Recommended 2003 YZ125 Performance Mods?

[theMotoMan]

What performance mods do you recommend for a 2003 YZ125? I live in Denver, so the altitude is apx 5000' and will be used for MX and Harescrambles. Since I ride the vet class, I'm willing to consider a big bore, although I don't think a stroker is worth the money.

Thanks in advance

 

[bclapham]

big bore. call Eric Gorr- he might even have one on the shelf ready to go.

 

[mxer842]

Pro Circuit pipe punches up the mid range nicely, and maybe just a touch of bottom end as well, doesn't do much for the top...stock gearing is a 49 which is about perfect coming out of corners and for jumps right out of corners for me as i can stay in 2nd, but it doesn't like to make the jump from 3rd to 4th without falling off the pipe so a 50 or 51 tooth should hopefully help out, but i haven't tryed one yet. Not sure what you are running in your bike in terms of jetting, but i rode Berthoud yesterday and ran a 380 main and clip in the 2nd from top, stock pilot...that setup seems to work about perfect throught the front range with only air screw adjustments, with or without the PC pipe as well. You'll never get the bike to be competitive with the 250f's so the best you can really do short of a big bore is have the bike properly geared and crisply jetted so it pulls hard out of corners.

You can always spring for porting and Vforce, but in my opinion, we'll always be at a disadvantage to the 250f's down the straightaways, so we have to make it up in corner speed, which has more to do with suspension set up and brakes anyways.

Sean

 

[Jeff Howe]

I hear the 00 powervalve spring helps a bit.

 

[Jman271]

I have done a few mods on the 03 YZ 125 that worked pretty good. For woods, you can take the head down from stock measured 1.25mm or .049" to about 1mm or .039". You will need to go up on the octane though since your CR will now be higher. Also, we dropped the needle to the 2nd position and ran a 430 main and played w/ the a.s. to get it just right. We are @ 0-1000 asl so your jetting will be diff a bit than ours. Next we took the PV gov assembly out of the large clutch cover. Looking @ the spring w/ the engaement gear to the left, take it apart and add another SS flat washer to the assembly to preload the tension on the gov a bit more. This helps remove a slight latency in the accutation of the PV closed, PV open and makes for a more even transition. This is only for the 03, the 04 came stock like this from Yami. Along w/ a VF3, and a shorty doma silencer, or PC shorty, etc, you should have a nice low-mid burst good for woods and tight MX for not alot of $$$ spent. If you ride faster MX tracks, you can still take a few thous off the head, but not too much or it will start to go flat on you up top. Then there is gearing, ignition tinkering, etc. Good luck and post back. ;)

ps- look at the rubber boot intake of the carby, it has a huge neg step in the flow from the carb into the reed cage. If you rough it up w/a porting tool or mounted stone, you can fill it in w/ a rubber epoxy that is fuel safe to remove any eddy currents that may impede the intake flow.

 

[theMotoMan]

big bore. call Eric Gorr- he might even have one on the shelf ready to go.

I am interested in a big bore, but was wondering which one is the most bang for the buck? It looks like the 134 is a bolt-on deal, but the 144 requires you to send in the whole motor? I was wondering if it was worth the trouble and how reliable it is. Do you need to change piston and rings more often with a big bore?

 

[mxer842]

Also, the suspension is really soft on the bike. I have the compression turned all the way in on both the forks and shocks and am running the max. oil height and can still bottom the forks on hard landings and the shock if i hit a berm hard enough. If I was planning on racing this bike next season I would invest in some stiffer springs and a revalve, but since this is just a bike i ride since my 250f blew, I'll save my money for an '06 250f and not invest in an old practice bike I had from seasons ago anyways.