Jump to content

Single dcoe jetting vs twin


Recommended Posts

The answer is eluding my feeble brain. I am running a Lynx/ 45 Weber set up. It runs great. I have a twin 45 pair I want to use because it looks better, sounds better and should perform better. Should the jetting be the same on the twins as on the single? The single is Spanish. The twins are Italian if that matters. Thank you everyone.  

Link to comment
Share on other sites

My brain says the jetting for the twin Italians (love that thought!) will be different for the following reasons:

With the twins, you are getting one carb barrel per cylinder.

With the single, you are using one carb barrel  for two cylinders. 

Do you know how the Lynx manifold feeds the cylinder head?  Are the runners combined, or does one carb barrel feet cylinders 1 & 4 and the other 2 & 3 (or other combination?) ?? 

I've always wondered about the pulsing effect as a result of the firing order on a 4 cylinder. Does it matter??

Sorry, more questions than answers.... 


Link to comment
Share on other sites

The jets are gonna be smaller when you fit two carbs vs one on the same engine for exactly the reasons pointed out by John. 


Dunno about Webers, but I have a "Tuning the Solex Carburetor" book that gives formulas for determining various jet sizes, based on engine displacement, number of carb barrels etc. that would at least give you a rational starting point to begin fine tuning.  I would be reasonably sure that a similar book exists for Webers.  


Just as an aside. dual 45s are a lot of carburetor for an M10, especially with a stock 264 cam and compression.  You're gonna have to fit smaller venturis as well as smaller main and air correction jets, or you're gonna have two quarts of gas & air going into a one quart engine...But those dual Webers do look cool...



Link to comment
Share on other sites

Thanks everyone for responding. More food for thought. I didn't go into details on the engine specs- it runs great with the single 45 but it has 9.5 cr and shrick 292. Tii exhaust manifold, 002 mechanical distributor, IE exhaust. From what I understand cylinders 1 and 4 are fed by venturi 1 while 2 and 3 are from venturi 2. Firing order 1342 so each cylinder alternates throats. Seems similar to what dual webers would do? Anyway, I will probably baseline with the Weber book and see what's in the nice working single carb and go from there. 

Link to comment
Share on other sites

Really get your base line from what most have done here, the standard baseline in the weber books is close, but you can do better. something along the lines of;


Choke 34 mm

Aux venturi  4.0 or 4.5

main jet 130

air corrector 180-190

e tubes f9 or f16

needle valve 175

idle jet 45 f9 

pump jet 40 or so.


That should get you close, as long as you set your float level for whatever carb Spanish of Italian to spec and have a GOOD fuel pressure regulator and good throttle linkage.







  • Like 3
Link to comment
Share on other sites

Martysson's numbers look real.


I find that the air correctors go even smaller on the race car,

and I can't really explain why, other than 'that's what works'


But I do spend some real time making sure it stays in the mid- 13's,

as that's what keeps the lap times down.




  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...