Carb Size Question

[BackroadsKansas]

This is my first post here so forgive me if I miss something. My question is... I built my first Predator 212 last year and was buying parts as I was learning and possibly got ahead of myself on the carb. I picked up a 28mm Mikuni type knockoff D slide for pretty cheap but its actually working pretty well but I can't help but wonder if I'm not gaining something I could have with a more proper sized carb (vm22, 24 etc.) considering my small stock sized valves. I have the AFR around what I think is perfect running mid 14s at a fast cruise and richer at heavier throttle around 12-13 (running a wideband o2 system from Rolling Wrench). I feel that scientifically if my AFR is proper then economy and higher rpm performance shouldn't change with a smaller carb because I'd be running the same air and fuel at the same volume because I'd be limited by my head and valves but I can't help but wonder if I'd be better off on the low end acceleration etc. with a more "proper" sized carb. I'm not looking for reasons to spend more money at this point either so if I'm fine with what I have then I'll leave it be for the time being. For what it's worth the engine doesn't bog if floored while rolling and warmed up, I can't really floor it from a stop because it'd wheelie too hard.

This build is a 3rd gen Predator non-hemi 212 with ported original heads (head measured 21cc), port matched intake and exhaust, staged short header with RVL 4104 muffler, billet +40 rod, hemi style -20 flat top piston around .005 in the hole, .02 head gasket, PVL flywheel at 32 deg timing, Black Mamba Jr. cam, chromoly pushrods, Champion rockers with #26 springs and obviously a gov delete. Compression should be around 10.23.

The engine is on a Baja mb200 with a Tav2, Juggernaut driver with 5:1 on the gears and 19x7-8 tires.

 

[ole4]

That is a pretty nice combination! If you ran a chikuni or a 24mm I would think it would pick up the low end response as the air velocity thru the 28mm carb will be low till engine rpm increases if you open the throttle fully at low RPM. Once the rpm's pick up it should work just fine. If you plan on adding a higher flowing head then by all means keep the 28mm carb. Also just learn to modulate the throttle by just opening it half way for a sec then start opening it up, that would take care of the initial air velocity issues.

 

[BackroadsKansas]

Thanks Ole4! , that's kinda what I was thinking but figured I'd get another opinion. Sounds like I need to decide weather I want to buy a big valve head or another carb. I'd rather buy the head and have that gain but I suspect the carb would be the cheaper route. I originally built the engine for a drift trike project so it was always at high rpm and had much deeper gears but this bike is an offroad/trail setup so having better response down low is probably a better design, not that I won't take it up to 70mph on occasion. I'll do some looking around and see if there are some affordable big valve options out there. Thanks again!

 

[SAS289]

Are you running a Juggernaut driver? What is the RPM range you are using for off road?

Wouldn't the carb be a better choice for your off road RPM range? My guess is that you would still run very well at higher RPM with any smaller slide carb.

 

[BackroadsKansas]

Yeah I'm running a Juggernaut. I took the tach off when I mounted the motor on this bike but I'm guessing 3500-4500 for most cruising and over 6ooo if I'm getting on it, but I run much lower if I'm climbing any kind of hill or rolling over rocks or logs or just trying to get anywhere without running out of fuel. I'd like to take it on some longer rides this spring so I may reinstall my stock driver for those trips if it's mostly flat land just to bring the rpm down. I really need to reinstall my tach and look at where I spend most my time.

 

[ole4]

I have plenty of big valve heads when you are ready.

 

[BackroadsKansas]

I picked up a Nibbi 22mm round slide and will run some 0-30, 30-60 and drivability tests once the weather's warm enough to dial in the afr, it's -5 and windy here in Central Kansas and I'm getting soft so it'll have to wait till later next week.

 

[Jnunez4o88]

Did you order the black mamba cam from small engine cams website and if so did they charge you shipping and how long did it take to deliver to your doorstep?

 

[BackroadsKansas]

I got mine from Chadz mini fab. I understand small engine cams doesnt make these very often and I had to look around quite a bit before I found someone that actually had one. One vendor tried selling me one they didn't have and I waited on that one about 3 weeks after they took my money before I canceled. I'm very happy with mine and it was worth the wait.

 

[Davis]

I have plenty of big valve heads when you are ready.

They work folks!