48ida jetting q's

[Richierich56]

Hi all.

In lieu of my new AFR gauge arriving, can anyone just give me an idea of whether this jetting will be a decent start point?

2276 motor, 86C, 44x37.5 heads with 52cc chambers, 1:4 rockers and 10:1 comp. Altitude is pretty much SL.

Carbs are EMPI 48's with 40 vents and:

165 mains;
200 airs;
60 idles;
F7emulsions

The carbs came off another motor and did have 150 mains in them which I guessed would be too small. This is just for a start point so any advice would be really appreciated.

[richie]

Looks a decent starting point, might want 65 idles, I would guess you would be inbetween 60s and 65s with that combo and 65s should make it drive smoother on the transition from idle to mains

cheers Richie

[neil68]

I had a very similar engine:  2332 cc, 48 Weber IDAs, Web 86C, Scat 1.4 rockers, 10:1 CR, 44 x 37 044 oval ports, 54 cc chambers.  The best change that I made was to ditch the F7 tubes that came with my IDAs and install F2 tubes.  I ended up with 42 venturies, but 40's also worked well.  Jetting was 65 idles, 210 air, 170 main and 2 mm needle valves.

[Richierich56]

Cheers guys - hopefully sounds like I'll be in the ballpark then. Why would 150 mains be in there to start with - are they not way small for 48's?

[neil68]

If I recall correctly, 150 mains and F7 tubes are the original jets that came with IDAs along with the standard 37 mm venturies.

There is a formula in the literature that suggests the main jet should be ~4.0 to 4.3 times venturi size, therefore 150 mains with 37 vents is close.  For 40 vents, you'd want 160-170 mains which I found to be fairly close.

I'm now up to 44 vents and 185 mains, with larger intake valves & cam.  The dyno will tell you what works best for your engine.