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Tuning a 38DGEV w/ a Wide Band AFR; Now Goes Lean at Quick Full Throttle

7K views 51 replies 2 participants last post by  The Cub 
#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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#2 ·
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
 
#6 · (Edited)
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
 
#7 ·
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..
 
#4 ·
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
 
#8 ·
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..
 
#5 · (Edited)
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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#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.
 
#11 ·
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...
 
#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...
 
#18 ·
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
 
#14 ·
"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.
 
#15 ·
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.
 
#16 · (Edited)
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.
 
#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
 
#19 · (Edited)
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.
 
#23 ·
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.
Thanks for the info,,,

Not sure on the last comment however. If your referring to the vacuum ported line off the Carb only, then, yes, no vacuum at idle should be seen, as long as your throttle valves are closed. I guess I'm not sure what happens at WOT. You definitely have air flow through the throat of the carburetor, so, I would think you would still have a decent vac. off the carb port since ported line operates on a venturi effect, but I guess I really can't say here. I believe the point of the discussion was simply, if you remove the retard function side of the distributer vacuum canister, and operate only a vacuum advance, where do you hook the the advance vacuum line to, the ported or manifold? Doing this has several advantages as was discussed earlier, but the main thing that is very noticeable, is to help with low end idle on a built or slightly built engine. Since at idle you have no Vac on the carb port you hook the advance side of the distributor vacuum canister to the manifold, where their is vacuum at idle. This pulls the timeing up a few degrees at idle helping your engine run smoother, but since the vacuum drops in your manifold during acceleration, the few deg. advance also goes away and your back on your performance settings in your distributor mechanical advance.

As far as the distributor and the vacuum advance set-up goes, I'm sure you already know this, I just repeated some things as to not confuse some readers who may not, or jumped in at the middle.. :eek:)

Thad
 
#21 ·
Just an FYI, but a naturally aspirated CIH always makes its best power between 12.8 - 13.2 AFR on the dyno.

You can be a little leaner at cruise of course, and at idle I’ll go as high as 14.2-14.5.

But at that AFR you need a very good ignition system to fire the lean mixtures.
 
#25 ·
Thanks very much Bob,, ,, Your power range for aspirated engine seem to fall inline with other charts I have found on line, some go as rich as 12.0. Although I do not know you personally, I do know,,, you know, what you are talking about,, Ill target your numbers.

I do have a question concerning the AFR read out,,,, With an engine set up like mine (see my first post) Is it common for them to be very erratic in their digital readings? I mean idle and even cruising down the road the engine seems smooth but the AFR reading is not very steady . You can watch it and get s sense of the mid point but just was wondering. I'm running a 2" exhaust with a shorty header, a Crain Fireball XR700 ignition with a hotter bosh coil. O2 senser is set about 4" back of the header mixing point at about the 1 o-clock region on the pipe. just wondering if I have something else going on.
 
#27 ·
The bigger the cam, the more erratic it will be.
 
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#30 ·
Yes, I’ve found that when tuning the 38/38 one jet size, (example 140 main jet to a 145) is like jumping two jet sizes on the 32/36. I believe that’s the reason Weber has the 142 main jet & the.47 idle jet. I just like taking it a step further.

I found once I got in the the float level right & my mains close, getting the idle circuit set up right as Harold mentioned on the speed screw throttle plate adjustment not pulling any vacuum on the port. Once the throttle plates open the progression holes add more air to the fuel in addition to the fixed idle air correctors, which reeks havoc on the transition. This is why proper mixture screw positioning is so critical. A lot of guys give up and can’t get past this point with the stumble. Too far in or too far out with those screws.
There’s multiple ways of tuning the idle/progression circuit
Here’s one of the best jetting threads I’ve read:


(See post 76) one of the guys pulled and tapped his idle air bleeds with a 10/32 tap. He has an assortment of allen headed brass plugs from 1.50 up to 1.90 in .5mm increments (1.9 is the factory supplied idle air bleed size) not at all necessary just pretty cool educational stuff.

Once I found the spot on the mixture screws that offers the smoothest transition instead of doing that or enriching or leaning my idle afr by moving the mixture screws off their sweet spot to fine tune, I decided to drill out the idle jets, I’ve got a factory .47 (14:1), .48 (13.5:1), .49 (12.9:1) & the factory .50 (12.2:1) and my mixture screws are sitting in the sweet spot. The car has smooth transition with any of the above the .49 is where I’m getting the best overall performance.

I got frustrated with the plastic float, you may have far better success than I did, if you seem to be doing everything right and still are frustrated it’s money we’ll spent to pick up the brass float. Maybe Gil at OGTS has a few used ones lying around. I battled with the plastic one for months, take your time enjoy the experience, it’s a lot of fun tuning with the afr.
 
#31 ·
I got frustrated with the plastic float, you may have far better success than I did, if you seem to be doing everything right and still are frustrated it’s money we’ll spent to pick up the brass float. Maybe Gil at OGTS has a few used ones lying around. I battled with the plastic one for months, take your time enjoy the experience, it’s a lot of fun tuning with the afr.
That's interesting you had tuning problems with plastic floats. I ve had no issues but recall the recommended Weber plastic float height setting would make my car run out of gas in 3rd WOT. I adjustment the float height to keep a slightly higher fuel level in the bowl and never had the issue again.
 
#34 ·
I haven't seen much out there on results of chaining pump jets to cure quick, full throttle acceleration. So, if I may try to gather any experience to help me trouble shoot this area, great.

I have a 2.0 L ( original 1.9 upgrade to a 2.0L), running flat top pistons and a street competition cam, with a 38/38 weber and 2" exhaust. I have an AFR gauge and I have been tweaking the carburetor for best overall performance. I feel I'm close for most driving conditions but I am at a loss on what to change for correcting a full lean condition under quick, full throttle acceleration, if not a pump jet? When I accelerate hard but in a slower progression, the engine appears to pull good with fuel ratios in the power range of 13.0 to 13.5, but, if I am cruising along and go to quick full acceleration the AFR drops to 15.5. to 16. It will build back to a rich condition of 13.3 or so as I keep going... but that quick lean dump is definitely taking something out of the performance. I already have #70 Pump jets in the carb and it appears that is the largest size they sell. I could try drilling them out, but I thought I would check to see if others have had this issue and what was done to correct that isolated lean condition on acceleration.

I have checked the power valve settings and its working fine for my vacuum range, accelerator diaphragm is new and fully functional as well. Also, I'm running 3.5 venturis and do have a set of 4.0,, I am going to try this change but I feel it will just move me richer in high end cruising condition, not quick acceleration. I just don't need it richer at high end steady state.

Thadman
 
#35 ·
Have you done the F66 emulsion tube swap already?

Stock 38 DGAS emulsion tubes (F50) tend to create a bad stumble just off idle that no amount of rejetting can work around on the Opel CIH engines.

These carburetors were originally fitted to a V6 engine, so changes to make them run well on the CIH may be necessary.

Otherwise, you can check on your accelerator pump arm to be sure it’s got a light preload (no gap) or it will lean-bog off idle from this.

On some really ‘built’ engines I’ve drilled out the pump (squirters) that were just too small. I think I went from .023” to .027”, IIRC.
 
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#36 ·
I’m running a 65 pump jet with very similar engine/exhaust specs to yours. In my experience once you get the mains, emulsion tubes & float level set up, go after the idle mixture screw setting first drive it around & keep adjusting until the lean spot is almost un noticeable, then try increasing the idle jets, I’ve found between 47-50 works best in my case with the mains I have listed below. Your last comment on the venturi probably won’t do you any good since your trouble isn’t when you’re on the main jets.
Also, when I moved the vacuum advance off the ported side to the manifold it helped smooth things out too. I’ve got my timing set up higher than normal which makes everything more sensitive and it eliminates that 6° jump on initial acceleration. On the 32/36 it didn’t make much difference but with the double barrel tip in it did.

Good point Rallybob made on the possibility of a gap before the arm engages for the diaphragm. That’s the easiest thing to check. Maybe your closer than you think.

Here’s how my 2.0 is jetted:
Idle 49’s (drilled out from 47’s)
Mains 145’s Air’s 185’s
F-66 emulsion tubes
Pump jet 65
Brass Float 38/48mm (with gasket)
Idle turns: 7/16 inner screw 1 outer screw
¾ turns in on speed screw
28-33mpg

HTH
 
#38 ·
Yup, if you idle at 14” of vacuum, I’d want the power valve to open around 12”.

6”-8” opening will cause a huge bog/delay.
 
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#40 ·
Thanks Rally Bob,,,
It's good to note from such a dependable source that I may be on the right track for some correction here.

What do you think about raising the float level as it pertains to the timing introduction of the enrichment tubes. I have not found any info that would tell me what air flow in CFM is required across the tips of these tubes that would draw fuel up the channels from the float bowl? I have looked at them during short quick accelerations in the garage but don't see much happening. I blew air down the body across the ends of these tubes using my air compressor set at around 30 psi and found them to work,, just not sure what that means in CFM as it pertains to drawing air in, across the whole intake body? Any words of wisdom here before I try this out also?
 
#41 ·
Trying to firm up the power valve spring was not that successful... I placed 3 nylon spacers at the spring bottom, 1/16 in. thick each, for a total of 3/16 in.,, and it may have moved up the vacuum tension about 1/2 or 1 in./Hg... not much, and that had very little impact on my instant full throttle going lean.. I can see how some may need to weaken the spring by sniping a wrap or two,, but it appears not much can be done to tighten the spring.. unless I am missing something. I checked to see if these valves are offered in different tensions but it appears they only make the one..

Back to the drawing board. Is anyone else, who has an AFR gauge in their car, see the same short-term drop in fuel before it builds back up when going full throttle,? Maybe this is normal for a carbureted engine..
 
#42 ·
Yes, it’s normal for a brief lean condition upon slamming the throttle. It’s not normal if you can feel it though. You want to tune it away as best you can.
 
#43 ·
It sounds like the one you have isn’t going to get you where you need to be. A new power valve spring opens at 14” of vacuum. If you buy a new one you’ll probably find yourself trimming the spring, as RallyBob mentioned in post #38 your goal being 12”. I like to get all of the known details ironed out before tinkering with the float level or jets.

One thing that made it easier for me to get things right was having a broad selection of jets. The way they designed these DG carbs you’re always going to see a lean spot on the progression stage. As Knorm65 just stated the trick is not to feel it. Right about when my AFR hits a brief 16.7:1 is about the peak of where it becomes noticeable, anything higher and I definitely feel the bog.

I really enjoyed the journey tuning with the air fuel ratio gauge, all the heavy lifting is behind you once you get that installed. It helped me to log my information on each different change, adjustments etc. along with the result along the way.
Don’t get too frustrated, keep taking your time, you’ll get there and it’ll be worth it!
 
#44 ·
Thanks for the info,, It sounds as if a lot of carb tuners out there have a draw down vacuum around 14,,, my original power valve I got with this used 38, drew back at around 6 ,, replaced it when I rebuilt it and this one also draws back the same at 6 . Could have a bad gauge I suppose but in any case the idle vacuum is still 7 or 8 in./Hg different than the pull down vacuum at idle using the same gauge,, so bad gauge or not,,,, it showing a larger separation.. But again I think the gauge is fine because it sounds about right at 14 at idle with the CAM I have...

Keep working on it is all I can do..
 
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