250f mod options

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#1
I have gone with the Big Gun pipe, and am now wondering what options are available for additional increases. My particular interest is in headwork. I am wondering about the amount of gain that is available by headwork. I am thinking that I would want to explore this option first to keep my bike within AMA limits. Ideally, I am looking for more mid to top without sacrificing any part of the existing powerband.

I did a quick search on the boards, but didn't see anything, maybe I missed this discussion?
 
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#2
Mr. Farley-

My fear is that going with an overbore will make the bike ineligible for AMA 125cc competition. I must admit, I don't have my rule book handy, but can't imagine that doing an over-bore and/or increasing cc wouldn't make the bike illegal.
I guess my question is more focused towards does anyone have any knowledge/experience of the amount of benefits that may be gained by headwork.
 
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#3
I haven't gotten my 250F cylinder head yet so I haven't had the chance to do in-depth testing. If it's anything like the 400 & 426 heads I'd suggest you look elsewhere for horsepower increases. The heads can be cleaned up and made a bit better, but there is nothing major hiding in the stock 400 design. It's VERY GOOD. Chances are the 250F head is even BETTER out of the crate. At 13500 rpm you can't screw up the cylinder head and still make good broad range power like this engine does. Blueprinting the cylinder head and deck heights will certainly pay some dividends but tend to be a poor value in terms of HP/dollar. I realize this won't stop "tuners" from charging outrageous prices for porting. caveat imperative of cavere

High end fuels (oxygenated or high energy blends) will meet the power requirements you outlined and still be AMA legal, but it's still not cheap and it takes some tuning work to realize it's advantages. No free lunch when the engine starts out this good :)

The camshaft is the area where there is the most to gain, but I haven't seen anything (SO FAR) that meets your requirements.
 
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#4
thanks again

That was the info I was looking for. BF said you were the techie in the group.

I believe that the design is extreemly good and thought that improvements would be very small. And also agree that there are numerous tuners that would disagree with me and be willing to charge a nice fee for a few angles on a valve edge.
I am curious regarding cam options, but then again have not found anything I feel comfortable with. Please post any results you come up with regarding cams.
I am a firm believer in stock for the most part. I ended up with the Big Gun pipe for it's nice mid range torque. I will have to say that it was a hard decision between Big Gun and Dubach's pipe.
 
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#5
It may be worth investing in a second pipe with a larger ID and shorter primary length for top end tracks . The stock pipe can be shortened to shift the torque a bit up in the rpm range, but the effect isn't as great as a new pipe with a larger ID. Running specific setups for specific tracks seems like the way to go for expert level riders running the 250F.

We'll be cracking open Okie's 250 to do a 267 kit soon, so I'll have more info at that point.
 
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#6
pipe-

.. the good thing is that Mike @ Big Gun would probably make that for my to test. I'll call him up and pester him for it. Will keep you posted if I get one.
 
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#7
I just installed Gorr's 2mm overbore kit in my YZ. I think it's legal for the 125 modified class. It's less expensive than most pipes and adds about the same amount of power. I can feel it mostly in the low to mid RPM levels. Top end seems about the same to me. I can now pull 2nd gear off the starting line on a consistant basis. I can also ride the bike more effectively in a lower RPM range. Overall, I think this is the second best mod after a pipe.

'01 YZ267F
Yoshimura pipe
49T rear sprocket


Mark
Mark
 
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#8
Try a good air filter, like White Bros or Uni. Then dial your jetting in. These are only small gains but, are also cheap mods. And they do add up to a larger gain overall.

vetwfo'er
 
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#9
Originally posted by vetwfo'er
Try a good air filter, like White Bros or Uni. Then dial your jetting in. These are only small gains but, are also cheap mods. And they do add up to a larger gain overall.

vetwfo'er
There is zero performance difference between the OEM and aftermarket air filters.
 
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#10
another thought-

..and I know I've already spent a lot time on another thread regarding this particular subject, but in speaking with the R&D guy from Pro Circuit at the races yesterday, his comment was to put a high compression piston in the bike. I then suffled over to the other side of the pits and asked the Vertex piston importer if they had that type of piston available yet. (This is the REM race on Sat at Glen Helen- pretty much anyone in the industry who races races here evry weekend). He said the Weisco boght out Vertex. Unfortunately Jay from Weisco wasn't here this weekend. (Whew!)

So, In a long winded sorta way- Mr. Rohrich, your thoughts? Also, would you still run pump gas, or would you have to switch to a 99 octane leaded fuel? I have always understood that you don't run race gas in Yamaha's 4-strokes.
 
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#11
Re: another thought-

Originally posted by thump chump
I have always understood that you don't run race gas in Yamaha's 4-strokes.
And toads cause warts.
 
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#13
Race Gas in Yamie 4-strokes-

Ahhh- but I have this info straight from the horse's mouth at Yamaha- Ed Scheidler. I was told that running race gas actually caused the valves to gum up and affect performance. One possible caveat though- this was on the YZ400. I have not heard this from Ed regarding the 250f. But I would presume the same holds true due to the similarity in head design.

But at the same time, does a high compression piston neccesitate the use of race gas as bumping up the compression in a 2-stroke does? I would believe so, but would like to hear from someone who has direct experience in this area.
 

holeshot

Crazy Russian
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#14
Re: another thought-

Originally posted by thump chump
(This is the REM race on Sat at Glen Helen- pretty much anyone in the industry who races races here every weekend).
Yeah, I heard that one of Rich's idols (initials J.W.) races at that jumpless, featureless, mudhole, geriatric rider's track.:p
 
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#15
Re: Race Gas in Yamie 4-strokes-

Originally posted by thump chump
Ahhh- but I have this info straight from the horse's mouth at Yamaha- Ed Scheidler. I was told that running race gas actually caused the valves to gum up and affect performance. One possible caveat though- this was on the YZ400. I have not heard this from Ed regarding the 250f. But I would presume the same holds true due to the similarity in head design.
With all due respect to Mr. Scheidler assuming ALL race gas will have the same characteristics and cause the valves to (in his words) "gum up" while pump gas will magically escape this dire fate is ludicrous. Anyone who is attempting to extract real performace from these engines and thinks they will do it with the random supply of chemicals sold as "pump gas" is sorely mistaken. Pump gas is fine for the weekend warrior who rides for fun a couple of times a month and won't ever touch the jetting. Racers who use pump gas while bouncing the engine off of a 13.5K revlimiter are missing out on power and response. The 250F and 4xxF engines might not blow up on pump gas, but you can be sure they are far from optimized under these circumstances.

For your situation where you see a major swing in altitude density between your two riding areas, it would be very easy to jet the bike for the cooler high density area using a leaded race fuel like VP12, Phillips B32 or something similar, and use an oxygenated fuel like the VP MR series or Phillips B35 for the warmer lower density area. The change of fuel may be enough to balance the air denisty change once you get it dialed.