Main Jets size for 3.0 Twinplug w/40 Webers?

[Spydermon]

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!

[jgparker]

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

[Wayne 962]

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

[jgparker]

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

[rs911t]

I double checked my 2.8 when I got home

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.

[gumba]

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?

[jluetjen]

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.

[Spydermon]

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.

[jgparker]

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.



Thanks,

JP

[Wayne 962]

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

[Mr Beau]

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.

[Steve@Rennsport]

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

Both play a big role in jetting selections.