Weber 38 on 87 mm spx 4wd 2.6l rejetting help!

[88maximus]

Tired of putting new plugs in from being so lean, how do you re jet and tune these carbs? Never done jetting or tuned a carb so bare with me please. I have a header no cat and have a Flowmaster as well. Any help is appreciated

[geezer101]

It might only need the lean/rich screw adjusted. Wind it all the way in and then back it out 1 3/4 turns. Drive it and back it out 1/4 turn each test drive until it feels like it's running properly. I think by the time you get to 3 turns it will have a flat spot on acceleration. If adjusting doesn't work, time to re-jet it.

[SubGothius]

With the main idle-speed screw turned in 1-1/2 turns (starting from it barely contacting the stop, with choke fully disengaged), verify the idle mixture screw in the carb base is turned out 2 full turns from fully-seated (just snug, never tight), then fine-tune that with the engine fully warmed up. You'd just turn it in 1/4 or even 1/8 turn at a time until the engine starts noticeably stumbling, then back it off to the last adjustment point you were at before it started doing that, then you can fine-tune the idle speed to ~8-900 RPM or so.

If the mixture screw winds up more than 2-1/4 turns out or less than 1-1/2 turns in, your idle jets need to be larger or smaller respectively. If it idles fine within that adjustment range but still runs lean on the street -- you'll probably notice lean-misfire stumbling on acceleration and/or high-throttle cruising -- you will need to change your main or air-corrector jets. The main/air jets only come into play at mid-high RPMs; below that, the "idle" circuit does all the work.

Main jets are big steps in tuning the mixture, and air jets are for fine tuning within those big steps; larger mains are richer, while larger airs are leaner -- i.e., when swapping to a smaller (leaner) main jet, also swap to the smallest (richest) air jet you've got and increment up from there to fine-tune it leaner, or going to a larger (richer) main jet should also start with the largest (leanest) air jet you've got and increment down to fine-tune it richer.

Note the Weber 38 is synchronous, meaning both barrels are the same size and open together by the same amount simultaneously, so they need to be jetted identically. Weber jets are sized in increments of 5, and typically you'd go one step at a time, but airs could take larger jumps if you know you're way off target still.