Carb Tuning - Richer? Leaner? - Lateral-g Forums

protour73 · #1 06-15-2013, 05:08 PM

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.

Vegas69 · #2 06-15-2013, 07:22 PM

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.

Track Junky · #3 06-15-2013, 07:47 PM

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

**GregWeld** · #4 06-16-2013, 08:04 AM

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

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.

Vegas69 · #5 06-16-2013, 09:16 AM

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

**GregWeld** · #6 06-16-2013, 03:13 PM

Quote:

Originally Posted by Vegas69

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

Okay --- fair enough!

**camcojb** · #7 06-16-2013, 10:11 AM

Quote:

Originally Posted by GregWeld

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

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.

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.