48IDA Jetting Start Point. 2276

[steve_pugh]

I know I know... Age old question of what to jet my carbs to... I will be having some Rolling road time, but with plenty of engines out there in a similar spec, I kinda hope that someone in the know would be able to tell me the best starting point set up so that we can fire up the engine and do a Cam Burn etc and test things before taking to the rollers for the hopefully fine tune last tweaks.     Also this saves time on the rollers swapping stuff about. 

The spec is:
Autolinea Race Case.
CB 82mm CW crank.
94mm AA Forged Pistons
FK10 Cam.
Scat Solid Lightweight Lifters.
42 x 37 Valves. 
9.8:1 CR
DPR Lightened Flywheel. 
Kennedy Stage 1 Clutch.
48 IDA carbs.   
Merged Header with Stinger.

This is a Race Motor but is in a street car and as such will likely see some sunday drives but it's by no means a daily. 

Thank you.

Steve     
   

[kb]

What's your venturi size?
2 or 3 progression holes in the carb? :-)

[spanners]

180m x 200 air x 42 choke x 60 idle 55 pump maybe 60 pump for track day/circuit racing.

[steve_pugh]

Current setup is:   (Edited)
37 Vents.
120 idle air
70 idle.  

120 Air in F7 tubes and 135 mains.

2 progression holes. But if this is worth havin a 3rd drilled then this will get done too.  

[Old Guy]

Current setup is:
37 Vents.
120 idles.

120 Air in F7 tubes and 135 mains.

2 progression holes. But if this is worth havin a 3rd drilled then this will get done too.  

120 idles? 120 airs? Really Steve?  I've never even seen a 120 idle jet.  I think the largest we carry is a 70.  Also, the 120 air seems mighty small.  I've never been a fan of an air corrector being SMALLER than the main. I think Spanners' suggestion is a good place to start.  F-7 tubes should be ok.  The engine will tell you what it likes from there.

[steve_pugh]

Current setup is:
37 Vents.
120 idles.

120 Air in F7 tubes and 135 mains.

2 progression holes. But if this is worth havin a 3rd drilled then this will get done too.  

120 idles? 120 airs? Really Steve?  I've never even seen a 120 idle jet.  I think the largest we carry is a 70.  Also, the 120 air seems mighty small.  I've never been a fan of an air corrector being SMALLER than the main. I think Spanners' suggestion is a good place to start.  F-7 tubes should be ok.  The engine will tell you what it likes from there.

Thanks guys.  They really are just as I got them.   I've never had a new pair of carbs before so it's not even like they have been run on something close to what I'm gonna run. 

[stretch]

Steve, that's a 70 idle jet.

[steve_pugh]

Steve, that's a 70 idle jet.

Ah.  Is the 120 an air corrector value for the idle?   

[stretch]

Steve, it's hard to tell from the pictures, but, it looks to me like you've got 70 idles / 120 idle airs.  135 or 185 main jets with 120 airs and F7 emulsion tubes.

[steve_pugh]

Steve, it's hard to tell from the pictures, but, it looks to me like you've got 70 idles / 120 idle airs.  135 or 185 main jets with 120 airs and F7 emulsion tubes.

Cool.  Glad I've got that sorted.  It's definitely a 135.  Bit blurry I know.  But i took it quickly just so I have a reference without forgetting! :-)

There is another post in this section about he relation of Venturi etc vs intake valve size which says:
"So with that in mind, we hear, select your Venturi size, general rule = 1 or 2 sizes below your inlet valve size, ie 42mm inlet valve, commonly no larger than 40mm venturi. Next based on Venturi choice select the main jet. General rule somewhere between  multiply by 4 to 4.3 times the venturi size, ie 40mm venturi = 160 to 172 mains fuel jet."


So should I up to 40 vents?     Spanners said 42. But that seems a bit big?

[stretch]

Steve, i'm not the best person to comment on this.  There are people on this forum with way more experience than me.  Hopefully a couple of them will help out.  I just wanted to make you aware of the jets, etc, that you have in your carbs now.

[steve_pugh]

Steve, i'm not the best person to comment on this.  There are people on this forum with way more experience than me.  Hopefully a couple of them will help out.  I just wanted to make you aware of the jets, etc, that you have in your carbs now.

No it's all helpful.  You have clarified that I know even less. :-)
At least I know what I have now. 

[leec]

I've tried a 70 idle, it runs but is a bit big. A 60 or 65 tops is best for street/strip

The 70 idle was when we stepped up to a 44 Venturi but it made the car harder to drive on the road at lower rev range

Lee

[spanners]

Oh sure, a 70 idle is huge, I would guess 60 would be the go, it should cut the cylinder  with about 1/4 turn remaining to go in, and kick it back in by 1/2 to 3/4 out from fully in, 1 turn out should be all smooth, then give it 1/4 turn, all this with the linkage off obviously.
If your porting and manifolding are correct, you can run up to valve size on the choke, I come down 2 mm for circuit racing where we need the best possible throttle responses, so with 44 inlets, I'd run a 42 choke, don't forget 42 inlets use the same seat  as 44,s  so you should be good with a 40 choke, assuming you don't want to leave power in your pocket, I run big squirters again for throttle response,  you may not need 60 squirt jets but it will be a starting point and about were you want to be, 50 sounds to small, so maybe 55, also 3 hole IDA' can use a size down on mains over the 2 hole ones.