Main Jets size for 3.0 Twinplug w/40 Webers?
 

What size main jets should I use with my 1973 slightly modified 3.0 twinplug engine. I have 40mm Weber intake with MoTech ignition. It doesn't seem to have power at higher RPM, I just pulled out the main jets and they're only 110's.

On the street I use a Sport exhaust (two in, two out with very little packing) ... but on the track I'm using open exhaust and it seems really lean. My friend has a 3.0 single plug with 40mm webers and bosch and he's running 160 main jets.

What size main jet should I use? Thanks for any advice you have!

Wow, that is small. The set-up may be for a much smaller engine. I'm running 180 mains on my 2.8. Also very important, what venturies are you running? 36 or larger I hope? I got my carb set-up advice from Richard Parr at PMO. The man knows his stuff, and can sell you all the Weber/PMO parts. I don't believe Pelican sells carb set-ups.

Good luck,

JP

We sell all of Richard's (PMO) stuff here. It's somewhere in the catalog - try the search engine.

As for jetting, you'll constantly be guessing without a dyno, or one of these Digital Air/Fuel Ratio Meters:

http://www.pelicanparts.com/catalog/shopcart/911M/POR_911M_fuelCI_pg14.htm#item51

-Wayne

Woops, I do try to get everything I can from Pelican. I couldn't find Weber jets or venturies in the search engine.

I double checked my 2.8 when I got home, and I ended up with 160 mains, not 180s. I can't find a recomended starting point for a 3.0 in any of the PMO instructions, but I'm sure there is someone on this board who can recomend one.

Is that "the corkscrew" on your Avatar?

Good luck,

JP

Good. I was about to chime in about that. You probably have 180 air correction jets. That's what I run on my 2.8, but I'm only running 155 mains. Thought about 160s, but haven't put the AFR meter to it yet.

I dynoed my 2.8 twin plug yesterday. I'm currently using 150 mains, 36mm chokes. 140 air correctors. It's running rich from 4000 - 5600 rpm. I figure I will have to go up in air correctors. Has anyone used the digital air/fuel ration meter Wayne mentions?

Did anyone ever say what sort of venturi's they were using? Because all the rest of the jetting will depend on which venturi you use.

Thanks guys for all the helpful information. I think the best thing to get maximum performance is to get it on a dyno.

JP - Yes, that is the corkscrew.

Does anyone know if there is a sniffer version of a Air/Fuel Ratio Meter available? I'd rather not weld bungs into my exhaust systms.

http://forums.pelicanparts.com/uploa...1085690655.jpg

Thanks,

JP

Yes, this unit does that. You simply get the bracket that attaches this to the tailpipe - that's what I do. It gives very accurate readings (drove around with it for a long time on my truck and 308).

-Wayne

You can't compare main jet sizes unless you are using the same venturi size and the same emulsion tubes. The three combined (along with a lesser extent air correctors) adjust your main fueling.

I would also ask what cams you have and what your compression ratio is?

Both play a big role in jetting selections.