Carb Tuning Advice

[996tt_Stevo]

My 3.0 SC Race Car had PMO 46's, it's running a little Rich at idle and little Lean up top... I take it I need to up the Main Jet for the Lean... any advice gratefully appreciated.

P.S. I know it will have to hit the Dyno again.

[Targa Time]

How do you know that your carbs are running rich at idle and lean on the mains? As I understand it the exhaust "pop" and carb "spit" can occur under both lean and rich conditions. Idle Air and Mixture screws should be able to adjust your rich idle circuit. The only way I know to check the mains for sure is an hour of high speed driving, then check the plugs.

[red 2.7]

I tried lots of things and I was just stabbing in the dark for 2 years till I got an Air fuel gage then I was able to tune my pmo's easily.

[996tt_Stevo]

Originally Posted by Targa Time

How do you know that your carbs are running rich at idle and lean on the mains?

Because it's showing it on the Dyno, and that's at 2 separate Dynos

[996tt_Stevo]

It might be nothing to do with Mains, but as I have said it is definitely running rich at bottom and lean up top... really just asking if anyone knows which jets or air correction should be changed before next Dyno

[1QuickS]

Smaller air correctors will enrich the top end.

Idle richness may be idle jets too large or throttle plates too far open thusly exposing first hole in progression circuit. Throttle plates should be about 1/2 turn open from fully closed and then use idle air corrector screws to get idle speed you want plus balance individual flows out. If you must open throttle plates to get enough air for idling with the result of exposing the first progression hole (remove carb to visually inspect for this) you can get more air at idle by drilling a small hole into the throttle plate (1mm dia, 4mm from the edge, opposite from the progression ports.)

If idle jets are reduced with a resulting lean transition you can decrease size of idle air bleeds to help enrich the top end of progression or select a different emulsion tube or increase fuel level in float bowls...a little to help activate mains earlier.

[2.70Racer]

Paul Abbott is the man

[dicklague]

Originally Posted by red 2.7

I tried lots of things and I was just stabbing in the dark for 2 years till I got an Air fuel gage then I was able to tune my pmo's easily.

My advice is to install an AFR gauge permanently. I like the Daytona Sensors Wego IV sold by our hosts here. Very accurate. Self contained, logs 2 hours of data as well as the digital readout.

[996tt_Stevo]

Originally Posted by 1QuickS

Smaller air correctors will enrich the top end.

Idle richness may be idle jets too large or throttle plates too far open thusly exposing first hole in progression circuit. Throttle plates should be about 1/2 turn open from fully closed and then use idle air corrector screws to get idle speed you want plus balance individual flows out. If you must open throttle plates to get enough air for idling with the result of exposing the first progression hole (remove carb to visually inspect for this) you can get more air at idle by drilling a small hole into the throttle plate (1mm dia, 4mm from the edge, opposite from the progression ports.)

If idle jets are reduced with a resulting lean transition you can decrease size of idle air bleeds to help enrich the top end of progression or select a different emulsion tube or increase fuel level in float bowls...a little to help activate mains earlier.

Excellent advice, Thank you very much Paul.

[996tt_Stevo]

Paul, sent you a pm