Originally Posted by STANIMAL
What/where did you drill the slide ? Why ?
This is very simple to answer:
after changing jets`and tuning mixture screw my idling was fine, but when I tried to open throttle a little bit I still had some caughing and backfiring, not much, not often, but sometimes. When the bike was warm, I didn´t had that at all. The problem was generaly solved.
As you can see the slides in the pictures, they suffered a lot from lean mixture. So I drilled the side holes (holes beside center holes) up to 3mm.
This allows the slide to move a little bit faster upwards, an allows higher lift of slide with throttle is already a little bit open.
From my other an earlier bikes I know very well how this carb works.
But drilling the slides is not good if you do not raise the stock needle at the same time. Drilling slides only do cause a leaner condition at lower rpms till the taper of the needle begins to work.
So I had to shim the stock needle up to 1mm, 2 shims, perhaps a thruxton needle doesn´t need that or only one shim.
At least you can say, the drilling and shimming makes a better Throttle response an eleminates my caughing completely, also in the time when I´m running with choke.
But let me tell you again one thing:
70% of solving those problem is not drilling the slides or raising needles: It is the correct tuning of the idle mixture screw. Bought my bike a few months ago with 30k miles on it, completely stock, carbs never touched. And my bike ran first time correctly with those screws 3 turns out (40 pilot jet) or 2 turns (42 pilot jet). I cannot imagine if there is any Bonnie in this forum with (40 pilot jet) 2,5 turns out + open pipes and uni filter running correctly in lower rpm´s at sea level.
But as you can see, I still had sometimes with that enriched slow circuit some caughing while warming up. Ok, I could have been riding like this, test ride was done and everything was ok at the moment the engine was warm. With drilling and shimming I eliminated this caughing completely. But be careful, caughing can also be caused by air leaks in the intake section, check this first before starting any other changes. Running without or with snorchel hadn´t had any influence of the slow jet circuit at all, bike was also with snorchel too lean
I would strongly recommend to do settings of idle mixture screws and choice of jets at first before any other changes, and do always one thing after the other and make test rides. After changing exhaust and pulling snorchle follow the major recommendations of main jet sizes here in the forum.
If you´ve finished all changes go to the dyno and see results while controlling AFR.