Question about changing Weber/PMO main jets.

[moneymanager]

My 2.7 has PMO carbs set up for the street with 140 mains. The car runs perfectly. It has been suggested that there might be a bit more power available with 145 or even 150 mains for track use.
Question: can I make this fairly small change to bigger mains without changing three or four other jets/adjustments at the same time or am I pretty much getting into a major retune?

[Steve@Rennsport]

Generally speaking,.......main jets are sized to venturi selection while factoring CR and displacement.

You can change MJ's without adjusting/tuning anything unless its a big big difference in jet sizing.

Remember, you can use Air Corrector jets to fine tune the AFR's at WOT near the RPM peak.

[moneymanager]

Thanks Steve. Are you suggesting fooling with the air corrector jets as an option to a bigger main?

[Steve@Rennsport]

Originally Posted by moneymanager

Thanks Steve. Are you suggesting fooling with the air corrector jets as an option to a bigger main?

It depends where in the RPM range and throttle position one needs to make a mixture correction. MJ's are changed to alter AFR's mainly between 3K & max RPM from 50% to WOT.

If its slightly lean or rich at peak RPM, then an A/C change does the trick without affecting mid-range.

[Eagledriver]

Unless you have numbers for your AFR's, you are wasting your time. I have a wideband O2 guage in my car and have found that my car was running richer than I thought. I've had to go to smaller main jets to get a good mixture. (12.5-13.2 at WOT). A change of mains from 150 to 155 results in about .7 change to AFR on my 3.0 SC motor. I also found that a track temp of 65 degrees results in .5 leaner than a track temp of 90 degrees.

-Andy

[James Brown]

afr is critical, one on each cylinder would be best but you can get by with 2, one on each side.

[moneymanager]

Thanks all, very helpful.

[moneymanager]

I'd like to resurrect this thread to ask at what rpm the air correction jets might be expected to make a difference. Steve says peak rpm, but surely a change would show up earlier. 5k? 6k? Thanks.

[Eagledriver]

I have found that the air correction jets make smaller changes than the main jets. I have also found that either one only shifts the "curve" up or down. It doesn't change the shape of the curve. Maybe the emulsion tubes can change the shape but I suspect not. I just find what I think is the best compromise. It ends up at 13.2 AFR for most of the RPM range and goes rich at low and high RPM. As rich as 11.0 at 7200 and 4000.

-Andy

[rbogh901]

Are you measuring AFR's on both banks or individually?

[moneymanager]

For the moment just one. Thanks.