets would not impact the idle & transition circuit but the idle jets would impact the main circuit.