[Fot] Weber vs Mikuni

[Rick Parent]

What Ralph said............
I have some experience with carbs on Spifires so I will share about 2 cents (meaning minimal, I could probably write a book) worth if you don't mind. Dual Webers look really cool but will move you up a class or in SVRA to the next group, my Dyno says don't do it, it's not worth it. Makuni's? forget it, price and parts are a problem, everyone at the track has Weber parts on their trailer. 1 1/2 SU's will do 9000rpm on a 1296 I know for sure but they are a pain to straighten out the fuel curve throughout the RPM range, don't try this one at home boys and girls, you will end up throwing them through the window. The easiest thing to do is purchase a single Weber manifold and 45 DCOE, a 40 would work as well and maybe better depending on your state of tune. I have a box full of manifolds, 2 are actually within 2hp of the dual Webers on the top with way more bottom and mid range one I fabricated and the other was purchased but only runs to 7000rpm. I would be glad to help with jetting when the time comes.
All the best! 

    On Friday, October 19, 2018 4:53 AM, ralph hansen via Fot <fot at autox.team.net> wrote:
 

  gee, my 1147 with HS's puled all the way to 8k with a 4:11 gear - stock bore block, head work, A6 cam - its not the carbs, its either timing or cam
    On ‎Thursday‎, ‎October‎ ‎18‎, ‎2018‎ ‎01‎:‎13‎:‎48‎ ‎PM‎ ‎CDT, van.mulders.marcel--- via Fot <fot at autox.team.net> wrote:  
 
 -"...hitting a wall at that point..." : is it possible this is rather pointing to the camshaft than to the SU's? If it are the carburettors, maybe the power would decrease more gradually above 6500 rpm.-In general one choke per cylinder is better, and 2 Webers 40 DCOE isn't overkill I think- what about trying HS6 SU's on your actual manifold?At what maximum rpm is your engine still reliable?Marcel
Van: "fot" <fot at autox.team.net>
Aan: "fot" <fot at autox.team.net>
Verzonden: Woensdag 17 oktober 2018 15:49:45
Onderwerp: [Fot] Weber vs Mikuni

I'm looking into switching out my twin HS4s on the 1296 race engine to either Weber or Mukinis. I know this has been done for years and I'd like to get close the best set up out of the gate. 

I don't think the SUs are up to the task of moving enough air with the new ported head and Stahl header. I've worked on the SUs and gotten them to finally deliver a good AFR , 12.6, at 6500 RPM under load but I seem to hit a wall (figuratively) at that point. I suspect that the openings just aren't big enough to deliver the mass of air required. (spreadsheet or other flow calculator link appreciated)
Biggest gate I suspect is the intake valve, 1 3/8" head (~35mm) HS4 is ~38mm at best.

Recommendations? I've seen all these setups at the track, each with pluses, minuses and overkill cause it looks cool. 

One weber DCOE 40 or 2? what jets, tubes etc for a starting point? Best manifold? 

2 or 4 Mikuni 40? jet size?
thanksmike

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