"Advanced" Weber Downdraft tuning

[sreams]

Okay. So I'm getting more and more familiar with the nuances of tuning 32/36 DGEVs on my M30B35. Obviously, changing the various jets allows for different mixtures at various RPMs/loads... but I am finding that there isn't enough detailed control using just this method.

For comparison's sake, my '66 Volvo has a pair of Mikuni HSR45 carbs that have fuel metering needles (kind of like SUs). By changing the needle profile, it is easy to change the mixture at a specific throttle position. This has allowed me to get a solid 15.5:1 ratio at 1/8 throttle high speed cruise (verified with a wideband O2 sensor and gauge), while still allowing a richer mixture as soon as the gas pedal is pushed down a little further for acceleration. My Volvo gets great mileage as a result, without sacrificing any power.

I haven't had much luck getting an efficient high speed cruise mixture with the Webers. When cruising at 75mph, I am at about 2800rpm. From all of the research I've done, and from experience changing out jets, the mixture in this situation is controlled entirely by the main jet. Trouble is, the main jet also fully controls mixture at 1/4-full throttle acceleration at the same RPM. Using a jet that gives me an acceptable fuel ratio during acceleration (about 13-13.5:1, 12.5:1 at WOT) results in my high speed cruise mixture being far too rich (about 13-13.5:1). Using a jet that gives a leaner cruise mixture results in an overly lean mixture during acceleration , and stumbling.

I understand that the air correctors control where the transition from the idle circuit to the main circuit occurs, but since the air correctors also control high-RPM mixture, I don't have a lot of room to change them much without screwing that up.

So... I've been reading about emulsion tubes, and I'm thinking the F50s that many are using are simply not right for an M30B35. Unfortunately, the part numbers for emulsion tubes are virtually meaningless, and I don't want to have to buy them all.

Any advice from anyone who has dabbled this deeply into Weber downdrafts would be appreciated. I'm thinking the potential is there for decent mileage (for an M30) with the right combination of parts, but I don't know that anyone has actually discovered that combination yet.

 

[DerSchwede]

Hello,
I think the 32/36s are too small for a 3.5. Remember the 2.5-3.3 carb m30s used 35/40 with 24/30 venturis. The 3.5 engine will always suck more air than a 2.8 and thus it might be hard to get the mixture lean enough under light load. Changing the emulsion tubes might be a way to get it better, but I doubt it will turn out as you want it to be...

I used to have a pair of 36/36 DCD on a euro 3.0S for many years and they worked fine for normal and spirited driving, but somewhat restricted the max power above 5.5k rpm

Best regards
Anders

 

[sreams]

Actually... I think I just figured it out. Turns out the power valves were completely shot and doing nothing. Their role is to lean the mixture at cruise. I took some good ones out of a spare set of DGAVs I have, and suddenly everything changed. I still have some tuning to do before things are right again, but I am now seeing much leaner cruise mixtures.

 

[sreams]

I think I've got it all dialed in now. The power valves were definitely the problem all along. Had to change some jets after getting this fixed. Now I've got a cruise mixture at about 15.5:1 which transitions to 13.5:1 under acceleration and 12.5:1 at WOT. Here's what I ended up with:

Primary
70 idle
145 main
160 air corrector

Secondary
70 idle
135 main
170 air corrector

From what I've read on this forum, I would have expected a larger secondary main jet, but a 140 has me going into 11:1 territory at WOT.

I may tweak it a bit more later, but I'm really happy with how it is working right now. Can't wait to see what difference it makes for mileage.

 

[gwittman]

I am in the process of rebuilding mine. The rebuild information on the jets are:

Primary
60 Idle
140 Main
170 Air Corrector

Secondary
50 Idle
140 Main
160 Air Corrector

This is exactly what mine had. I am running a 2.8L engine but the rebuild information was for 32/36 carbs in general, not for any specific application.

 

[sreams]

I've really learned a lot about Weber downdrafts in the last couple of days. Things that aren't clearly defined anywhere that I can find.

I was playing with idle jets yesterday (mine were too lean as measured by the number of turns the idle mixture screws were turned out), and noticed that they had a huge impact on WOT mixture. going from a 60 to a 70 idle jet made mixture at WOT richer by an entire point. It had little to no effect on cruise mixture, however. As a result, I'm finding that I can fine tune how much leaner cruise is as compared to WOT by changing idle jets. For example... a richer 70 idle jet might give me a cruise mixture of 15:1 (fine) and a WOT mixture of 11:1 (too rich). Changing to a 60 idle jet leans the WOT mixture without affecting cruise mixture, for a more reasonable 15:1 and 12:1. Changing the main jet will move both mixtures by about the same amount.

The 32/36 probably is too small a carb for an M30B35, as I'm finding that the idle jets that give me the best range of mixtures are too lean for a decent idle. I currently have my idle mixture screws turned out 3 full turns for the best idle, which is out of spec.

Knowing this... I'm starting to think decent mileage may be possible with the Weber 38s using the methods I mentioned above. Weber Carbs Direct has them for about $200 each, which is tempting.

BTW... my final settings are as follows:

Primary

60 Idle
145 Main
160 Air Corrector

Secondary

65 Idle
125 Main
170 Air Corrector

-Scott

 

[Mike Goble]

I've got a pair of 32/36 Webers on a 3.5L and used the following jetting setup, one that I found at http://www.bmw2002faq.com/content/view/62/32/

PRIMARY
idle 60
main 140
air correction 145

SECONDARY
idle 55
main 170
air correction 175

AND -PLUG THE SECONDARY ENRICHMENT HOLE AT THE TOP OF THE CARB AS MY DIAGRAM SHOWS . DON'T ASK WHY- JUST DO IT AND SEE THE HUGE DIFFERENCE IN ACCURATE FUEL METERING THROUGH OUT ALL RPM AND THROTTLE OPENINGS.

The carbs work great, 20 mpg on the highway with excellent throttle response.

 

[sreams]

I've got a pair of 32/36 Webers on a 3.5L and used the following jetting setup, one that I found at http://www.bmw2002faq.com/content/view/62/32/



The carbs work great, 20 mpg on the highway with excellent throttle response.

I think there is potential for better mileage than that, though. I just took a trip from Sacramento to LA and back and averaged 19mpg at an average speed of 70-75mph... but my air/fuel gauge read 13.5:1 the entire time I was cruising. This is much too rich for cruise. I'm seeing 15.5:1 now, and the throttle response is great (with a bit larger accelerator pump nozzle). I'll get back with mileage numbers, but I expect a significant improvement. I went through similar steps years ago with my '66 Volvo 1800S (with a "hot" 2.1L 9.5:1 compression 4 cylinder), and highway mileage went from about 20mpg to nearly 30mpg. I don't expect quite as much from this engine, but I do expect a significant bump from 20mpg.

Also... the site you referenced refers to jetting for a 2.0L 2002 engine. Each of your carbs is servicing 3 pistons and 1.75L, so I would assume different jetting would be appropriate.

 

[Mike Goble]

Carburetors are demand driven devices, they don't know the displacement of the engine. If I draw 200 cfm of air through my carb, it doesn't matter if it goes into 3 liters or 5 liters.

 

[sreams]

Carburetors are demand driven devices, they don't know the displacement of the engine. If I draw 200 cfm of air through my carb, it doesn't matter if it goes into 3 liters or 5 liters.

Absolutely, but the cfm demands of a 5 liter engine at a given rpm are great than that of a 3 liter engine. There obviously isn't just one jetting for the DGV carbs that works with every engine, and our engines are measurably different from the 2002 engines in more ways than one. Just changing to a more open exhaust can result in the need for a change in jetting.

 

[jmackro]

I think I've got it all dialed in now. The power valves were definitely the problem all along. Had to change some jets after getting this fixed. Now I've got a cruise mixture at about 15.5:1 which transitions to 13.5:1 under acceleration and 12.5:1 at WOT.

sreams:

This is an interesting thread. I have three questions:

- When you see air/fuel ratios that are too low (e.g., too rich), you obviously need to use smaller main and idle jets depending on the rpm range where the problem occurs. But how do you arrive at the appropriate size for the air correctors?

- I am familiar with DCOE's, but not the DGXX's. DCOE's don't have a power valve - what is the function of that part?

- Where do you get DGXX parts? E.g., who are you buying your new jets and power valves from?

 

[sreams]

sreams:

This is an interesting thread. I have three questions:

- When you see air/fuel ratios that are too low (e.g., too rich), you obviously need to use smaller main and idle jets depending on the rpm range where the problem occurs. But how do you arrive at the appropriate size for the air correctors?

This is the best site I've found on the topic:

http://www.datsun2000.com/tech/weber_tuning_methodology.htm

The air corrector has its largest impact on high rpm mixture and on the tip in point for the high speed circuit.

- I am familiar with DCOE's, but not the DGXX's. DCOE's don't have a power valve - what is the function of that part?

It adds fuel to the mixture during idle and during acceleration, and closes (leans the mixture) when it sees vacuum at cruise. I've always read that DCOEs don't like vacuum advance, and I bet this is why. Lean cruise mixtures take longer to burn, and so vacuum advance (which advances timing only at cruise) compensates. Since the DCOEs don't lean the mixture at cruise, there is no benefit to vacuum advance. It also explains why mileage drops with these carbs.

- Where do you get DGXX parts? E.g., who are you buying your new jets and power valves from?

I find that webercarbsdirect.com has the best pricing on Weber carbs and parts. Buying carbs outright is particularly less expensive with them as compared to most other suppliers I've encountered.

-S

 

[Mike Goble]

[FONT=Trebuchet MS, Arial, Helvetica]The factory power valve operates by dumping extra fuel into the secondary emulsion tube well to allow a richer mixture during full throttle and allows the engine to run lean at other times to pass emissions. The problem with this system is that the fuel is basically un-metered, making jet selection difficult for racing engines at full power. Additionally, these power valves typically malfunction allowing the engine to run rich all the time. [/FONT][FONT=Trebuchet MS, Arial, Helvetica]

[/FONT]
http://www.thelolaregistry.com/DIY/Weber.htm

 

[Stevehose]

DCOE's love advance, but don't have a single power valve per se since each barrel services it's own cylinder. Leaning out my cruise mixture is the dragon I am chasing at present. Smaller idle jet lowers it somewhat, but then acceleration hesitates and leans out too quickly and I get popping at 3k rpms. Presently my accel and WOT is around 13-13.5 which for DCOE's is pretty good. Mains and air correctors doesn't seem to have any affect on cruise AFR so I am thinking of emulsion tubes may need a tweak or my accel pump jets are leaking.

I've always read that DCOEs don't like vacuum advance, and I bet this is why. Lean cruise mixtures take longer to burn, and so vacuum advance (which advances timing only at cruise) compensates. Since the DCOEs don't lean the mixture at cruise, there is no benefit to vacuum advance. It also explains why mileage drops with these carbs.

 

[sreams]

The factory power valve operates by dumping extra fuel into the secondary emulsion tube well to allow a richer mixture during full throttle and allows the engine to run lean at other times to pass emissions. The problem with this system is that the fuel is basically un-metered, making jet selection difficult for racing engines at full power. Additionally, these power valves typically malfunction allowing the engine to run rich all the time.

I agree that the power valve is pointless on a racing engine. Nobody is cruising or trying to pass emissions in a race car.

I will add, however, that the power valve is not just for "emissions". It serves to lean the cruise mixture for better efficiency. There is no need for a 13:1 mixture while cruising, and without a working power valve, that's what you'll get.

Leaning out my cruise mixture on my '66 Volvo from 13:1 to 15:1 yielded about 6mpg better mileage. In my case, it had nothing to do with emissions.

 

[Mike Goble]

The power valve has nothing to do with how lean I can get my cruise mixture, it has to do with how rich I can get my power mixture. A properly working power valve is closed during the cruise mode.

 

[sreams]

The power valve has nothing to do with how lean I can get my cruise mixture, it has to do with how rich I can get my power mixture. A properly working power valve is closed during the cruise mode.

Right. Effectively, it's the same thing. A working power valve means your cruise mixture is leaner as compared to your mixture during acceleration, or that your mixture under acceleration is richer as compared to your cruise mixture. It's like saying Tom is shorter than Bill... Or that Bill is taller than Tom. It means the same thing.

If the power valve is not working it is always open. If it is working, it closes and leans the mixture as compared to when it was open. It has everything to do with cruise mixture.

On my car... Power valves not working = 13.5:1 cruise mixture. Power valves working = 15.5:1 cruise mixture.

 

['69 2800cs]

Scott-thanks for posting all this detail. I've got a 3.5 on 32/36's as well and I'd guess my jetting is wrong, power valves are inoperable, etc, etc. More stuff to do...

Does your 3.5 rev freely to redline? Mine will go there smoothly but it all feels kinda pointless after 4000 rpm. I'd like to get a little high end zing.

 

[sreams]

Scott-thanks for posting all this detail. I've got a 3.5 on 32/36's as well and I'd guess my jetting is wrong, power valves are inoperable, etc, etc. More stuff to do...

Does your 3.5 rev freely to redline? Mine will go there smoothly but it all feels kinda pointless after 4000 rpm. I'd like to get a little high end zing.

It does, but it certainly didn't when I had larger mains on the secondaries. If other 3.5s are anything like mine, I wouldn't be surprised if many of the ones with Weber 32/36s are running rich at WOT. I've been tweaking more, and now I've got the following jetting:

Primary

60 Idle
140 Main
160 Air

Secondary

55 Idle
125 Main
185 Air

Believe it or not, I'm still running a little rich at WOT once the secondaries open (about 12:1).

Now... here's where it gets especially interesting. Because it's more convenient, I've been testing without the air filter housing installed. With it in place, my WOT mixture gets much richer. I'm seeing 10:1 with the housing installed... even without any filters in it. And it isn't just the gauge. I can feel the car flatten out as it goes rich, instead of pulling hard through the revs like it does without the housing there. It's starting to look like I'll need to go to something like a 110 main on the secondaries to get a workable mixture at WOT with the filter housing in place.

-S

 

[sreams]

Just took my first long trip since I made the changes.

Sacramento to Truckee and back, 176 miles, and I'm pretty certain I'm seeing just a bit better than 25mpg averaging 70-75mph. I think it will do better in cases where I'm not climbing from 0 to 7000 feet and back, so I wouldn't be surprised to end up with 27mpg or so. Obviously, I'm pretty thrilled with the result so far. Will keep you all updated, especially once I get a chance to do a long trip on level ground.

-Scott