Carb Tuning - Richer? Leaner?
 

Exactly when is lean, too lean without engine damage? I know rich is always the better default but at what cost to fuel economy.

I'm in the process of fine tuning my carb (Pro Systems 750cfm double pumper), and at cruise (55mph, 5th gear, 2000rpm) the Wideband O2 sensor is showing about 15-15.8. With just the slightest touch of the gas pedal, it goes to 13.7-14.4. I'm 15-17"s of vacuum at idle with a 6.5 power valve, thinking I should be at an 8.5.

The lean condition was a bit worse, mid 16 on the gauge before I jumped from a 74 jet to a 76 on the primary. Throttle response is really good and it gets into the secondaries nicely.

I'd go to a 7.5 pv just to be safe. That will keep you a hair under your low vacuum number at idle.

Get your AFR's in the 12.9-13 range at peak rpm and you should be fine.

Well --- Neither of the above gurus answered your question...:lol:


Running "lean" at cruise will not damage your engine.... Cruise is LIGHT LOAD... flat street -- just motoring down the road. Lean WILL damage your engine under LOAD.

15 A/F ratios at lightly loaded cruise conditions are fine.

I answered the question I knew the answer to, know it all.:lol:

If it's not surging at cruise/light load then it's fine. 15's at cruise won't hurt the engine. As above, at WOT with a naturally aspirated combo 12.8-13.2 A/F is normal; I usually lean towards upper 12's though just as a safety factor, even though some combos make a bit more power at low 13's A/F at wide open throttle and full load.

Okay --- fair enough!


:lol: :lol:

Thanks a lot for the great responses Todd, Gaetano and Greg. :thumbsup:

Jody too!!............. that's good detail on the WOT numbers, I need to check those.

Scott... I have a Wideband air/fuel gauge in my car and at cruise (light load) it always measures in the 14.5-15.5 range. At wide open throttle it measures 12.8-12.9. This is what was programmed into the ECM and it works great.

The main thing is to keep the AF ratio in the high 12's to low 13's at wide open throttle like the other guys said. When I hit the throttle hard the AF drops immediately into the 12's.

Jody and Dale, you guys were right on!!

I switched to 76 jets from 74's, and jumped up to the 8.5 power valve and it's damn near perfect. At cruise now AFR is at 14.3 to 14.8

At WOT, AFR is at 12.8-13.3. I went from the pink that the carb came with, to the blue with a very positive change, but I might change again to the orange cam. If I didn't have the wideband O2 sensor and just went by "seat of the pants" feel, it's damn near perfect for my driving style.

:morepower :hapdance: :excited: