Jump to content

Weber 38/38 tuning questions.

Recommended Posts

Looking for some tuning help on my Weber 38/38 downdraft. Just installed on the car and it’s running very lean. So lean, it doesn’t even register on my AFR gauge; above 18. The set up is a 38/38 with electric choke. 145 mains, 50 idles, 175 air correctors and F66 emulsion tubes. I changed the air correctors and emulsion tubes to 175 and F66 per the research I did on “good setups” for the carb. Elevation is about 1,000’. The manifold has been widened for the carb and is a peanut shape opening. It has an itg foam filter. And I’ve tried it from 1/4 turn out on the mixture screws all the way to 3 turns out, just to see if that would bring it out of the lean territory.


Engine seems to run ok, meaning idles around 800 and revs without any flat spots. Maybe it’s just getting too much air for the set ups I’ve been researching? The gauge and o2 sensor are operating fine. They are fairly new and were good with the 32/36 I took off.

So not sure where to go except to start experimenting with the jets, but that could get pricey fast. Any ideas or recommendations are welcome. Thanks.

Link to post
Share on other sites

Does the car run rough?


A too rich condition (or cylinder not firing) can actually read lean, if the fuel isn't burned, O2 isn't used up, so it reads lean.


Also make sure there are no exhaust leaks ahead of the sensor.


But it's most likely as Jim says, I'd check the gauge, do a free-air calibration.

Link to post
Share on other sites

Car seems to be running well, so could be a faulty o2 sensor? Doing research, what the gauge is reading sounds like a sensor problem. But is it common for a gauge and sensor to work one day and then not, without touching anything or knocking the sensor around? Seems coincidental that it would stop working when I changed the carb.


But I do agree that if it were actually running that lean, it would not be running well at all. I guess I could invest in a new o2 sensor and see if that does it.


Link to post
Share on other sites

When I've had them fail, they've failed completely-

either full rich or full lean, or the heater's popped.


18:1 isn't unstartable, but it shouldn't be running 'well'.


How's your float level?




Link to post
Share on other sites

Checked the float level tonight and it’s 35mm and 45 mm. I’ve read it should be  51 on the low side, but the tab won’t allow it to bend that far and still maintain 35 mm when the float is high.


The internet has many stories of the AEM wide and gauge pinning all the way lean with three dashes on the readout and it’s the sensor, not the engine. The plugs were a little grey, but didn’t look too bad. I’ll try some new sensor parts and see if that was it.


And on an unrelated note, I replaced the water pump among some other things and the pump housing is about 1/16” or so thicker than the previous one, so now the fan blades are hitting the bottom of the lower radiator tank slightly. I heard that bending the blade tips is a solution, but is that really a good and safe one? And for all wondering, I made sure to get the alternator, water pump and crank pulley all aligned for the belt. Just have the radiator spacing issue now. Any ideas?

Link to post
Share on other sites

No way it could be that lean and "run fine" IMO. 

It's gotta be the sensor. If you have a vacuum gauge you can hook it up to the intake manifold and get best lean idle with timing light and carb settings. Iterative back and forth. 

I confirmed that the best vacuum at idle I can pull is at the 12 on the afm. Doesn't go lean through the rpm band but I still have to do some drive testing.

Its a 38/38 so idle isn't its sweetest spot. 

  • Like 1
Link to post
Share on other sites

I agree with others, there is no way a 2002 engine will run ok at those AFRs. 

If it was , your down pipes would be glowing bright red  and your plugs would look like you just pulled them out of the box. 



Link to post
Share on other sites

Changed the Bosch sensor and looks to be all good now. Changed to the previous jet setup and readings are 12.8-13 at warm idle, 13-13.5 cruise and 11-12.5 under full throttle. Engine seems to be running the well and idling steady, so sounds like it was just the sensor. Thanks for all the replies.

  • Like 3
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.