A summary of what I've learned about downdraft Webers...

[sreams]

After much experimenting with my 32/36 DGEV Webers, here's a summary of some things I've discovered that might be helpful to others.

My CS has an M30B35 with the original (old) exhaust manifolds, and the original Zenith intake manifolds. The outlets of the intake manifolds have been port matched to the larger intake ports on the 3.5 head.

I've installed a wideband O2 sensor and gauge in order to help with fine tuning the carbs.

Before having the help of the O2 sensor, I always felt like accellerating beyond about 4000rpm was somewhat pointless. Power seemed to drop off at that point, and I've read about others experiencing the same thing on these forums. I also felt like I wasn't getting the mileage that should be possible if things were setup correctly.

So here are my observations:

1) The power valves provide for the difference in mixture between cruise (leaner) and accelleration (richer). If the power valve assemblies no longer hold a vacuum (age and/or backfires through the carb can damage the diaphragms), the needle valve will always be held open, which means extra fuel is always being dumped into the circuit, even during cruise when it shouldn't be. Both of mine had failed, and replacing them leaned out my cruise mixture from 13:1 to about 16:1. This makes for a very significant increase in mileage, especially on the highway.

2) Every recommendation I've seen for jetting DGVs on an M30 has involved main jets in the 140-150 range for both the primary and secondary. I've found that a secondary main jet in that range is far, far too rich for my engine. With 140 mains on the primaries and secondaries, I see about 13:1 while accellerating with about 3/4 throttle (primaries open 100%, secondaries still closed). Accellerating at WOT, however, pegs my O2 sensor at 10:1 (it won't show any richer). Dropping to 110s on the secondaries gets me closer to 12.5:1... and the engine sings right up to the redline with this setup.

3) The stock air filter housing has a -huge- effect on WOT mixture... even without any air filters installed, and even with the top cover removed. A jetting that yields 12.5:1 at WOT with no filters or housing will yield 10.5:1 with the air filter housing in place. Trouble is, cruise mixture stays the same... so if you lean out the main jets to compensate for the air filter housing, your cruise goes way too lean. This threw me off for quite a while because I was testing jet combinations without the housing in place. When I ended up with something I liked, I'd replace the housing only to find that the car had lost upper end oomph. Long story short... I removed the air filter housing altogether and went with standard Weber filters (the taller 2.5" ones, which just fit). They work great and solve the mixture issue.

After all is said and done, my current jetting is as follows:

Primary
60 Idle
140 Main
160 Air

Secondary
55 Idle
110 Main
185 Air

 

[jmackro]

Very interesting - thanks for posting this. Two follow-on questions:

- Can you explain what instrumentation and data logging system was used. Innovate LC-1 perhaps? Did you add a throttle position sensor? Was RPM information fed into the data logger via a signal from the coil?

- I'm not challenging your result, but I am surprised that a stock air filter housing with no elements or top provided significant air restriction. And that an aftermarket Weber filter (K&N? Weiand?) was that much less restrictive. Do you have any idea why this should be so?

 

[sreams]

Very interesting - thanks for posting this. Two follow-on questions:

- Can you explain what instrumentation and data logging system was used. Innovate LC-1 perhaps? Did you add a throttle position sensor? Was RPM information fed into the data logger via a signal from the coil?

I didn't use a data logging system. Just an air/fuel gauge getting a direct reading from my O2 sensor. Like this:

http://www.ebay.com/itm/AEM-DIGITAL...Parts_Accessories&hash=item35aa3365fe&vxp=mtr

I'm driving and reading the gauge in different situations. I can tell where the secondaries open by feeling it in the pedal. The tach in the dash is pretty accurate according to my timing light (which shows rpm), so I'm using that as well.

- I'm not challenging your result, but I am surprised that a stock air filter housing with no elements or top provided significant air restriction. And that an aftermarket Weber filter (K&N? Weiand?) was that much less restrictive. Do you have any idea why this should be so?

Just a guess... but I think it has a lot more to do with a change in the dynamics of air flow than any particular restriction. The air filter housing creates a large, flat, horizontal plane that may completely change the dynamics of air being drawn into the carbs. The filters I'm using remove that surface almost completely.

http://67.192.23.28/ebayimgs/assets/wcd/additionalparts/F 2000.jpg

In any case, I'm no expert on the science of air flow, but I can not only see the change on my air/fuel gauge immediately when the housing is in place, I can also feel a significant drop in power as the mixture goes overly rich.

 

[David]

Thanks for posting. Sorry for the noob questions.

For a dual carb car, don't you need two sensors (one per carb) ?

Or, is it true that the mixture screw only really impacts the idle (sub 2k RPM), and from their the jetting impacts performance--and so if your jetting is the same on each carb you only need one sensor in the exhaust pipe?

On a similar note, I was told that the plastic spacer between the carbs and intake manifold can leak where it meets the manifold, and that its best to make it a solid piece by filling it in with resin. Not doing so may result in a slow air leak developing. Anyone else know about this ?

 

[sreams]

Thanks for posting. Sorry for the noob questions.

For a dual carb car, don't you need two sensors (one per carb) ?

In a perfect world, you'd have an O2 sensor for each cylinder. That way you could tell if the mixture was different for the outer cylinders as opposed to the inner cylinder for each carb. I think one is enough to get meaningful data so long as your carbs are in a similar condition and setup the same.

Or, is it true that the mixture screw only really impacts the idle (sub 2k RPM), and from their the jetting impacts performance--and so if your jetting is the same on each carb you only need one sensor in the exhaust pipe?

This. Even so, it is relatively easy to make sure the idle screws are set the same on each carb by first bottoming them out and then counting how many turns out they are.

On a similar note, I was told that the plastic spacer between the carbs and intake manifold can leak where it meets the manifold, and that its best to make it a solid piece by filling it in with resin. Not doing so may result in a slow air leak developing. Anyone else know about this ?

In my case, I doubt I have a vacuum leak at all. Otherwise, I'd be fighting a lean condition, not a rich condition.

 

[Nicad]

Does your car pull till redline now? I remember my stock Bavaria with a worn out (smoking) engine really had clean power all the way to redline.

 

[echappe]

This is good information as most of the fine tuning and data logging has been on the various fuel injection options as installed on the M30B35. I have weber 38's on mine and it pulls hard all the way to redline. Downside is the fuel mileage is probably in the 12 to 14 MPG range. My foot probably has a lot to do with it though.
Where did you get the intake manifold ported and what may I ask did it cost?
Ernest

 

[aalto]

Thanks for this information. I've been a bit disappointed in the performance of the new-to-me M30b35 in my coupe. Similar setup to yours but with a set of stahl headers instead of a the typical exhaust manifolds. Off the line the car seems to make good power but, like you mention, very little beyond about 4000 RPM.

I am currently running the short weber air filter setup and was planning to switch to the stock air filter housing (for engine bay aesthetics more than anything) since I have the adapter plates on hand. Perhaps i'll hold off until i do a bit more experimentation and investigation.

 

[sreams]

This is good information as most of the fine tuning and data logging has been on the various fuel injection options as installed on the M30B35. I have weber 38's on mine and it pulls hard all the way to redline. Downside is the fuel mileage is probably in the 12 to 14 MPG range. My foot probably has a lot to do with it though.
Where did you get the intake manifold ported and what may I ask did it cost?
Ernest

I did the porting myself. I have a compressor and a grinder. I used a gasket to mark where material needed to be removed.

 

[sreams]

Does your car pull till redline now? I remember my stock Bavaria with a worn out (smoking) engine really had clean power all the way to redline.

It certainly does now. It would just barely get to the redline before. Now it pulls hard the entire way there.

 

[John Buchtenkirch]

Do you have a name & phone number on where the jets & various Weber parts are available ? My coupe has the exact same too rich problems with the downdraft Webers. Thanks in advance ~ John Buchtenkirch

P.S. I also always thought my old Bavaria pulled harder but that was over 30 years ago so maybe it was my imagination :confused:. After reading this thread I’m sure now that It was true.

 

[Stevehose]

Pierce Manifolds.

http://www.piercemanifolds.com/

I wish I owned stock in them :razz:

Do you have a name & phone number on where the jets & various Weber parts are available ? My coupe has the exact same too rich problems with the downdraft Webers. Thanks in advance ~ John Buchtenkirch

P.S. I also always thought my old Bavaria pulled harder but that was over 30 years ago so maybe it was my imagination :confused:. After reading this thread I’m sure now that It was true.

 

[sreams]

Do you have a name & phone number on where the jets & various Weber parts are available ? My coupe has the exact same too rich problems with the downdraft Webers. Thanks in advance ~ John Buchtenkirch

P.S. I also always thought my old Bavaria pulled harder but that was over 30 years ago so maybe it was my imagination :confused:. After reading this thread I’m sure now that It was true.

I'm getting Weber parts from here:

http://www.webercarbsdirect.com

They also have really good prices on the carbs themselves. They've got the 38 DGES carbs for $220 each. Tempting.

Pierce has a great website and great selection, but the 38s are $100/each more from them.

-Scott