Thinking of an air-fuel meter to tune carb - any experience?

[lippy]

A follow up on this topic. Wideband meters can be great, but they also can drive you pretty far off course if you don't use them in context. After a ton of fiddling with main jets, air bleeds, restrictors, etc, I still couldn't get this carb dialed in. As it turns out, AFR readings for engines with big cams with a lot overlap are useless and inaccurate at low load/RPM because a lot of unburnt fuel and air pass through the engine, and the Wideband meter reads lean. Because I used the meter to adjust the idle screws, my idle mixture was rich and this was affecting my tune at all conditions. Once I learned this and used the wideband only for WOT and cruise under load, and tuned idle using idle speed and vacuum, the carb tune fell into place and I think I'm very close now. To be fair, Brent and one of the tech guys at Holley warned me that the best way to tune is to read plugs, and not to use a wideband. Brent also added that this is one of the reasons he specs racing plugs, as they are easier to read with unleaded gas.

[Ace23]

Originally Posted by lippy

A follow up on this topic. Wideband meters can be great, but they also can drive you pretty far off course if you don't use them in context. After a ton of fiddling with main jets, air bleeds, restrictors, etc, I still couldn't get this carb dialed in. As it turns out, AFR readings for engines with big cams with a lot overlap are useless and inaccurate at low load/RPM because a lot of unburnt fuel and air pass through the engine, and the Wideband meter reads lean. Because I used the meter to adjust the idle screws, my idle mixture was rich and this was affecting my tune at all conditions. Once I learned this and used the wideband only for WOT and cruise under load, and tuned idle using idle speed and vacuum, the carb tune fell into place and I think I'm very close now. To be fair, Brent and one of the tech guys at Holley warned me that the best way to tune is to read plugs, and not to use a wideband. Brent also added that this is one of the reasons he specs racing plugs, as they are easier to read with unleaded gas.

Lippy,intersting thread. Did you continue with the wideband tuning and if so did you get the results you were looking for?

[lippy]

Got it in the ballpark with the wideband. But you can get wrapped up in your shorts with too much data, so I took it off when I got close. Drove the car for awhile, cut some of the plugs with a hacksaw for a good read, and the only change since then is leaning out all four jets by one number. Car runs and idles well, and the plugs are clean.

Run on is a different matter. Since my car needs to idle at 1200 to oil the solid rollers, it's prone to run on simply because of the idle speed. I think Chas suggested I put the car in second, put the parking and regular brake on, and let out the clutch slowly to drag down the idle speed. At that point I cut the ignition and let the clutch out fully. Sounds scary but it isn't. Very easy and solves the problem with a racy engine.