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 PAir of SU 1.5 carbs on a 998cc engine

 Created by: MF46
Orig. Posting Date User Name Edit Date
Apr 2, 2021 07:20PM 1963SV3  
Apr 1, 2021 08:07AM MF46  
Apr 1, 2021 07:55AM Spank  
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 Posted: Apr 2, 2021 07:20PM
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Member since:Oct 23, 2020
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While 1.5s might be a bit of an overkill for a 998 - depending of course on the actual state to tune - they shouldn’t present any real problem.   

Vizard recommends the same spring/needles for a modded 998 whether fitted with twin HS2s or 4s or even a single 4.(blue/E3). 

Because twin SUs essentially work independently of each other, the real benefit of twins over singles is the efficiency of the manifold.  Each carb sees the demand of its particular pair of cylinders.  So the instantaneous demand on one of a pair is pretty much the same as the demand on a single (allowing for the relative efficiencies of the manifolds).

Depending on the state of tune of your particular engine the twins may not buy you much.  However, if they’re what you have, then there’s no reason they won’t (can’t be made to :) work.

IMHO, once you move away from completely stock, you’re not going to get anywhere near optimum without a dyno ...and someone who knows how to work it :)

Cheers, Ian

 Posted: Apr 1, 2021 08:07AM
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Thank you, I suspected as much :(

 Posted: Apr 1, 2021 07:55AM
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It is certainly possible to get your 998cc engine running "right" on dual 1.5" SU carburetors, but that setup is certainly outside the "norm" for what most 998 engines are capable of utilizing.

It's sorta like can my 13 yr old son wear my hand-me-down suits? Yes, we can make my old suit work... but the tailoring fees to get it to fit him correctly are likely a bit unreasonable and that's assuming there's someone out there with the tailoring skills and willingness and time to do it.

The SU has a variable venturi / opening size by design and there's a good chance that your 998 engine will never create the demand to ever see the carbs fully open at Wide Open Throttle, so you'll be using only the thicker part of the needle range on them.

You'll have to just play with it and don't expect many people to be able to offer specific advice on what needles to run or what you need to do to make them work because you'll be in a very limited family of people to have ever tried to do this.