Welcome to Lotus Stig.
Agree completely with the above comments. Get the basic information on your current setup and post on the Sidedraft forum in the above link. Prior to posting, suggest going through this “White Paper” that Keith Franck wrote:
An Alternative Way of Extracting the Best Tractability and Performance from the Weber DCOE Carburetor
© 2006 Keith D. Franckhttps://www.rx7club.com/attachments/1st ... _paper.pdfNote the White Paper was written a while ago, and he has greatly improved his products and understanding since. The basic idea is he improves the ability of the emulsion tube to lift fuel at part throttle, which allows the main jet to be sized for higher rpm and throttle opening without leaning out in the transition or running an overly rich idle jet to ‘hide’ the lean spot. He then uses the air corrector size to tune in when the main jet ‘comes on’; larger A/C and the mains start lifting fuel earlier to banish the flat spot.
Here is a short list of the basic info they will want to know to get you started
Idle jet size
Main jet size
Air Corrector size
Emulsion tube
Choke size installed
Fuel level (Keith recommends 25 mm, which is slightly higher than stock. See measurement details in his paper)
Accelerator pump spec you have it.
The particulars of your Twin Cam best you know, but I would not get too hung up on the details. Basically is it over-bored or stroked? Which cam is installed? If you don’t know I don’t think it will be a big deal as Keith is so familiar with various Twin Cams he can probably get pretty close.
I am also new to tuning Webers, and got great help from Keith on his forum. He makes custom idle jets and emulsion tubes that can really help with exactly the issue you seem to be having, which is commonly caused by going lean in the transition from the idle/progression circuit to the main circuit. He will send you his stuff by post, and can probably make a few recommendations right away to get you started.
I am not familiar with your exact model 45’s, but you may be able to tune them for your TC. It really depends on the details of the progression holes that your carbs have. It will be critical to know what choke size is installed though. The guys on the forum will likely have experience with your 45’s and be able to advise how you can effectively tune around the existing progression.
Another key point before trying to tune out the dreaded flat spot. Ensure your fuel supply is in good shape and delivering steady fuel pressure, the timing and ignition is spot on, and you have a wideband O2 gauge installed to read air fuel ratio (AFR) while making test runs.
Note even with Keith’s VP tubes you will need a selection of main jets and air correctors to test and optimize. You can get a good selection in kit form, and fill in as required from eBay.
For Keith’s idle jets, each jet assembly has air corrector holes that can be selectively closed off, so you hopefully only need one size available to tune your circuit. Note you can remove the main jets & emulsion tubes completely and test drive on just the idle circuit to confirm your idle jet size is working OK to around 2500 to 2800 rpm at part throttle, at which point it will crap out. You can then order the same size W jet from Keith if you are unable to fully tune out the transition with his VP tubes.
Hope this helps. Apologies for going on if you have tons of Weber experience with other marques. Basically the key message is your non-stock 45’s may be tuneable for your TC, but it is best to get a clear starting point and for sure know the installed choke size so you know what you are tuning to. I had never imagined tuning a carb a year ago, but I am now very happy with my newer 45-151’s, although my engine is decidedly non-stock and larger displacement.
HTH