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'01 YZ250F questions

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My last question post was answered and is long gone down the line, so I thought I would just open a fresh one:

I am putting a YZ250F into a Birel 125 shifter kart - supposed to be legal F125 for SCCA (thus WCMA) Solo II class. However, since my last Yam dirt bike was (still IS) a '79 TT500, I am not familiar with the new stuff at all.

Does the '01/'02 share it's countershaft chain wheel spline (sprocket mount) with any other Yamaha (WR, newer YZ, or otherwise)? I am still trying to find a 428 conversion sprocket, but it would be nice if something else shared this spline that USED a 428 chain stock (sorry, my local dealer is kind of new and not at ALL helpful for this stuff).

There are two rubber tubes from the top of the cam box. One is about 1/2" and is ovbiously a breather that vents overboard. The other is under 1/4". What the heck is it for? (you guessed it, I got the motor, not the bike).

I need a strong pressure pulse to run a fuel pump. In our sprint karts, we use the crankcase pressure variation to drive the pump, but there is no easy looking place to get that signal from the YZ. Any suggestions? (no, the cambox with that great big breather pipe won't have enough pressure change).

Regional final in two weeks - big box of bike and kart parts on the floor - getting into a full scale panic mode.

Pat

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you can't just mount a low pressure car fuel pump.
Not car pumps, these are vacuum/pressure diaphragm pumps used on karts, snowmobiles, etc. They use the pressure pulse from the crankcase to pump fuel to the carb (our tanks are too low for gravity feed). It is done all day long in many different applications, but you need a good "signal" strength.
I believe that little hose is for the oil
Oil as in supply, return? OIl for what (the pressurized oil to the head comes from a metal banjo fitting, shared with the gearbox supply, I am talking about the rubber line beside the big vent hose).

Thanks - Pat

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on the old (ie steel frame) 250f's the small line was just a vent from the oil tank in the frame to the head. since 06 that is now gone because the tank is below and in front of the cylinder.

your search for a pulse may be had to come by. the big hose in the head is from the piston, but i have no idea how it will work for you.

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I spoke with Ken at MTD Karting today (they sell shiny new WR250F kart engine packages FROM YAMAHA!!) who solved my sprocket problems (we are pretty sure). The Yam factory engines use the signal from the carb throat, which, to my surprise, is strong enough with WOT to keep the fuel pump ahead of the engine needs! I may tap into this (blank on my carb) port, or just use the signal from the cambox vent and see what it does.

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on the old (ie steel frame) 250f's the small line was just a vent from the oil tank in the frame to the head. since 06 that is now gone because the tank is below and in front of the cylinder.

your search for a pulse may be had to come by. the big hose in the head is from the piston, but i have no idea how it will work for you.

I finally took the time to pull the cambox cover, and now see what you meant. The passage for the large vent tube goes straight to the crankcase, whereas the little line is just venting the cambox. These will both have to go to a catch can, but I think (make the HOPE) that the pulse down the big tube is so large that I should be able to tap into that to send a signal to the pump diaphragm. Actually, I am somewhat surprised that something that is intended to be in such a dusty or wet environment is so open to the crankcase! Makes me wonder how much power could be recovered by making that vent even less restrictive in flow, as it must provide as much air to the crankcase each rev and expell as much each half rev as the inlet ports do to the cylinder every other turn - yes 4x the flow (actually, 4x the displacement, but no where near that amount of flow due to the total volume below and tiny porting).

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um, no.

all it has to do is allow 250cc (oem size) worth of air to move. that volume need not be totally expelled or sucked in each stroke. just moved.

on a running motor it will not lift dust. but from a dead motor, kicking it with the tube in a puddle will suck water. big no no.

if you use a tube that will hold more than 250cc's then that is all you need.

rpm just determines how many times the air column must change direction. at high enough rpm, it comes to a human perceived standstill.

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Like rims on the freeway? or are those just spinners 👍

what are you going to do about the oil sump. its contained in the frame. I know Dr.D makes a wetsump kit for the alloy bikes, maybe one of those will work

um, no.

all it has to do is allow 250cc (oem size) worth of air to move. that volume need not be totally expelled or sucked in each stroke. just moved.

on a running motor it will not lift dust. but from a dead motor, kicking it with the tube in a puddle will suck water. big no no.

if you use a tube that will hold more than 250cc's then that is all you need.

rpm just determines how many times the air column must change direction. at high enough rpm, it comes to a human perceived standstill.

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um, no.

all it has to do is allow 250cc (oem size) worth of air to move. that volume need not be totally expelled or sucked in each stroke. just moved.

Yes, but on the demand side of the equation, the inlet port must move 250 cc every 720 degrees (i.e. 250 cc of displacement, whether it is fulfilled or not) whereas the vent must only move 250cc every 180 degrees, thus literally one liter of air must be displaced (by crankcase demand) in the same 720 degrees. The much lower CR of the chamber (due to gearbox and crankcase volume) means the actual delta P is lesser, but the flow (one of the oft-negected parts of crankcase design - especially in multi-cylinder cases) is still created by a demand of that much displacement (relative to time)
on a running motor it will not lift dust. but from a dead motor, kicking it with the tube in a puddle will suck water. big no no.
While you didn't catch the mathematical concept, you are expressing, in practical terms, what my concern was (for bikes, not a problem for karts I should forever hope!). How many times do you expect that a WR might get stalled in a submerged situation? I know that the only thing I prefer from my KL&KLRs (oops, did I say a dirty word on the Yam forum?) over the TT is that they have electric start that saves me trying to kick under a foot of mud. I can just see an enduro WR rider punching the button and doing as you say, because the first 250 cc IS almost fully expelled under water over the first 180 degrees, the NEXT 250cc is inhaled between 180/360 (as you mentioned, if the tube volume is under 250cc) and the whole thing starts again from 360 - 540, less breather tube volume) and completes the next suck cycle of the fourth 250cc of DISPLACEMENT between 540 and 720. You have to add the absolute values and ignore the sign. (+250 -250 +250 -250 = 1000cc displaced)

if you use a tube that will hold more than 250cc's then that is all you need.

rpm just determines how many times the air column must change direction.

except for that pesky inertia thing. Get some water moving inward at a good clip, and it will carry on surprisingly well. On top of that, the stock ZY breather tube is only about 50cc
at high enough rpm, it comes to a human perceived standstill.
there's that inertia thing going on again. However, while the NET result is a virtual standstill, there is a considerable pressure wave bouncing around, and that is what I need to harness to drive the fuel pump. Since I am not tearing this engine down, can you give me some idea of how well (port size) the crankcase communicates with the cambox? The little vent line may be a better source than trying to "T" into the top end of the big breather (than will pick up a bit more resistance to flow from it's required catch can). It is not flow that I need, just pressure change (which the cambox should have in abundance...IF it has big enough holes down the chain drive to the gearbox and/or crankcase).

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