I have a 1999 YZ250 that I bought as a 300 big bore. I replated the original cylinder back to OEM 250, did a fresh top end and put the carb back to factory specs (main jet 172, pilot jet 50 and clip 2nd position down). The 250 cylinder is ported and polished and stamped PA. I did not do anything with the "power jet" as I'm not familiar with this set up. I tried the air screw anywhere from 1 turn out to 3. 1st and 2nd gear sound like the choke is on. Blubbey and no power, like it's starving to get into the power band. 2nd and 3rd gear a feather of the clutch will get it to take off. Otherwise , takes for ever to get the power to turn on. Where should I start?Mix is 40:1 with Cam 2 and Klotz pre mix.
Someone said the power valves may have been modified to accomodate the 300 cylinder though it dropped right into the 250 cylinder. Any chance that could be the issue?
Pilot jet 80?! Try like a 48 pilot (sea level to 3k feet).
What needle/clip position?
Going from a 300 back down to a 250 certainly could be an issue, though I can't really be specific. I probably would have just bought an OEM cylinder.
My bad....pilot jet 50. I fixed it above. Needle is NV3F, 2nd position down. Moved it to 3rd position down and I think it got worse though I was messing with the air screw. Plug is nice light brown hue, almost copper colored.
The bike had the original 250 cylinder (OEM) which is what I put back in it. Though without knowing to what degree it was ported/polished, I suppose it could need additional tweaking from stock.
Im in NE Ohio, about 800'.
Sorry, I misread your original post about the cylinder. If there isn't a mechanical issue with the power valve, I'd really go over that carb and make sure everything is clean/in good shape. It might need a rebuild.
Get the N3EJ needle and run that on clip 2. That was the original needle for the bike.
Klotz and Cam2 is a heavy oil and way too much octane. That will make the bike richer. Also, 40:1 will make the bike slightly richer than the recommended 32:1.
Yep, the power valves were likely trimmed back for proper clearance on the larger 300 bore. So they now might have poor sealing and blowby with the smaller bore. This will make the bike run poorly on the bottom end.
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Isn't 32:1 a richer mix than 40:1?
This is were I got my "stock" specs:
Ignore my needle comment. Forgot the 99 had the PWM carb.
Going richer on the oil mixture has the effect of going leaner on the gas to air ratio. Since you put more oil in, there is less gas, per volume. It’s a very small difference from 40:1 to 32:1, but it is a difference.
Anyone else have input on the powes valves? I've read the 300 big bore requires removing marterial from the inside of the power valve to cLear the piston/rings. But I dont see how this would affect the airflow if the opening intl the exuast port remails the same?
The center powervalve (the guillotine style one) needs to ride very close to the piston and rings in order to hold compression in and properly alter the exhaust port height as it is intended to. Since the 300 bore is about 7mm bigger, the face of that valve has to be trimmed back about 3.5mm for proper clearance. If you then put this valve back in a 250 bore, it will have about 3.5mm of excess clearance to the piston which may allow compression blowby and not properly lower the exhaust port at low rpm's like it is supposed to. It's sort of like when the powervalve is gunked up with carbon and gets stuck open.
So you think the PV isn't either opening properly at low Rpm or is opening to much at lover Rpm? AND no amount of jet tweaks will fix it?
LCH- Did it run correctly before? Or was it a new-to-you bike that you are just getting put back together and dialed in?
It has nothing to do with it opening or closing. If the valve was trimmed for the bigger bore it is no longer the correct size for the smaller 250 bore.
Post a pic of the inside of the bore, the exhaust port area with the PV fully closed.
Notice how this is designed to fit very close to the piston. If yours was machined bigger for the bigger 300 bore it will no longer fit close enough to the piston in a smaller 250 bore.
New to me bIke that I purchased as a 300. It was running good as a 300. Since then I've put factory toothed sprockets, original 250 cylinder w/ top end and factory jet settings. It ran better as the 300. I didn't need the extra aggression so I wanted to turn it back to a 250 to mellow it out some. I think PV is the next step
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