Hello guys,I have a technical question about my project,so I search for the right place to ask and find this forum.First of all I have to apologize to you guys for my bad english I will try to explain as much as I can so you will be able to understand me.
So my question is about my engine head,
I'm planning to raise the cc of my city bike so I've decide to upgrade,I'll upgrade the cylinder from 52.5mm to 56 next I'll upgrade the head valves from 21/23mm to 24/28 mm.
Next I'll get 28mm carburator with 28mm manifold.my main question is,I'm thinking to make some modifications of the head entrance I mean the entrance of the fuel/air mixture,so I'm thinking from the start of the entrance to make it 28mm and as it goes further in to the head to make it smaller to like 26mm or maybe 24 my goal is to make it like a funnel,so my question is does this will work or will not be a good idea to do it,I'm thinking that if I do that I'll increase the fuel pressure before reaching the valve and will increase the speed of the fuel also will increase the suction effect.im not a engineer or something I'm just thinking and that's why I'm searching for help from you guys.So what do you think is this a good idea to do and will this helps me in some way or it will make it worse.

We're not an engineering forum so you may have posted in the wrong place, but for what it's worth my approach on the half a dozen overbored engines I've done has been to minimise the restrictions rather than increase them. You want to get as much air in as possible and there's usually enough of a bottleneck at the inlet valve anyway.

Engines vary in their design so your one may be worse or better than average but if you've increased the capacity, put in larger valves and are using a larger carburettor, the inlet manifold would generally want to be bigger rather than smaller. Get the dimensions from an engine factory produced with the cylinder capacity and valve sizes you're using and work from there. As I said I've only modified around half a dozen engines like that so I'm no expert but my rule of thumb is that gas flow is everything.

It's a horizontal engine and the carburator is at the top,the gravity helps pushing that fuel/air mixture in there too.Im on the cross roads I down to try it with a spare head to see it if it will work,but yeah you have a point there more restrictions means slow moving mixture.....I'll try with the spare head that I have.If you want and anybody is interested about it I can post the results after the experiment.

I'm not so sure a 'downdraft' carb makes that much difference. They were very popular long ago for the reason you give but that may have been more marketing than engineering. There's more to inlet manifold design than just a pipe connecting the carb to the engine and the diameter, length and surface finish all have a role to play in mixture control and gas flow. It's very easy to guess these things and you might get lucky, but having gone to all the trouble of the bore and head work you might want to approach it more scientifically. There's plenty of books that'll point you in the right direction - A. Graham Bell's Four Stroke Performance Tuning for example is one I have. Or you could ask on an appropriate forum on Facebook, but there, I suspect, if you get ten replies you'll have twenty opinions.

The only way you're really going to know what works and what doesn't is to run before and after checks on a dyno. Much quicker and more accurate (although more expensive) than just running it up the road.