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jdeitch

Weber 38/38 Tuning

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(edited)

I spent some time searching the FAQ for a setup I could use for baseline setting for my Weber 38/38 but I know jetting is dependent on engine specs so I was hoping on getting your help on a good starting point for my jetting.

I recently switched from dual 40 Dells to a 38/38 downdraft as an experiment. The 'out of the box' performance and drivability is acceptable, but I know there is MUCH room for improvement.

This is my current engine setup:
Stock M10 bottom end with piano tops. E12 head with Schrick 292 cam and springs. IE heavy duty rockers and locks. IE shorty header. OEM intake manifold port matched to 38/38 base gasket. IE intake manifold water bypass. Complete MSD ignition. New Weber 38/38 carb.

I have a wide-band O2 kit I bought as part of the group buy here, but I haven't figured out where to mount the O2 sensor yet. I have the weld in bung. (40 inches from closest spark plug?) I'm also not sure if my pending 5-speed conversion might cause some exhaust rework.

Thanks in advance for you help and support.

I got a ton of feedback when I was setting up my Dells. Now I hope to get the same great support for the Weber 38/38. No haters please. I still have my sexy dual Dell setup and the water bypass is still installed. I can revert to sidedrafts if/when necessary. ;)

 

BEFORE:

post-39742-0-69666100-1414631872_thumb.j

 

AFTER:

post-39742-0-58829200-1414631898_thumb.j

Edited by jdeitch

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Well, before the helpful advice starts pouring in, I have a couple of questions.  

 

In a recent most you made, I read that you consider the bypassing the manifold water jacket to be an improvement and I am curious as to why.  It is my understanding (from working on VWs with one centrally located carb and long (frosty) runners) that without heating, the manifold is subject to chilling due to the vaporized fuel passing through and this makes it harder for the fuel to stay in suspension, which can lead to pooling and an incomplete burn.  I know it is beneficial to have cold air coming into the carb, but I thought the heated intake was a good thing, no?  Which brings me to the next question, has anyone run the stock air filter housing on the 38-38?  

 

The wide band will obviously come in handy while dialing this in, but it may be a good thing that you have waited, in that I have read of at least one person who had to relocate the bung, to accommodate the five speed conversion.  I don't see myself making the swap any time soon, but I would like to place the bung in such a way that it works with either transmission.  Maybe someone can shed some light on that concern.

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Suggestion: Start by removing carb top, documenting jets and checking/setting float level at 'up' and 'drop' positions. Write down idle jet numbers and position of idle volume screws (aka 'mixture' screws), too. I believe you'd be wise to start with low speed calibration, achieved via the idle jets and idle speed screw (getting a decent idle with the speed screw set 3/4 turn or less may be the first challenge with your engine build). When you've got the low-speed settings right, then move to tuning the main jets and air correctors (sure - there's not much detail there - but an approach which will help prevent issues down the road...) -KB

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(edited)

I just had a rebuilt engine installed about 600 miles ago, and it sounds similiar to yours.  I have an IE Shorty Header, brand new 38/38, new 9.5:1 IE pistons, but stock tii dizzy with points.  OEM intake manifold has been machined out to match the carb.

 

The jetting set up on my new carb by Ken Blasko, in Long Beach, who did my rebuild, is Mains 132, Air Correction 195, and idles were originally 45s but I've changed them to 50s, and the car runs very nice.

 

This might give you a starting point to work from.

 

Bob Napier

 

r.napier AT mchsi DOT com

Edited by Napes

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I've got the exact same motor as you and I recently put in an IE-sourced 38/38 along with one of the WBO2 sensors from the group buy. I was running an old, crappy chinese knock-off 32/36 which had numerous problems, mainly a choke that didn't work at all. So I needed to go with a new carb, and figured the 38/38 would be wise, given the theoretical potential of my engine. After about 3-4 months, I'm convinced that I need a new spanish 32/36. The 38/38 is dialed in about as well as it can be, but for a DD, it's still just a bit too stinky rich, and too sensitive to jetting. Plus it just doesn't give me any more bang than I felt with the 32/36, besides a more sensitive throttle. 

 

The jets I ended up running are 50 idle, 140 main and 185 air corr, symmetrical primary/secondary (not really pri/sec on a synchronous carb). I can go down on the idle and the main and it will idle fine, with a slightly better idle AFR, but it starts to hesitate when I apply throttle. My AFR at idle is about 13.2ish and under load at full throttle 12.7ish while at cruise on the freeway (4000 or so rpm) it sits at 14.5-15.0. Overall I don't think the WBO2 helped me all that much tuning this carb. My nose and butt dyno gave me enough info, plus my car really only likes the one jet combo, so the AFR readings were pretty much irrelevant.

 

As for the water jacket bypass, my 38/38 runs much better when warm, and I strongly suspect the water jacket is helping. Can't prove this though. Good luck. 

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codigoRojo - before giving up on the 38/38 you might try going down to 135 mains and up to 195 air corrections and see if that improves things.  As you can see from my post above that's about what I'm running and I love the performance I'm seeing.

 

Bob Napier

 

post-10-0-57748500-1414603260_thumb.jpg

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I too remember getting stuck between 135 and 140 mains- the 135's were fine when you were on them, but

made the lean- out in transition worse....

 

As to the oxygen sensor, you can actually put it quite a ways downstream and it'll be fine.  They're self- heating,

so if they're too cool, they just pull a bit of juice.

 

On a 'stock- type' 4-2-1 manifold and downpipe, I put mine 12" downstream of the 2-1 join,

and on a 4-1, 12" downstream of the collector.  

What's more important is to 'clock' it as high in the pipe as you can- the sensors hate water,

and the higher up they are, the less likely they are to get wet.

The E30, which spits water like crazy when it's warming up, has eaten 3 sensors in 3 seasons so far...

 

hth

 

t

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I have a stock air cleaner assembly on a 38/38.  I bought the air cleaner (powder coated gray) from Coupe King, and at my request they cut it to fit the 38/38.  It fits perfectly.  

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The jets I've been running with since the carbs was new are: 145 MAIN, 170 AIR CORRECTION, 45 IDLE.  

I was able to find a VW place locally that has some jets in stock and I have just installed: 140 MAIN, 190 AIR CORRECTION, 47 IDLE.  I will try this setup for awhile.

Based on the feedback so far, it appears 140 MAIN, 190 AIR CORRECTION, 50 IDLE may be the best setup for me.

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(edited)

Well, before the helpful advice starts pouring in, I have a couple of questions.  

 

In a recent most you made, I read that you consider the bypassing the manifold water jacket to be an improvement and I am curious as to why.  It is my understanding (from working on VWs with one centrally located carb and long (frosty) runners) that without heating, the manifold is subject to chilling due to the vaporized fuel passing through and this makes it harder for the fuel to stay in suspension, which can lead to pooling and an incomplete burn.  I know it is beneficial to have cold air coming into the carb, but I thought the heated intake was a good thing, no?  Which brings me to the next question, has anyone run the stock air filter housing on the 38-38?  

 

The wide band will obviously come in handy while dialing this in, but it may be a good thing that you have waited, in that I have read of at least one person who had to relocate the bung, to accommodate the five speed conversion.  I don't see myself making the swap any time soon, but I would like to place the bung in such a way that it works with either transmission.  Maybe someone can shed some light on that concern.

I have left the water bypass in place primarily to ease the transition back to dual side draft carbs, if desired, easier.  I understand what you are saying about fuel temps and pooling in the intake manifold, but in the HOT desert Southwest I never considered this being an issue.  I will have to do some more research.  I do also use a heat isolation spacer between the carb and the intake manifold.

 

I have a very rare JAM adapter that allows the factory air cleaner to be used on a Weber carb.  I'm currently running a small K&N filter.

Edited by jdeitch

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I too remember getting stuck between 135 and 140 mains- the 135's were fine when you were on them, but

made the lean- out in transition worse....

 

As to the oxygen sensor, you can actually put it quite a ways downstream and it'll be fine.  They're self- heating,

so if they're too cool, they just pull a bit of juice.

 

On a 'stock- type' 4-2-1 manifold and downpipe, I put mine 12" downstream of the 2-1 join,

and on a 4-1, 12" downstream of the collector.  

What's more important is to 'clock' it as high in the pipe as you can- the sensors hate water,

and the higher up they are, the less likely they are to get wet.

The E30, which spits water like crazy when it's warming up, has eaten 3 sensors in 3 seasons so far...

 

hth

 

t

Thanks for the info on the O2 placement.  I would have not considered the moisture issues you mentioned.

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(edited)

I just tuned a 38/38 that was poorly configured for a 2.2 MM stroker, BY MM...  He was only making 117HP and was in the +/-10s on the dyno's AFM off idle all the way through. Drove like that for 2000 miles before finding out..  luck he has rings.

We set him up with 135 mains, 170 air correctors.  'stock' tubes.  and adjusted the float level to have about 3/16" of fuel in the little kick out area of the bowl, below the float hinge.. had the fuel level a bit too low for a minute, and the motor would starve at 5k on a two or three shift WOT pull..  I also installed the group buy AFM into his dash pod. (Peel off the black film on the LEDs if you do this installation. Its too dim if you don't.)

We have his AFR at 13-15 average, from idle. to cruise, to 6k.  Runs like a stuck pig.  Waiting to hear what his next dyno pull gets him.

Edited by eurotrash

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I just tuned a 38/38 that was poorly configured for a 2.2 MM stroker, BY MM...  He was only making 117HP and was in the +/-10s on the dyno's AFM off idle all the way through. Drove like that for 2000 miles before finding out..  luck he has rings.

We set him up with 135 mains, 170 air correctors.  'stock' tubes.  and adjusted the float level to have about 3/16" of fuel in the little kick out area of the bowl, below the float hinge.. had the fuel level a bit too low for a minute, and the motor would starve at 5k on a two or three shift WOT pull..  I also installed the group buy AFM into his dash pod. (Peel off the black film on the LEDs if you do this installation. Its too dim if you don't.)

We have his AFR at 13-15 average, from idle. to cruise, to 6k.  Runs like a stuck pig.  Waiting to hear what his next dyne pull gets him.

Thanks for the info.  I read about your exploits on that engine.  What did you use for idle jets?

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I've got the exact same motor as you and I recently put in an IE-sourced 38/38 along with one of the WBO2 sensors from the group buy. I was running an old, crappy chinese knock-off 32/36 which had numerous problems, mainly a choke that didn't work at all. So I needed to go with a new carb, and figured the 38/38 would be wise, given the theoretical potential of my engine. After about 3-4 months, I'm convinced that I need a new spanish 32/36. The 38/38 is dialed in about as well as it can be, but for a DD, it's still just a bit too stinky rich, and too sensitive to jetting. Plus it just doesn't give me any more bang than I felt with the 32/36, besides a more sensitive throttle. 

 

The jets I ended up running are 50 idle, 140 main and 185 air corr, symmetrical primary/secondary (not really pri/sec on a synchronous carb). I can go down on the idle and the main and it will idle fine, with a slightly better idle AFR, but it starts to hesitate when I apply throttle. My AFR at idle is about 13.2ish and under load at full throttle 12.7ish while at cruise on the freeway (4000 or so rpm) it sits at 14.5-15.0. Overall I don't think the WBO2 helped me all that much tuning this carb. My nose and butt dyno gave me enough info, plus my car really only likes the one jet combo, so the AFR readings were pretty much irrelevant.

 

As for the water jacket bypass, my 38/38 runs much better when warm, and I strongly suspect the water jacket is helping. Can't prove this though. Good luck. 

 

same boat, similar engine, and same conclusion. Plus very similar jetting. Before I go down the twin sidedraft route (which is inevitable as I now have a Ti badge for my NK) I might try drilling a hole in the throttle plates to improve the idle. Seems to me it's VERY tricky to get these to idle right on the idle circuit. It's a good carb for high performance alone, but a bit crap for an all road round driver. 

 

I think you'll find your experiment will be a flop, unless of course your old dellortos were in a bad state. 

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