I'm not a machanic so I was wandering if someone could explain the two carbs & valves. sorry if this seems obvious to most.Im just wandering

 

Good question, vgab! Seeing at a Honda 400 cc dirt bike on the showroom floor for the first time, I asked the salesman, "Is that a TWIN?"

The answer is, "No." The thing has two exhaust valves & pipes & etc.

Now, for FIVE valves; hmmmmmmm. Lessee, two carburetors; that leaves three valves . . . THREE exhaust pipes? Or three exhaust ports into a single . . .

Perhaps not; we need a knowledgeable insider to satisfy our curiosity!

Tree Farmer

 

Yamaha has used the 5 valve system since the mid-late 80's in there street bikes, and more recently in the YZ400/YZ426. It was called Genesis back then, but it appears Yamaha has dropped that moniker. It uses 3 small intake valves, and 2 exhaust valves. The 2 exhaust pipes are to scavenge all of the exhaust out of the combustion chamber, allowing more in to the cylinder on the intake stroke. Two carbs are more than likely for better throttle response, and more top-end.

 

Tree Farmer, you're still alive!!!! I was shocked that you didn't have the answer to this question.

Two carbs flowing into three intake valves, hmmm. Methinks there is a common intake runner for the center valve. The left and right side valves get flow solely from the same side carbs, but the center valve can pull from either/both carbs.

I agree with the throttle response suggestion above. Could you imagine the low speed response characteristics from a 46mm carb??? Two 33's would roughly equal that.

Yamaha may have a detail view of the 5 valve head somewhere on their website.

Chuck Kreuzer

 

How come 33mm + 33mm = 46mm???

 

I think the area of a circle is proportional to the square of the radius, Scooby.

Lessee: The area of twin 33's, proportional to (16.5) X (16.5) X (2) = (Oh, somebody else do the math; never was any good at long multipication.)

The cross-sectional area of a 46, proportional to (23) X (23) = (How close is this to the other number?)

Looks like the products are pretty close, as SandmanBlue suggested.

The two small carbs should atomize fuel more uniformly through the operational range than a single big one, I imagine. Then again, maybe they're PROGRESSIVE, like the multiple carburetors on hot rods of yesteryear . . . one carb alone feeds the engine a low- to mid-rpms, then, the twin cuts in; or, stomp it and you open both . . . the result was a very responsive and fuel-efficient engine for "normal" driving, with the ability to gulp vast quanities of mixture when the need arose.

Anyone know whether the Raptor carbs are synchronized or progressively linked?

Tree Farmer

 

Update: The home-schooled kid down at the post office and general store said the sum of the squares of the two 33 radii was 524.5; the square of the 46's radius, 529. That's pretty close. Looks like SandmanBlue's onto something.

Tree Farmer

 

This is going to be very basic(any more that that and you will need somebody well versed in physics, flow design and math. Here goes, two small valves flow better than one large valve with same total circumference. Three smaller valves flow even better. This also allows for a better, more even placement of the incoming fuel mixture. Same goes for the two exhaust ports, two head pipes, and to some extent, two carburators. Also(and very important) smaller valves are lighter so they will be able to rev faster, and higher and will suffer less stress than one big valve.

 

3 small intake valves will give you a bigger intake area than 1 big valve within the given space in the head . that = more air / fuel

2 header pipes = better air flow

1 main carb 2nd carb will be a vacuum carb


paul
south africa

98 blaster
99 banshee
00 raptor - on order

 

You know...

You never see technical stuff like this in the Polaris threads. If someone mentions anything technical there, it confuses them.

Sorry, but it seemed relevant to me...