a CD jetting conundrum

[robsanab]

I have a new (now is new-ish I suppose) Weber 32/36 with CD's prescription. Although it has been running very good I have suspected that it is just too rich for my car (live on the NW coast, so typically under 100ft elevation)... everything is new, tuned and timed and yet I get very poor gas mileage. I have 5 speed, crane ignition, 3.64 and I've gotten like 13 city with 22-24 strictly highway for the past few years since I've had this new carb (my old leaky one with stock jets, and 4 speed at that, got better MPG).

I just came from a quick trip over the mountains, where I pushed the car VERY hard through passes of 5500 feet and then drove for a half a tank at 1500 to 1800 feet on the other side, and then back over again... maybe 30/70 ratio of city/hwy driving. Ran super strong even at the top of the pass with no need to tweek the idle or mixture adjustment screws AND I got like 26 mpg!! So now I know my jetting is off.

I know there are many variables and what ifs... but I would think that i wouldn't have to significantly alter what CD has done here. In order to reduce my rejetting experimentation, could anyone recommend a good starting point... meaning what should I start with as for a combination of idle and main jets to change out?? I know they work together, so I hesitate to just reduce the jets by say 1 size... or is that what I'll have to do?

for a refresher, here are CD's jet sizes...

Float Level 40mm

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 DIFFERANCE IN ACCURATE FUEL METERING THROUGH OUT ALL RPM AND THROTTLE OPENINGS.

http://www.bmw2002faq.com/content/view/62/32/

[c.d.iesel]

(nt)

[robsanab]

when your weather cools down to 30's - 55*F temps, and damp-

your power will be mint given all the data so far.

your MPG's highway agree with my car - 25 - 26 at 70-80 MPH

if you want MPG's to improve city, partial throttle, and idle?

then play with the primary AIR and MAIN jets

secondary jet changes only if high rpm (above 3500rpm with 1/2, 3/4 to full throttle fast sport driving) power is 'off'

CD… please clarify or elaborate if you would. Specifically which way should I go with each of the primary air and main jets? I’ll get a variety of each in that direction.

I disagree with you that everything will be mint during our wet winters… as I said I’ve lived with this through several seasons… the weather does not make a difference. Over this time I’ve also had my pro, who knows ‘02’s and webers, tune it a time or two… so I know it isn’t due to my lack of skill or knowledge. And to the contrary, the gas mileage doesn’t agree with your car. Maybe my description wasn’t very clear… I only got the better 26 mpg from mostly freeway and in the mountains on this recent trip. At home (90% of my driving), winter or summer, it’s closer to 16mpg combo city/hwy (13 strictly city… my best with conservative highway driving has yielded 24mpg). Again I’m pretty certain it can’t be much else because my previous stock jetted 32/36 eeked out 27 once on the highway… the only change has been I had a 4 speed back then.

So you may be right in that the secondary may be right on, if not maybe still a bit rich. How do I adjust the circuit to bring the primary and secondary closer together?

It always runs good and strong, I am just getting tired of the gas mileage and I’m ready to do something about it… it’s either go back to stock jetting and play from there OR stay with your well tested prescription and tweak just a bit. I would much rather work with what I’ve got.

thanks much

[Chamonix72]

You might also try giving Patrick at Midnight Motorsport a call. Since he is relatively local to you, he can get you his prescription, which might be more suited for this area.

www.midnightmotorsport.com

[robsanab]

You might also try giving Patrick at Midnight Motorsport a call. Since he is relatively local to you, he can get you his prescription, which might be more suited for this area.

www.midnightmotorsport.com

good idea.... talked to patrick before about my future EFI plans.

May end up doing that... for now i just want to try to work with what i've got. Seems like this setup is so unique no one quite knows where to modify it. Thought those on the forum who have this set-up may have a known tweak that moves me in the right direction.

[TobyB]

If you really want to see what's going on, plunk for a wideband o2 sensor.

There are some inexpensive DIY options that are probably worth it.

Problem is, you don't know exactly where you're rich/lean, so knowing

which circuit to tune isn't going to be trivial.

Most cars will be pretty similar- but there are often several transition/main/air

combos that you can get to work pretty well.

that's my take.

t

[roadhog0]

Patrick's prescription is pretty much dead on. 27 Highway and 20 average with a lot of puttering around.

[PatAllen]

i did CD prescription too just for fun, and it ended up way rich. i had to reduce the jetting much to the point it is almost as it was before.

fwiw, i went from 170 main 2nd to 165. WOT afr went from 11.5-11.7 to 12.2-12.5, way much better and the car flies.

took 1st air jet from 145 to 160 just to see how far it would affect the uber rich condition before transition (10:1 sometimes !!)

So now it seems and feel way much better than ever, the richest i see before transistion is like 11.7 but that only occurs on some very unique circumstances and dont last very long.

The only thing i would say is part throtle high speed is a bit lean, 14-14.5, but this is a very light throtle and the car doesnt surge so i guess this is just good for good mpg. As soon as it is floored for any slight acceleration it falls to 13-12.7. 80kph kick down from 3rd to 2nd never felt this good.

I guess i could use 155 instead of 160 to richen it up a litle from 14 to 13.8

[robsanab]

i did CD prescription too just for fun, and it ended up way rich. i had to reduce the jetting much to the point it is almost as it was before.

fwiw, i went from 170 main 2nd to 165. WOT afr went from 11.5-11.7 to 12.2-12.5, way much better and the car flies.

took 1st air jet from 145 to 160 just to see how far it would affect the uber rich condition before transition (10:1 sometimes !!)

I guess i could use 155 instead of 160 to richen it up a litle from 14 to 13.8

So, your idle circuit was pretty good? That sort of makes sense in my case as well. The rich condition is in the transition. That’s why I am suffering mostly around town (mpg is much worse there than on hwy) and yet my adjustment screws seem to respond properly within spec.

Pat, to reiterate… you changed only the following:

1) Primary air correction from 145 to 160 (or as you said 155 could do it)

2) Secondary main jet from 170 to 165.

3) no change to the secondary air?

4) no change to the primary main?

[PatAllen]

i did CD prescription too just for fun, and it ended up way rich. i had to reduce the jetting much to the point it is almost as it was before.

fwiw, i went from 170 main 2nd to 165. WOT afr went from 11.5-11.7 to 12.2-12.5, way much better and the car flies.

took 1st air jet from 145 to 160 just to see how far it would affect the uber rich condition before transition (10:1 sometimes !!)

I guess i could use 155 instead of 160 to richen it up a litle from 14 to 13.8

So, your idle circuit was pretty good? That sort of makes sense in my case as well. The rich condition is in the transition. That’s why I am suffering mostly around town (mpg is much worse there than on hwy) and yet my adjustment screws seem to respond properly within spec.

Pat, to reiterate… you changed only the following:

1) Primary air correction from 145 to 160 (or as you said 155 could do it)

2) Secondary main jet from 170 to 165.

3) no change to the secondary air?

4) no change to the primary main?

thats correct, i went from CD prescription to the setting above. i think the primary could be reduced a bit, i have a 137 and a 135(instead of 140), mated to a 155 air i think it will be perfect. But the way it is now i made very good mpg when i went to bav auto show and shine. Almost 30mpg all the way thru on a Automatic 02. This is not negliegeable performance i would say in a 40 year old car.

[jdt10768]

nt