"Advanced" Weber Downdraft tuning

[jabloomf1230]

I just put a Metric Mechanic 3700 Sport engine (M30 rated at 270 HP) in my 1972 3.0CS with dual Webers. Here's my jetting (old settings for the original 3.0L engine are in parentheses):

PRIMARY
idle 75 (60)
main 160 (140)
air correction 180 (170) - although I did try 200

SECONDARY
idle 60 (50)
main 150 (140)
air correction 170 (160)

It runs a hair rich, but performance and idle are excellent. I also changed the pump jet from 50 to 60, but I doubt that it did much.

 

[TodB]

On the MM sport engine, have you Dyno'd it or are those results provided by the engine maker?

 

[decoupe]

270 would be crank hp.

 

[jabloomf1230]

Hagerty has a recent article that's relevant to the disparity:

http://www.hagerty.com/classic-car-...tures/News/All-Articles/2013/08/13/Horsepower

I could be wrong, but the stock 3.5L M30 engines were rated at 218 bhp, so the MM versions are at least that. If I put it on a dyno at some point, I'll post the results. The original 3.0L engines were rated at anywhere from 185-208 bhp as I recall.

 

[jmackro]

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.

Mike:

I know this is an old thread, but it has become pertinent to me now. I tried following the link to the 2002forum, but it seems broken. Can you shed any light on exactly what hole the poster recommended plugging? Did you try that fix and if so, did it improve things? Any ideas of how to search the 2002forum to find the thread where that recommendation was posted?

 

[Mike Goble]

http://s226.photobucket.com/user/cd02md/media/WEBERprescriptionplughole31photoB.jpg.html

http://s226.photobucket.com/user/cd02md/media/WEBERprescriptionplughole31a.jpg.html

http://www.bmw2002faq.com/topic/102031-installed-the-synchronous-link-weber-3236/

You can google Chreighton Demarest for some more posts like this.

I did this mod summarily when I was rebuilding the carbs about 8 years ago and haven't touched them since. The throttle responce is very smooth, I don't use a choke, got thousands and thousands of trouble-free miles on them.

 

[sreams]

Personally, I'd recommend -against- plugging the hole. That circuit is there for a reason. It provides for the difference in mixture between cruise and acceleration. With it plugged and jetting set to compensate, you'll be running unnecessarily rich at cruise, killing gas mileage with no upside. My M30B35 runs DGEVs without the holes plugged. There are zero flat spots of any kind, and cruise mixture approaches 15.5:1. Personally, I think some plug the whole as a crutch for an improperly jetted carb.

 

[jmackro]

Personally, I think some plug the whole as a crutch for an improperly jetted carb.

That might answer another thing that was puzzling me in this thread. Several posters list the jet sizes they are running, and most of them have larger main and idle jets on the primary circuit than on the secondary.

But the post that suggests plugging that hole (from the 2002faq and quoted in #7 above) recommends running smaller jet sizes on the primary circuit than on the secondary. Would that be how he improperly compensates for the blocked-off passage?

 

[Dave G]

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.

Thanks for the great detail on your Weber 32/36 DGV carb observations! I'm in the process of installing the 32/36's on my rebuilt M30B35 swap.
1. Did you end up trying the 110 main secondaries and/or resolve the WOT richness and lack of pull with the air filter housing on?
2. What is your elevation and have you had any issues in the Sierras? I'm at 4900' in CO, right on the edge of Weber's recommended minor jetting changes