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Discussion Starter · #1 ·
I have a carburetor tunning question that I hope to gain some insight from others out there who have tunned a Webber 38 DGES using an AFR gauge. Two questions really.

  • Should you tune the idle circuit (at engine idle) using the stochiometric AFR number of around 14.9, rather than the Webber recommended ear/listening method? (With newer fuels and all my actual AFR idle target is around 13.9 to 14.5).
  • The other question regardless of the answer, is there a method for moving between the two barrels on a 38 DGES when adjusting the idle mixtures needle valves?
These are my basic question… the rest below is back ground and general INFO

When I adjust the idle to the AFR numbers, the engine idles rough, clearly not the best idle according to what Webber recommends. However, this rough idle setting brings me closer to the AFR range at transition and low load cruising RPM and it seems to pull nice. When I tune to hearing smoothest idle, the engine runs rich, around 12.5 on the AFR at idle and the whole low end cruising engine circuit runs richer than I would like 13 to 13.5.

No vacuum leaks, power valve has been checked for my engine vacuum range. Carburetor jetting is currently;

  • 3.5 Aux. Venturis
  • 50 idle Jets
  • 155 mains
  • 185 Air Correctors
For a smoother idle, I’m about 1- 7/8 turns out on the engine side idle needle and 1- ¾ turns out on the outboard side. I can move these in by ¼ turn to move to the AFR range, the engine only looses about 50 RPM but rough… I'm always around 1/2 to 5/8 turns in on the idle adjustment screw ( I generally shoot for 1000 RPM with the hotter CAM)

Gen Info on engine: Recently rebuilt with about 7,000 miles on it. It’s a 1971, 1.9L block bored out to 2.0L, flat top pistons, larger valve size with a combination cam @0.430 lift and 268 duration (so it already has somewhat of a rougher idle than stock with a lower vacuum signal). I am running 90 octane, nonalcohol fuel. I have removed the Vacuum retard but did keep the vacuum advance on the distributer. The engine is timed with no Vac. Advanced hooked up to 36 Deg Advanced when mechanical advance is all in.

Thanks
Thadman
 

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I am surprised that you can get a 1.9 CIH with a 38DGES to even RUN at idle with a AFR of 14.7:1. Mine idles nicely at 12.5:1.

Idle jets seem a bit big at "50", but whatever works for your engine is the thing. It is a combination (float level, idle and main jets, air correctors, and emulsion tubes) that need to be iterated to get the best operation. The AFR is best used to look for changes and trends, NOT an absolute reading

This is what I do (pretty much the Weber procedure):

1) Adjust each idle mixture screw to obtain the highest manifold vacuum and highest, smoothest idle.

2) Check the mixture screws settings. If more than two turns out, replace the idle jet with one "5" larger If less than 1 1/2 turns out, replace the idle jet with one "5" smaller.

3) Repeat.

Similar with main jets and air correctors. Seat of the pants, spark plug colour, fuel efficiency, AFR reading; in that order from most to least important.

What is your idle manifold vacuum? Have you checked your power valve spring? Has to be done absolutely first for this engine (which is almost exactly what I run)

HTH
 

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By the bye, if you open this Forum, there are a half dozen "stuck" threads on jetting Weber downdrafts carbs, and two on using an AFR...
 

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I have a carburetor tuning question that I hope to gain some insight from others out there who have tuned a Weber 38 DGES using an AFR gauge. Two questions really.



No vacuum leaks, power valve has been checked for my engine vacuum range. Carburetor jetting is currently;

( I generally shoot for 1000 RPM with the hotter CAM)

Gen Info on engine: Recently rebuilt with about 7,000 miles on it. It’s a 1971, 1.9L block bored out to 2.0L, flat top pistons, larger valve size with a combination cam @0.430 lift and 268 duration (so it already has somewhat of a rougher idle than stock with a lower vacuum signal). I am running 90 octane, nonalcohol fuel. I have removed the Vacuum retard but did keep the vacuum advance on the distributor. The engine is timed with no Vac. Advanced hooked up to 36 Deg Advanced when mechanical advance is all in.

Thanks
Thadman
Total advance is great but you may need more initial advance for a smoother idle and better throttle response. There are threads in the forum of various methods members have used to limit total advance.

Harold
 

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Mite be worth trying smaller main jets. I have a very simular engine to yours and it runs fare better using 140 or 145 main jets .

I tried 155 mains for one ride and it was like I lost 20 hp
 
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I am surprised that you can get a 1.9 CIH with a 38DGES to even RUN at idle with a AFR of 14.7:1. Mine idles nicely at 12.5:1.

This is what I do (pretty much the Weber procedure):

2) Check the mixture screws settings. If more than two turns out, replace the idle jet with one "5" larger If less than 1 1/2 turns out, replace the idle jet with one "5" smaller.

HTH
I believe 1 turn is recommended by Redline for the 38 DGAS/DGES carbs.

Harold
 

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Discussion Starter · #7 ·
By the bye, if you open this Forum, there are a half dozen "stuck" threads on jetting Weber downdrafts carbs, and two on using an AFR...
Thanks Keith,

Yes, lot of good info in the forum and have read through them,,, maybe should do it again..

It appears as if the AFR doesn't mean much in your tunning practices,, I get that, it's not absolute but you would think if pure clean combustion occurs at 14.9:1 you would be closer to that number at best idle than 12.0:1..... I have read enough on jetting a 38 to a 4 cylinder to know that You cannot follow redline tunning suggestions completely since they were made for 6 cylinder engines to start with. I think I will tune to best idle and then watch the AFR at cruising and transition, if it's to rich I'll try a smaller idle jet,, even if best idle is at over 2 turns out.. Keep fiddling..

Thanks again for your input..
 

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Discussion Starter · #8 ·
Total advance is great but you may need more initial advance for a smoother idle and better throttle response. There are threads in the forum of various methods members have used to limit total advance.

Harold
Thanks Harold,,,

The 1971 distributer has a full in mechanical of 28 to 32 deg... That was the year they started to tune them down a bit for emissions and so on.. So, timeing to 36 deg advance at full in mechanical is giving me about 7 to 8 initial, then when I hook up the vacuum advance it will pull that to 12 deg.. With the vac. advance I was thinking I had to much but not hearing any pre-ignition,, so that's where I am at for now..
 

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If you're pulling vacuum from the ported tube on the carburetor at idle then you're no longer in the idle circuit.
Higher vacuum is produced when there is little or no load on the engine and may be okay.

Harold
 

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Discussion Starter · #10 ·
Mite be worth trying smaller main jets. I have a very similar engine to yours and it runs fare better using 140 or 145 main jets .

I tried 155 mains for one ride and it was like I lost 20 hp
I'll keep that in mind,,, Right now however, the upper performance end seams to be OK,,, I would think the lower end and transition RPM is controlled more,, if not completely,, by the idle jet size and not the mains, since the fuel feeding this circuit comes in from the lower part of the emulsion well , below the emulsion tubes, and should be at float level with little to no demand from the primary circuit pulling fuel away. But it is a balancing act I know, so if I'm still rich down low and can give up a little on the upper end, I'll give it a try..

Thanks again.
 

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Discussion Starter · #11 ·
If you're pulling vacuum from the ported tube on the carburetor at idle then you're no longer in the idle circuit.
Higher vacuum is produced when there is little or no load on the engine and may be okay.

Harold
I moved my advance to the manifold were the vacuum is stronger at steady state but drops to nothing during hard acceleration. In this condition, hard acceleration, the mechanical takes over and keeps me in the 36 deg. advance range. Once the car settles in, advances through the manifold adds a bit more advance for fuel economy.. But it's something to think about, maybe that extra advance is what causes the need for a rich mixture at idle.. Hmmmm...
 

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I moved my advance to the manifold were the vacuum is stronger at steady state but drops to nothing during hard acceleration. In this condition, hard acceleration, the mechanical takes over and keeps me in the 36 deg. advance range.
I would think the distributor vacuum would be the same from either one once under way. The distributor just won't see it until you crack the throttle. There will be little to no vacuum from either port under WOT regardless of how you have it hooked up.

Harold
 

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Discussion Starter · #13 ·
Anytime you are pulling and building speed you are higher in vacuum in your carbureted port than in your manifold. So,, under these conditions, if you had the lines switched you would pull on your distributer vacuum advance during acceleration adding 4 to 7 degrees advance above your mechanical and this would be too much for general driving, with standard fuels and such, even with a moderate performance engine (potential 40 to 42 deg. advanced during hard acceleration). With your vacuum advanced hooked to the manifold, you are providing a little advance at idle which helps tunning low end and at steady state cruising. The added advance helps with cooler head and valve temp. soon as you hit the gas, manifold vacuum neutralizes removing the 4 to 7 deg. advance you see at idle or steady state and your performance mechanical distributer settings take over. Most hot rods that wanted some general drivability were set up this way back in the day.

In 1971, for meeting emission standards, manufactures added a vacuum retard to the distributers as well and ran the advances off the ported vacuum side and the retard off the manifold.. but for this to work you need to dumb down engine performance and have both hooked up... another story here...
 

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"With your vacuum advanced hooked to the manifold, you are providing a little advance at idle which helps tunning low end and at steady state cruising. The added advance helps with cooler head and valve temp. soon as you hit the gas, manifold vacuum neutralizes removing the 4 to 7 deg. advance you see at idle or steady state and your performance mechanical distributer settings take over. Most hot rods that wanted some general drivability were set up this way back in the day."

Perzactly. That's how I set up all my old cars.
 

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Bob said: "I am surprised that you can get a 1.9 CIH with a 38DGES to even RUN at idle with a AFR of 14.7:1. Mine idles nicely at 12.5:1."

I doubt you are going to get a carbed engine to idle at 14.9:1. They are just not as precise as the modern feedback computer controlled engine management systems.

Even and ecu controlled engine WITHOUT feedback to the ecu won't like to idle there. My 83 745i is that way. It is ecu controlled fuel injection, but does not have "open loop" and closed loop". There is no O2 sensor from the factory (unlike the U.S. models). I put an afr meter in the cabin (along with fuel pressure, oil temp and vac/boost) so I would always know what is going on.

It runs 14.6 to 15.2 at cruize, idles at 12.9 to 13.1 and under heavy boost, can get down to 11.0:1. Probably a little rich there, but I can't get my RRFPR to adjust fuel pressure any lower at 10 psi boost.
 

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In reading your posts it looks like you have a very good understanding about what you’re doing. I have a 2.0, I set up my cam with a 4° advance bushing using 405 lift/256 duration cam, less aggressive than yours. I can get my 38/38 to idle nicely at 14.4:1 and that’s about it, no higher. My only problem with this set up was at closed throttle on fairly aggressive deceleration was way too lean going above the 18:1 burn limit and too much stinky sputtering from the exhaust. I’ve got it at its happiest at idle between 12.6-13:1 and it runs about 13-16:1 closed throttle deceleration the engine sounds fine with no little sputters and a much cleaner burn.

On tuning, I too had a struggle with everything let me just put it that way. I prefer the f-66 e-tubes, but if you make that change be sure to raise the float level, I recommend using the brass float. I didn’t get the good consistency on the plastic float. The main jets do have plenty of effect on the progression/transition circuit. My 2.0 seemed to like it between the 47-50 idles, I tried many, many combinations on the mains and ended up using the 145 mains with the 185 air correctors. The 47 idles were a bit too lean and the 50’s were just a bit on the rich side. I ended up drilling the 47’s to 49’s and she purrs like a kitten with smooth transition and good power all the way through. I tended to start lean, find the happiest setting on the idle mixture screws for the smooth idle and transition & work my way slowly to being just enough on the rich side to get the good performing afr numbers.

It also likes a lot of ignition timing, just over 13° of mechanical advance at 800-850 rpm idle with 35° all in at 3,500 rpm’s. Add the 7° vacuum advance brings it right at 42° all in on the highway. I discovered that moving the vacuum advance to the manifold offers a much smoother stop & go. With that much initial advance, a zero to 7° of additional advance going up & down with the throttle opening and closing at low & higher speeds the cylinder filling is just a lot smoother. I’m averaging 29 mpg combined. My happy place with this little car is to get its best power and gas mileage as both priority one, Having the afr gauge lets me look at its performance, driving at 13.5:1 is where it’s frequently at. in the 14’s on the highway.

Here’s what took me so long to understand, with the Weber there’s a lean spot towards the top of the progression stage, that’s normal, it’s when you’re cruising on the highway and saving fuel. The CIH likes fuel so what I’ve found, the way I have it set up, I do most of my driving with plenty of power 12:1-14:1 driving under that lean spot. I used to get hung up on trying to stay lean all the time. That’s where the afr numbers were tricky, jetting leaner gave me good afr numbers 13-16:1 the engine ran smoothly but because I was driving more on the mains on or past the lean spot with more throttle my performance and mileage didn’t fair as well. As previously mentioned there’s a fine line in giving it too much fuel, you can be doing 12.4:1 with a spongy pedal and using too much throttle. The air fuel ratio gauge doesn’t do it all, especially when you’re on the rich side. Here’s how the 38/38 I have is set up. I use 91 octane California e-10 gas.


Idle 49’s Mains 145’s Air’s 185’s

F-66 emulsion tubes

Pump jet 65

Brass Float 38/48mm (with gasket)

Idle 12.8:1 turns: 7/16 inner screw 1 full turn outer screw

¾ turns in on speed screw

Cruise 50 mph steady/tip in 12:5 to 13.5:1

part throttle decel 11.2-12.5 mostly 11.8-12.3:1

Cruise highway 14.5-15’’s mostly upper 14’s occasional 16-17:1 blip on tip in

part throttle highway decel 12.6-13:1

Light acceleration 12.5-15.5:1

Aggressive acceleration 12.6:1- 13.5:1

Hard decel throttle closed 13.5 to 15.8:1

WOT 12.6:1

Good all the way around

5/14/22 changed distributor bushing from 5.56mm to 5.4mm, not much of a change. 34° total 12.5° idle

7/5/22 re adjusted distributor to 13.5° at idle & 35° total add 7° vacuum advance for 42° total part throttle. Moved from ported to manifold.
 

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Anytime you are pulling and building speed you are higher in vacuum in your carbureted port than in your manifold. So, under these conditions, if you had the lines switched you would pull on your distributor vacuum advance during acceleration adding 4 to 7 degrees advance above your mechanical and this would be too much for general driving, with standard fuels and such, even with a moderate performance engine (potential 40 to 42 deg. advanced during hard acceleration). With your vacuum advanced hooked to the manifold, you are providing a little advance at idle which helps tuning low end and at steady state cruising. The added advance helps with cooler head and valve temp. soon as you hit the gas, manifold vacuum neutralizes removing the 4 to 7 deg. advance you see at idle or steady state and your performance mechanical distributor settings take over. Most hot rods that wanted some general driveability were set up this way back in the day.

In 1971, for meeting emission standards, manufacturers added a vacuum retard to the distributors as well and ran the advances off the ported vacuum side and the retard off the manifold.. but for this to work you need to dumb down engine performance and have both hooked up... another story here...
My experience back in the day switching vacuum sources causing pinging, so I switched back. I hook mine up the way I suggested. According to most articles I've read your recommendations are correct. Who knows, I may try it again. Goodness knows, I don't like having to use 89 octane in an low performance car.

Opel was using vacuum retard before 1971 in the US models.

Harold
 

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Carbs do not run well at 14.7 idle on any car (unless it’s a severely undersized carb). Opels like 12.0-13.0:1 in my experience. 12.8:1 is the sweet spot for me! The Opel CIH is notorious for over scavenging and needing significantly more fuel than stoicomeric at all times.

I aim for around 12.5:1 everywhere with my DCOE.

With a carb that has smaller venturies and more velocity you can afford go leaner, but you aren’t going to get good cruising or idle performance leaner than 13.5:1 even with a 32/36 in my experience.

There’s more to it than just tuning to 14.7. There’s also emulsion, atomization, temperature of fuel (and exhaust/ combustion temps), and varied mixture requirement of center cylinders vs outer cylinders on a CIH to consider.


Also I noticed you are talking about vacuum advance during hard acceleration. During hard acceleration you will not have ANY vacuum advance unless your carb is undersized. Full throttle should have close to zero vacuum at the manifold and carb port. Vacuum advance only operates at partial throttle. Never at idle, and never at WOT.
 

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Discussion Starter · #20 ·
In reading your posts it looks like you have a very good understanding about what you’re doing. I have a 2.0, I set up my cam with a 4° advance bushing using 405 lift/256 duration cam, less aggressive than yours. I can get my 38/38 to idle nicely at 14.4:1 and that’s about it, no higher. My only problem with this set up was at closed throttle on fairly aggressive deceleration was way too lean going above the 18:1 burn limit and too much stinky sputtering from the exhaust. I’ve got it at its happiest at idle between 12.6-13:1 and it runs about 13-16:1 closed throttle deceleration the engine sounds fine with no little sputters and a much cleaner burn.

On tuning, I too had a struggle with everything let me just put it that way. I prefer the f-66 e-tubes, but if you make that change be sure to raise the float level, I recommend using the brass float. I didn’t get the good consistency on the plastic float. The main jets do have plenty of effect on the progression/transition circuit. My 2.0 seemed to like it between the 47-50 idles, I tried many, many combinations on the mains and ended up using the 145 mains with the 185 air correctors. The 47 idles were a bit too lean and the 50’s were just a bit on the rich side. I ended up drilling the 47’s to 49’s and she purrs like a kitten with smooth transition and good power all the way through. I tended to start lean, find the happiest setting on the idle mixture screws for the smooth idle and transition & work my way slowly to being just enough on the rich side to get the good performing afr numbers.

It also likes a lot of ignition timing, just over 13° of mechanical advance at 800-850 rpm idle with 35° all in at 3,500 rpm’s. Add the 7° vacuum advance brings it right at 42° all in on the highway. I discovered that moving the vacuum advance to the manifold offers a much smoother stop & go. With that much initial advance, a zero to 7° of additional advance going up & down with the throttle opening and closing at low & higher speeds the cylinder filling is just a lot smoother. I’m averaging 29 mpg combined. My happy place with this little car is to get its best power and gas mileage as both priority one, Having the afr gauge lets me look at its performance, driving at 13.5:1 is where it’s frequently at. in the 14’s on the highway.

Here’s what took me so long to understand, with the Weber there’s a lean spot towards the top of the progression stage, that’s normal, it’s when you’re cruising on the highway and saving fuel. The CIH likes fuel so what I’ve found, the way I have it set up, I do most of my driving with plenty of power 12:1-14:1 driving under that lean spot. I used to get hung up on trying to stay lean all the time. That’s where the afr numbers were tricky, jetting leaner gave me good afr numbers 13-16:1 the engine ran smoothly but because I was driving more on the mains on or past the lean spot with more throttle my performance and mileage didn’t fair as well. As previously mentioned there’s a fine line in giving it too much fuel, you can be doing 12.4:1 with a spongy pedal and using too much throttle. The air fuel ratio gauge doesn’t do it all, especially when you’re on the rich side. Here’s how the 38/38 I have is set up. I use 91 octane California e-10 gas.


Idle 49’s Mains 145’s Air’s 185’s

F-66 emulsion tubes

Pump jet 65

Brass Float 38/48mm (with gasket)

Idle 12.8:1 turns: 7/16 inner screw 1 full turn outer screw

¾ turns in on speed screw

Cruise 50 mph steady/tip in 12:5 to 13.5:1

part throttle decel 11.2-12.5 mostly 11.8-12.3:1

Cruise highway 14.5-15’’s mostly upper 14’s occasional 16-17:1 blip on tip in

part throttle highway decel 12.6-13:1

Light acceleration 12.5-15.5:1

Aggressive acceleration 12.6:1- 13.5:1

Hard decel throttle closed 13.5 to 15.8:1

WOT 12.6:1

Good all the way around

5/14/22 changed distributor bushing from 5.56mm to 5.4mm, not much of a change. 34° total 12.5° idle

7/5/22 re adjusted distributor to 13.5° at idle & 35° total add 7° vacuum advance for 42° total part throttle. Moved from ported to manifold.
This is all great info,,, thanks,, I was never one to think to much info is bad,, you play with it all.. :eek:)

The AFR gauge on my 2.0L, 1971 GT is new to me this year . I updated my exhaust with a shorty header (from OGTS) and 2" pipes this past winter ( this alone make a great improvement across the board). I added the O2 sensor at that time. Being the engineer that I am, I was thinking if 14.7 to 14.9 is perfect combustion,,, than that is were thing should be,,, Probably should have digested more on some of the material I was reading before I sent the question out.. I have since gone back and found several tables of AFR targets under different engine conditions and the 12.5 to 13.5 AFR reading at idle for aspirated engines seems about normal,, makes me feel a bit better on what I see in my own GT best idle settings..

Thanks again
 
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