YAMITO 421 help from the owner and gurus

[Phantom]

I need some help since I'll be going to build a 421cc upon reconsidering my option to ditch my original plan on going for the 392 but now the 421 got my interest.
There are some question about the 421cc that I may need to get some clarifications to those folks whom may have worked and have further knowledge about it that can give me an absolute answer.

The YAMITO project bike that I'm currently working with will mostly be used more for street/road and rarely on track and I want to achieve getting a 80-100hp but I do not know what brand of cylinder I should get to achieve the target HP point that I want some said go for the Athena ones as they are better suited for road while others said to just grab CPI/Serval over the Athena.

I am wondering is it still safe to use the stock gearboxes when hitting 80-100hp?
There are some answers that make it unclear for me as many said it's still okay to use the stock gearbox even if its 100hp but going higher I'll be needing the billet gearboxes, while others said going over 80-100+HP I definitely be needing the billet gearbox for good while other have suggest to just go grab a 1st-3rd close ratio conversions and hope for the best while these mixes answers/suggestion it does still confuses me about this concern also the fact that the prices of a complete gearbox is not a joke to begin with.

how's the maintenance with these 100hp engines?
are they really that fast to wear and tear as many claims it is faster to go maintenance when having higher HP on the engine than some lower built hp's in mind

what would be the best carb size set up for a 421?
as I have two mikuni 35mm carbs that I may end up re-jetting up but others said go for 38mm instead.

is it worth getting the cool heads over the stock included ones on the 421 kits?
or its better to skip getting cool heads and just focus on other necessary parts that I may need mostly

what are the best mods I need to achieve the HP I am targeting with?

I have found someone who will do the Dyno, custom Titanium made chamber, new wire loom, engine cradles and such for my build.

Will the stock clutch hub housing be good to go? or do I need to grab some billet clutch hub instead?

so far here are the parts that I have on hand :
vforce 4 reeds
Zeeltronic pcdi-10t
Zeeltronic handheld and usb type programmer
Heavy duty Clutch pack
Heavy duty springs
Straight cut water gear
Slick shift detent arm
Billet Water Pump Cover
Billet impeller
straight cut primary gears
Aprilia RSV4 front fork set up for both my bike and my own weight
Ducati Panigale V4 Ohlins Rear shock
Brembo M4 front and back


Parts that I'm about to grab :
Hotrods +4mm crank
421cc big bore kit maker still unknown which will be
Driveline ball bearing type lock up clutch
Genuine Output Gearbox Bearing
Genuine Output Gearbox Needle Bearing
Genuine Input Gearbox Bearing
Genuine Input Gearbox Needle Bearing


any other suggestions will greatly help me out.

[jerele]

Hello.
It would be a really interesting project.
And I encourage you to do it.
However, I think a 64x54 has nothing to envy to a "big head".
Youtube: "Crazy RD 350 YPVS Onboard !!!"

Greetings.

[Phantom]

Hello.
It would be a really interesting project.
And I encourage you to do it.
However, I think a 64x54 has nothing to envy to a "big head".
Youtube: "Crazy RD 350 YPVS Onboard !!!"

Greetings.

Hi there!

Just be needing some clarifications and surely will be a go project I am not willing to let go nor die.

Cheers!!

[(F5)]

I certainly can't answer all your questions. People seem to get away with stock gearboxes. The std cooling is a bit of a joke but with a decent radiator should be ok, I doubt a cool head will actually improve anything, but I'm a cynic. Ideally it should come in the back of each barrel, around the exhaust ports properly and up into the head. Std setup is like a bucket with a lid. A hose in the top on one side and an outlet on the other side top.

Anyhoo, I just touch 100hp with my 496 Cheetah at 10,000, or just below from memory. I run 35pwk . Mine is a roadbike so midrange, packaging and even fuel consumption drove my choice. I'm on stock clutch machined for extra plate to stop the slip. I lengthened the clutch pull arm to make lever pull acceptable.

It's all about the pipes and where you aim peak power as to how peaky it will be and how long it will last. My 496 I was told would reach 120hp in race spec. That's interesting but not my use.

Dont be greedy. Dont rev it too high to get impressive numbers. I'd shoot for 80 with a 421, you could get more, but would be plenty in a 125 chassis.