engine and drivetrain Weber running lean - air pressure - thoughts?

[GreenSwede]

When I get other things sorted I'll deal with the fact I'm still running a wee bit lean on high revs. Particularly on 3rd and 4th gear.

 

I spoke to a guy who is specialized in getting more oomph out of VW's. He brought up the thought of air pressure around the engine at speed. And I do know that if I forget to latch the hood, the hood lifts at a not very high speed:)

 

Anyway, his thinking reasoned that at higher speed more air is "forced" into the carb, thus creating a different conditions for the jetting. And at those conditions the carb can't supply enough fuel to get it right.

It does sound reasonable, I think. A way would be to try feeding the carb air from a place that has a more constant air pressure, like inside the car? I would really like to try, but that would mean building something air tight around the carb and feeding that with a hose..

 

What do the experts think about the sense of the theory?

At least it's interesting, if not running forced induction the conditions should be constant.

Edited January 14, 2015 by GreenSwede

[danco_]

I kept up with your previous thread regarding the tuning of your 38/38 with the AFR guage some months back. I know you were 98% dialed in, but weren't happy with AFRs at WOT.

 

 

Is this the same problem you were experiencing from the last thread, or is this a new development?

 

 

Care to post your current jet setup? Idles, mains, air correctors, emulsion tubes, etc. 

 

I'm in the exact same boat as you, but I'm also at sea-level in Southern California...conditions a bit different than yours. I believe you were something like 600 feet above sea level?

[jrkoupe]

all i want to add.....is that telling Us it hapPens in 3 and 4 th gear is not so impt..

its more so what rpms does it haPPEN?..

[GreenSwede]

I kept up with your previous thread regarding the tuning of your 38/38 with the AFR guage some months back. I know you were 98% dialed in, but weren't happy with AFRs at WOT.

 

 

Is this the same problem you were experiencing from the last thread, or is this a new development?

 

 

Care to post your current jet setup? Idles, mains, air correctors, emulsion tubes, etc. 

 

I'm in the exact same boat as you, but I'm also at sea-level in Southern California...conditions a bit different than yours. I believe you were something like 600 feet above sea level?

 

Ah! Yes, same problem. To get more right on top, I had to richen it up at the bottom, which seemed to make me lose some grunt. 

 

And sorry, I don't have the specs handy. I don't even think I wrote down the last version. I'll get to it, and probably continue that same thread:)

And I think I'm about 200 feet above sea level. 

all i want to add.....is that telling Us it hapPens in 3 and 4 th gear is not so impt..

its more so what rpms does it haPPEN?..

 

Sorry. As I remember it, it starts leaning out at around 5k and is too lean around 5500-6000 rpm.

 

Edit: Nope. At 5500, up until about then I'm at 13 AFR, then it slowly starts leaning out hitting 13.5 at 6500 rpm.

And here's the old thread: http://www.bmw2002faq.com/topic/152708-weber-38-wb-readings-lean-on-anything-else-than-wot/page-4

Edited January 14, 2015 by GreenSwede

[ndog]

what intake manifold and filter setup do you guys have?

[TobyB]

Yes, the stock 2002 grille opening creates positive pressure under the hood.

 

The racers will block 3/4ths of the grille to fix that.

 

You could, too- the radiator doesn't need it.

 

I spent some time with a manometer meant for air handlers (it measures very low differential pressures)

and I don't think that the hood's pushed up as much as pulled up by the negative pressure on top

of it- it's 'lifted' up, not pushed.

But I won't swear to this- it's not trivial to get this data to be accurate!

 

"it starts leaning out at around 5k and is too lean around 5500-6000 rpm."

 

have you tried a smaller air corrector?  Different emulsions?  There is a fabulous diatribe on the

Weber site about how the emulsions work, and it's really enlightening....

 

t

[Hans]

I read somewhere that you'd have to be going several hundred mph to gain anything from ram air plumbing because the vacuum creted by the engine is so strong. What a ram air duct will do is bring cooler air into the carb.I'd be playing with jetting and timimg.

[GreenSwede]

what intake manifold and filter setup do you guys have?

 

The air cleaner that came with the previous 32/36 - sheet metal/filter/sheet metal.

[GreenSwede]

Yes, the stock 2002 grille opening creates positive pressure under the hood.

 

The racers will block 3/4ths of the grille to fix that.

 

You could, too- the radiator doesn't need it.

 

I spent some time with a manometer meant for air handlers (it measures very low differential pressures)

and I don't think that the hood's pushed up as much as pulled up by the negative pressure on top

of it- it's 'lifted' up, not pushed.

But I won't swear to this- it's not trivial to get this data to be accurate!

 

"it starts leaning out at around 5k and is too lean around 5500-6000 rpm."

 

have you tried a smaller air corrector?  Different emulsions?  There is a fabulous diatribe on the

Weber site about how the emulsions work, and it's really enlightening....

 

t

 

Really? Cool. Or hot, I guess. I'll contemplate blocking some air out. Just for fun.

 

Smaller airs, yes. They got me richer at lower rpms, 3-4k. Even though they shouldn't, I guess. I haven't gotten inte emulsion tubes. However, playing with airs and mains does not seem to get me closer, so maybe I should play with E-tubes.

 

Also, I've constantly been changing jets for both barrels to the same. One could fiddle with that, one a tad leaner, or so..

[jimk]

I read somewhere that you'd have to be going several hundred mph to gain anything from ram air plumbing because the vacuum creted by the engine is so strong. What a ram air duct will do is bring cooler air into the carb.I'd be playing with jetting and timimg.

Engine vacuum has no effect in developing any "ram air".  It's called "Velocity Pressure" off the street.  At 60mph (in std air) it gains you a whopping 0.0627psi "ram air" and increases by the velocity squared. So at 120mph you get 0.25psi ram air.  Is it worth it?

[GreenSwede]

Engine vacuum has no effect in developing any "ram air".  It's called "Velocity Pressure" off the street.  At 60mph (in std air) it gains you a whopping 0.0627psi "ram air" and increases by the velocity squared. So at 120mph you get 0.25psi ram air.  Is it worth it?

 

If anything, concerning the theory and situation I brought up, I would like to get rid of any ram air-effect:)

 

My thought was that at 70km/h (40-45mp/h?) the hood lifts if not latched, and would that increased air pressure amount to something that could set the jetting off at redline at 3rd and 4th gear?

I never considered the lift theory Toby brought up. Now I don't know:)

[jimk]

It's the air flowing up and over the hood and steeply set windshield that creates a low pressure area above the hood and so why the hood will lift up a bit at highway speeds if left unlatched.  Toby didn't say where the low pressure side of the manometer was located but if it was in the car, I doubt it there would be much differential pressure under the hood with respect to the cabin.

[Stevenola]

What kind of fuel pump do you have? a lot of times if not enough volume, at high rpms it will not keep up with demand and will lean out.

 

Is your float level to spec?  Not too low?

 

Have you tried going up a size on the mains just to see if it affects the high rpm regardless of what it does to mid range rpm's?  If it does affect it then the mains aren't letting enough fuel in under high load and it will lean out.  But this of course will affect lower ranges by making it richer but it will at least confirm wheter or not enough fuel is getting from the pump and mains.  Because the air corrector did not richen you up at high end may indicate it's simply running out of fuel.

 

So if you end up going richer on the mains you'll have to shift the progression curve leaner, this usually involves a smaller idle jet but first things first.

 

Typically it's best to jet the main first then work back that but it's often the last thing adjusted and then causes a chain reaction of frustration and despair.

 

Check everything before messing with emulsion tubes.

 

First thing I'd try is checking to see if enough fuel is there at high rpm's.

 

My $.02

[GreenSwede]

What kind of fuel pump do you have? a lot of times if not enough volume, at high rpms it will not keep up with demand and will lean out.

Is your float level to spec? Not too low?

Have you tried going up a size on the mains just to see if it affects the high rpm regardless of what it does to mid range rpm's? If it does affect it then the mains aren't letting enough fuel in under high load and it will lean out. But this of course will affect lower ranges by making it richer but it will at least confirm wheter or not enough fuel is getting from the pump and mains. Because the air corrector did not richen you up at high end may indicate it's simply running out of fuel.

So if you end up going richer on the mains you'll have to shift the progression curve leaner, this usually involves a smaller idle jet but first things first.

Typically it's best to jet the main first then work back that but it's often the last thing adjusted and then causes a chain reaction of frustration and despair.

Check everything before messing with emulsion tubes.

First thing I'd try is checking to see if enough fuel is there at high rpm's.

My $.02

Stock mechanical fuel pump, but new. Well, 2 years and few miles.

The float level is to the specs I could find. Can't remember off the top of my head.. 35 or 38 mm's? A good spec would be helpful?

I also changed the needle valve to .250 (from .200, I think)

As I recall larger mains fixes the high rpms (5500-6500 or so), at least almost to afr 13, which is the target. But with those I got really rich on low rpm full throttle, not to speak of beating around town. Can't remember afrs now, but 11-12, I think. Around 3-4 k rpm.

I'm not sure I follow - you suggest setting mains first, then idles, then airs? Airs set by checking around 3-4k rpm?

E-tubes are F50, by the way. Fitted when delivered.

Very thankful for all responses, by the way!

Edited January 15, 2015 by GreenSwede

[Stevenola]

Ok so it seems your fuel pump is up to the task because you can get to 12.5-13 afr with a bigger main jet.

 

Typically the air corrector doesn't have a large affect on afr until the upper range of the rpm.  However, the ac can influence when the main jets come in.  So one possibility is a small ac is causing your mains to come in too early and mixing with your idle jets which havent stopped contributing yet.  What size main and ac are in it now?

 

The ac is a very fine tuned adjustment and should be kept at stock relationship to the mains for initial tuning.  I am not familiar with 38's but for DCOE's the formula is to add about 50 to the value of the mains for the ac as a starting point.  So if you have a 135 main you's start with a 180 or 190 ac. 

 

Next I would examine the idle jet behavior.  On DCOE's you can remove the mains and run on only the idle jets up to a point.  Once the idle jets crap out and start to pop or afr goes above your desired amount, just before this rpm is when the mains have to come in to play.  Too early, and it's rich. Too late, and it's lean.  A close overlap is what is desired.  So if possible with your carb, carefully drive on the idle jets to see when they start to crap out under slight load and no load.  Do so in a low traffic place because if you quickly step on the gas the engine will die.  Usually it's around 3k rpm under light load that they run out.  Lower rpm's the more load is put on them.

 

If you have some smaller idle jets try them with the bigger main jets to see if this helps your lower rpm and cruise afr's.  If you are cruising under light load in 2nd or 3rd gear at 3000 rpm and you start to pop/sneeze then your overlap is not correct.  Once you mash the throttle to WOT the idles are not involved and it is all main jet, this is why the mains should be set to your desired afr and the other jets tuned around it.  At least this helped my situation. 

 

 

Stock mechanical fuel pump, but new. Well, 2 years and few miles.

The float level is to the specs I could find. Can't remember off the top of my head.. 35 or 38 mm's? A good spec would be helpful?

I also changed the needle valve to .250 (from .200, I think)

As I recall larger mains fixes the high rpms (5500-6500 or so), at least almost to afr 13, which is the target. But with those I got really rich on low rpm full throttle, not to speak of beating around town. Can't remember afrs now, but 11-12, I think. Around 3-4 k rpm.

I'm not sure I follow - you suggest setting mains first, then idles, then airs? Airs set by checking around 3-4k rpm?

E-tubes are F50, by the way. Fitted when delivered.
Very thankful for all responses, by the way!

Edited January 16, 2015 by Stevenola