engine and drivetrain Weber 38 Tuning

[72bmw2002]

I have some general questions about the Weber 38/38,

 

Current Set-up:

90mm 9.5:1 compression

292 Regrind .008intake .008 exhaust valves

Weber 38

Mains: 140

Air:170

Idle:50

Float:36mm ~ min

25degrees at 2400rpm

tii manifold

It doesn't diesel here

 

its running a decent idle but it can be better.

 

I ran a vacuum test and I am getting 11-12 (it rapidly jumps) which is too low, as it should be around 15 and up. I pour sprayed water on the manifold to head area and no reaction. I sprayed some warm water around carb base, no reaction. I plugged the brake booster and no reaction. 

 

I met this guy that has been working on carbs for years (so he says) and says that I should maybe raise the air correction jets a step higher, because the cam needs more air to breath at idle, but then I from a source that is trusted that air correction jets won't effect vacuum, and neither do the mains.

 

I brought up that weber 38 tuning papers says the idle speed screw should not be out more than a 1/2 turn after touching the arm and the air mixture screws should be around 1 1/2 turns out. And his reply was idle speed can be set to what I want it to be, as far as how high I want the RPMs to be, and air mixture should be 2 1/2 screws out.

 

So I am left to wonder if that is true, If I set the speed to 1/2 turn then I idle is horribly low about 600, but If I set it past that the throttle plate will be open causing less vacuum. what seems to be others experiences?

 

 

 

[TobyB]

You mean, idle fuel mixture, right?  Yes, there's a range for it, but it turns out that Weber has used

several thread pitches, so the answer is "it varies"

The throttle plate position does matter, but it matters for the progression jets,

that add 'progressively' more fuel as the throttle opens.  They're tiny holes in the bore of the carb at throttle plate level.

And ideally, they are covered (just) when the car is idling.  You can bend, file, spindle, drill or otherwise mutilate

the throttle plate to get them to be in the right position when the car's at idle.  It's not easy.  I never have...

 

Your jetting numbers look reasonable.  The air corrector will only affect higher speed mixture.

 

 

If your valve lash is accurate, look at your timing.  Seriously.  Change it and watch the vacuum numbers move around.  Quite a bit.

The dropping vacuum needle can be a leaking valve, a misfiring spark plug, a plugged fuel injector (not your problem!) or maybe a 

distributor that's wobbling badly.

A long time ago, I spent $4.95 on a vacuum gauge and a real mechanic read it for me as he adjusted 

the engine.  He even set the valves (running!) with it.  It's a really useful tool.  But one thing he never adjusted

(beyond keeping the tach where it should have been) was the idle screw.  All it does is keep the engine idling.

Lots of other things were how he 'adjusted' the vacuum.

I still have the gauge.  I maybe use it less than I should.

If you want to add another useful gauge to it, a wideband o2 meter is also very instructive.

 

hth

 

t

 

['76mintgrün'02]

here is an interesting link about using a vacuum gauge.

http://www.secondchancegarage.com/public/186.cfm

scroll down to the illustration of the various scenarios.

#8 sounds similar to what you are seeing...

I am wondering why you set your timing to 25 degrees at 2400 rpm and not 1400.

I thought the higher setting was for the tii, but I could be mistaken.

Tom

[jim57759]

Can't wait until I get to adjusting the carb!  New block was just bored yesterday and soon to be reassembled so I can get my hands on it.  Think I will order a wide band oxygen sensor this morning and re-read all the great posts here re. final tuning.  

[psalt]

10 hours ago, 72bmw2002 said:

what seems to be others experiences?

 

My advice is to get a wide band O2 sensor, focus on what's happening under load, and forget about the idle.  What happens at idle is largely irrelevant, unless you are running a stationary engine.  I have 30 years experience with 38/38's on 02's and they are all far from ideal.  The problem is that if they are set up to provide the correct 13-13.5 AFR under load, the cruise AFR will be way too rich.  The best idle set up I've found is to do what MM recommended.  Close down one idle screw completely and idle it on one jet with the correct idle shut down solenoid.  Usually they run best at around two turns out, at 15 in of vacuum.  Have you done a compression test, and are you running a stock distributor with vacuum ?

[Dave_PA]

I was beyond fortunate to have Paul Wegweiser help with setting my 38 up. We couldn't figure out my vacuum leak. Turns out it was the vacuum off the brake booster. The gasket was about the size of a quarter. It made a huge difference.

 

Also just ordered a wide band from the group that Jason recommended. http://www.wide-band.com/default.asp

 

 

[roadhog0]

3 hours ago, psalt said:

The problem is that if they are set up to provide the correct 13-13.5 AFR under load, the cruise AFR will be way too rich. 

I havent found this to be true. Usually it's lean at cruise (about 16), and a number of us have complained about the same. Wide open/load it runs fine at 13 then back to lean for cruise.

[jimk]

Back to the question of best idle.  It's an iterative process.  Idle mixture setting, idle speed setting, then repeat the idle mixture and idle speed.  On a 38/38 this works.  On a 32/36 there is only one mixture screw.

Adjust one idle mixture screw at a time for best setting as follows.  With the screw driver in one hand and the other on the engine, select either mixture screw.  First turn it out 1/2 turn.  This tells if it is already too lean.  Keep feeling the engine.  If it didn't get any smoother bur the speed increased, things are getting better and give it another 1/2 turn.  If it gets better, give it more out until there is no change in speed or feeling with the hand.  Then begin to turn the screw in 1/4 turns at a time and keep feeling the engine. When the engine begins to labor (by your feel with roughness just beginning and the idle speed falling), it's about there and back out the screw 1/8 turn.

Shift over to the other screw and repeat the process.  When that's done, recheck both sides again because the engine uses fuel from both sides in a mixture.

Set the idle speed screw for the idle speed you want, then recheck both idle mixture screws.

 

If you have hesitation when applying load from an idle condition, as when proceeding from a stop, then the idle (progressive circuit) jets are too small.  If these are upsized, then you will need to repeat the idle mixture setting process.

 

Making high speed adjustments are a complete different subject and the above suggestions for a wide band is the new tech way to find the right stuff.  Suggest a good book on Weber carbs for that because there is a lot of magic amongst the high speed jets, emulsion tubes and power jets.

[kbmb02]

I'll ask a question based on one of the things I usually explore:

 

72BMW2002: With 25 degrees of ignition timing at 2,400, what is your ignition timing at your preferred idle speed? (say, 900 or so)

[TobyB]

Quote

 Close down one idle screw completely and idle it on one jet with the correct idle shut down solenoid.

 

This is not, perhaps, a bad idea.  However, I'd be curious if the carb fuelled all 4 equally at idle.

Time for 2 o2 sensors- or even 4!

Also, I wonder if dicking around with different sized progression ("idle") jets on the bores might

help with the transition problems a number of us have had with this carb.

 

t

[psalt]

3 minutes ago, TobyB said:

 

This is not, perhaps, a bad idea.  However, I'd be curious if the carb fuelled all 4 equally at idle.

 

 

I don't think so, the stock manifold is horrible for distribution.  Closing down one side of the idle circuit designed for a +2.5 liter V-6 is what Metric Mechanics advised with the 38's they sold in the 1980's.  I've found if you have a fresh engine with correctly finished bores, seated rings and good valve seal (+150 psi), you need the idle cut off solenoid on a road car idling above 800 rpm on pump gas.  Most of them do not have a provision for dual solenoids. I've also found if you have a fresh modified engine with high compression, a ported head, a decent cam and headers, you need at least a 150 main jet to stay below 13.5 AFR at WOT on a dyno.  If you are tuning for idle at the ice cream store, maybe not important.  The 38/38 is the wrong carb for a stock 02, the 32/36 runs much closer to ideal out of the box.  It was the right carb for an African Peugeot.

[jimk]

28 minutes ago, TobyB said:

Close down one idle screw completely and idle it on one jet with the correct idle shut down solenoid.

The progressive circuit uses fuel from both the idle screw bypass and the jet.  Shutting down one side would hose that up for sure.  I can't understand where the ideas contrary to Weber design come from. Nobody thinks, it seems stuff is just pulled from where the sun doesn't shine.  I never had any trouble following their design.

[jimk]

19 minutes ago, psalt said:

The 38/38 is the wrong carb for a stock 02

Other than maybe having heard this from someone in line at McDonalds, why is it wrong? (I don't own one anymore and don't run carbs except for the lawn equipment, but I'd always like to add to my information base).

[TobyB]

Quote

The progressive circuit uses fuel from both the idle screw bypass and the jet

 

The progression circuit draws fuel through the progression jet, as does the idle circuit.

And emulsion air for both comes through the emulsion hole in that progression jet.
 

There are 2 progression jets, progression circuits, and idle jets on a 38/38, one for each barrel.

 

When you're in idle or in 'progression', you're drawing all your fuel through the progression jets until

the mains start to kick in.  You can be cruising at 55 on the freeway before they do...

 

The carb's designed to be synchronous, but with a common plenum intake, it doesn't HAVE to be.  So

potentially, you could use one large progression jet on one barrel, and one small on the other.

Or just use one idle screw, as above, and leave the other closed.  The progression circuits

would both work, after they got exposed to airflow. 

 

Heck, for that matter, one could 'de sync' the carb,

and let one barrel close completely, to draw through only one.  Then tune THAT one to get up on

the main by the time the other was flowing.  Thus crossing over the functions of the circuits to band- aid each other...

Hey, but isn't that the way the 32/36's supposed to work???  (I, for one, am not sold on the 'sync link'.  But I haven't tried it)

 

All of this aside, the problems I had with the 38/38 were the opposite- it would run fine on the

progressions, it would run fine on the mains, but in the transition band between them it would

run lean.  The carb's designed for a higher flow RATE than a 2l engine on a plenum.

The original V6 didn't draw all that much more air, but its maximum pulse velocity was greater,

and that got the mains flowing earlier.  There are certainly ways to fix this-

but at that point, I had a race car with dcoe's and putting a 32/36 back on the street rat for street use

with a mild engine was an easier solution.  After I blew up the 'good' engine...

 

The 38 I had had no idle cutoff solenoid, and the thread for the progression jet carrier was too small to accept one.

And I never needed it, even idling at 900 on pump gas with a moderate cam at 9:1 with a 121 bigger- valve head

The stock 2bbl manifold isn't brilliant, but it's not horrible- well, until you get up to 5500, and then the even flow in both

bores of the 38/38 does noticably help keep things relatively even.  Not perfectly.  I would only race one if the rules said I had to.

But for a street/toy car that's not 10/10, it's not the biggest choke point...

 

Huh.  Did I have coffee?

 

t

 

 

Edited April 15, 2016 by TobyB

[psalt]

7 hours ago, TobyB said:

 

 

potentially, you could use one large progression jet on one barrel, and one small on the other.

Or just use one idle screw, as above, and leave the other closed.  The progression circuits

would both work, after they got exposed to airflow. 

 

Yes, that is right.  To suggest that closing one idle screw messes up the progression circuit is incorrect.  The idle jets are staggered at 45/55 to reflect this.  The need for a cut off solenoid really is determined by the actual compression, heat, and octane.  All of the engines over 9.5 compression I've built needed one to prevent bucking at shutdown.  Unfortunately, it is a different part and a 32/36 solenoid will not work.

 

All of this, and the power valve modification, are addressed in MM's 1986 "The Tuning of the Weber 38 Carburetor".....

Edited April 15, 2016 by psalt