More Bottom out of '04 YZ125

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Jun 19, 2000
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#1
My son is getting terrible starts on his new '04. We came off an '03 KTM that he was able to pull 2nd gear starts on. He can't on the YZ and he says 1st gear starts are worse then 2nd gear starts. We are running the bike stock with a 50 tooth rear sprocket.

Jetting:

Pilot - stock
Main - 430
needle - 3rd position (alternate needle that comes with bike)

Anyone have good results with bolt-ons (pipes, vforce ect..) in getting a little more response off the bottom.
 
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#2
I always did 1st gear starts on my '02 and ran 13/51 gearing. A pipe and adding the vforce won't give much more bottom if any always seemed to add a tiny bit more to mid/top my advice practice more! :-)
 

atc3434`

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#3
More practice will always help. More power isn't going to be the end all of making it faster. I can pull second gear starts on my '01 YZ, and they don't get any more pipier than that. Just scream and clutch. IMO, a 125 will never have good bottom, there is no way to "Really" improve it notably. So just work with the power where it is and have fun!
 

elf

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#4
you should try a phathead! Its a head with different combustion chambers,that have higher compression ratios. I put one on my sons 2000 yz 125 and it works great. I have a 03 125sx and his bike is just as fast as mine. Also his bike has more bottom end and pulls harder out of corners than mine. I know your bike is a different year but higher compression generaly gives any motor more low end.
 
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#5
doesnt higher compression do more for top end ?
by increasing compression you make HP
to get bottom end would you add a longer crank ?

big bore short stroke = high revs and HP

could always use a lighter rod in stock length

slip clutch to keep bike in the HP range

YZ's deliver smooth power, keep it where it makes the most

just some suggestions
 

Jaybird

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#6
I tend to agree with atc3434. A 125 will never have the brute low band power you are wanting. I have a '99 yz125 that has great low end power by charectoristic, but there is a limit to what it can do.

I would suggext you not throw money at the bike's low end. So many do that and are dissapointed, not to mention broker.
The cheapest and best way to gain torque on the low end of a small bike is to do it with the sprockets. Realise that changing the ratio for a gain on bottom end will have an equal negative effect on full out. I like a 4:1 ratio on 125's (13/52).
 

atc3434`

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#7
Right on with the gearing Jaybird, definitely my ratio of choice. Best modification you can make to a 125, IMO.
 
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#8
Not looking for brute force just a little more off the bottom. My feeling is if the KTM could pull 2nd in the dirt, should be able to get the YZ to pull 2nd gear starts, without resorting to modifying the head or porting the thing. I was thinking about going up one on the rear sprocket, two would be too much, Texas has some fast tracks.
 

atc3434`

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#9
Thats why there are 5 gears in the tranny!! Just goofing... a tooth you'll feel, 2 would really make a difference... and really, is there a track with a 5th gear topped out section. ;)
 
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#10
There are other companies out there that make them - but these guys have great prices. I don't know if they are legal in whatever ? that you race. So check with your promoters.

We put one on a CR125 and the diff in power was more than just noticable, the kid is selling his bike now (after a year), and he's taking the PhatHead off and taking it with him LOL The bike pulls much harder with higher compression, but with higher compression you lose a little top - normally. We didn't notice it because the CR125 is ported for about 12billion RPM so it just kept pulling and pulling and pulling. It was the best single thing we did to the bike - that and the new Three Stage Boyesen Rad Valve, which worked wonders. I love intakes, and have tried both big name brands, and both have great spots that they work on certain bikes etc, but the new three stage Rad Valve really really impressedme. If you drop PhatHeadRacing a line, tell them Mikeee said "Hey'.

MP
 
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#11
surely your 04 should pull 2nd gear starts.my2000 dose (13-49)gearing too .stock engine<crisp jetting helps.maybe a bit more wfo.running in the vets class against 250s & 450s. maybe i,ll stay on my 2000 than getting a 04!
 
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#12
The 03 model which is very similar, had the same dilemna - 1st or 2nd. Never quite got it right. Many clutches later and numerous timed starts, then got it right. Without expensive mods, just PC pipe, vforce and ports polished, the bike rocked off the line in second. The trick is to use ONE larger tooth rear sprocket with a launch control, PC is good.
Anton