Jetting Webers for a warmed up 2.7L

nabilious · 05-25-2010, 04:31 AM

So before it blew up on me, the engine was perfectly dialed. Idle jets are 52's and mains are 140. After a complete rebuild its up and running with a couple exceptions. The only thing I changed was add a Mod-S cam in place of the regular S cam. the car has 36/35mm ports, S dizzy, ect. Now as I'm breaking it in, I'm noticing a couple of things that I'm having trouble eradicating. If I get after it in 2nd gear, 3rd, or any others, after the tack swings past 5.5k there is a slight miss as it winds out. The jetting was spot on before the rebuild, with the exception of the Mod-S cam. Could that mean its a little lean on top? baaaaaataaaaaataaaaaaaataaaataa. is what is sounds like, the 'T's' are the light miss. It use to be really crisp so maybe the little extra cam is enough to cause it to be lean? I have the timing set at 34 degrees @ 6k rpms and I've adjusted the valves ect.

One last issue, while trying to balance the carbs left to right, I cant seem to get rid of this wobbly engine note from 2.5k to 3.1k ...Whomp whomp whomp kinda sounding loopsidedness....if that makes sense.
Any pointers would be appreciated, Its so close to being right and my rebuild (my first went really well!) I'll include pics when I download them later.
Cheers,
Nabil

0396 · 05-25-2010, 05:21 AM

great question.. - sub

dicklague · dicklague's Garage

Any idea what the air fuel ration is? Might be a good place to start.

nabilious · 05-25-2010, 07:45 AM

no idea, but my setup isnt terribly unique so someone probably has some jetting suggestions. My Idle mixture screws are out about 2 3/4 turns. I dunno how you would find that under full load at 70 mph's anyway! ;-))

Trackrash · Trackrash's Garage

What size venturis are you running? What size air corrections?
I think 140 mains with 32 mm venturis is probably too rich.
I would try 135 mains and see if it clears up.

nabilious · 05-25-2010, 08:08 AM

The venturi's are 34's, the air correction jet is a 170. It ran beautifully with this recipe at one point in its life, now that I rebuilt it and changed the cam's to something slightly more aggressive, its giving me a hard time. I was leaning more towards the lean side of things, if it was rich it just wouldn't like to rev, but it revs but with some slight missing in the top end when your foot is all the way down. What do you think about my timing being set at 34-35 degrees at 6000rpm? to much? too little?
Thanks
Nabil

andrew15 · 05-25-2010, 08:27 AM

blocked idle or main jet?

Taking the carbs off and putting them back on may have disturbed some sediment or particles that are now blocking a jet somewhere.

I've had both int he recent past and they both produced a light miss (idle down low and main at 4-5k) and a bit of stumbling on accel.

1QuickS · 05-25-2010, 03:52 PM

Check the interfaces between the throttle body and the secondary venturis, if the interfaces aren't flat or the little spring that keeps them in close contact isn't doing its job then you can get uncontrolled fuel delivery into the main circuit. When I remanufacture Webers I address this issue by sanding the fuel interface surfaces of the aux venturis flat using some 320 grit sandpaper. Also if the aux venturis are the tall kind then check to see that they are reasonably well fitted as they tend to vibrate and get loose.

Paul Abbott
www.PerformanceOriented.com

Trackrash · Trackrash's Garage

It sounds just like mine did when I went too rich on the main jets. It ran fine until you put your foot down above 4 to 5 tousand RPM. You might want to look at the plugs.
35 degrees is probably too much advance, depending on your compression ratio.
Early Es and Ss ran 30 degrees total advance. I don't think this is your problem however.

2.70Racer · 05-25-2010, 07:37 PM

I run a 2.7 w/solex cams and 9.5 JE pistons at 5,500 ft in Albuquerque.
34mm venturi's, 200 air correction jets, 50 idles, 145 mains. F3 emulsion tubes.
This is a track car that I spent lots of time getting the jets right for the high altitude.
At or near sea level I would expect 180 airs and 150/155 mains, idles probably 52/55.
My timing runs about 30 degrees at 6,000. The best I can get from the stock distributor.
The floats are dead right, fuel pressure 3.5 lbs.
Float level is mandatory. Set with the glass gauge. Shafts are snug.
Double check the float level and try some 155 mains. Jets are cheap.