Keeping up with the 250f's

Hey guys I am helping a younger rider that has been riding for 3 years now. He is on a 2012 125 SX and is into his 2nd season of racing this year. His skills are improving quickly! I have been trying to coach him what I have learned through the years and what other pro riders have taught me. He is a small rider (140 lbs max) so the 125 does get him around the track quite well. He is pretty fearless when committing to jumping. I am teaching him that corner speed is where races are won.

He is racing 250C and schoolboy class in our local mx series. Most of the class is stacked with 250f's. Today's tracks also cater a lot to 4 stroke bikes with jumps right out of corners.

We have already been working on suspension set up and have been working with factory connection on valving. It's close to being about as good as we can get it on his budget.

Our next move is to increase the power of the 125. We are looking for more power everywhere in the rpm range. Getting out of corners faster is what we are trying to focus on to stay up with 250f's. With the amount of research I have done, I have concluded that 2 inexpensive mods that have good results are cutting the head for the correct squish band clearance, and boring the carb.

I plan to contact Cycle playground and question if it will have a noticeable gain of power. We don't have the budget to build a full race motor on race gas because this bike gets atleast 5-10 practice hrs a week put on it.

If any of you have suggestions or have any experience with these mods please chime in! I am open ears on suggestions. Thanks!

Doesn't that bike have a 39mm carb? If so then I dont' think you'll find someone who'll bore it larger. Strangely enough the larger motors use smaller carbs often and those are the carb's they bore, 36mm.

 

 

Getting the head done will improve everything and for the cost it's a no brainer. Next up would be porting the cylinder and matching the cases which helps, not as cheap but it doesn't require you to run race fuel. 

Does he have stock exhaust on it?  For me, one of the biggest upgrades was a full FMF exhaust which gave it more mid range which is where you'll be coming out of corners. 

I believe it's a 38mm but could be wrong. I haven't measure it yet. Porting is an option we are considering although it tends to be a little more $$$. The bike does have a stock exhaust on it. I figured for the price of a full exhaust, there are options that give a better power to money advantage.

As the old saying goes "if you can't beat 'em, join 'em"....has he thought about selling the 125 and getting a 250F ?  Might be a lot cheaper in the long run.  Sounds like the track layout stacks the deck against the 125.

As the old saying goes "if you can't beat 'em, join 'em"....has he thought about selling the 125 and getting a 250F ? Might be a lot cheaper in the long run. Sounds like the track layout stacks the deck against the 125.

probably not cheaper but it may help his results.

What's wrong with holding the throttle on longer and getting on the brakes later?

Answering the original question, a port job will give you better gains and potentially move the power somewhat lower in the RPM range....

Sell it and get a 150, it'll hang with 250f's.  It's more work to race it as fast as 250f but it'll make him a better racer. 

What kind of fuel do you currently use?  

 

My first thought is that you might be fighting an uphill battle if you are wanting affordable mods that net more HP without running better fuel.

We are using premium pump fuel. Race fuel is too expensive to be running as many hrs a week as we are. 

 

 

As the old saying goes "if you can't beat 'em, join 'em"....has he thought about selling the 125 and getting a 250F ?  Might be a lot cheaper in the long run.  Sounds like the track layout stacks the deck against the 125.

He has considered this, but the maintenance and cost of racing a 250f is greater throughout the season. 

 

What's wrong with holding the throttle on longer and getting on the brakes later?

 

This is the biggest thing we are working on. Like most young racers, he had a real problem coasting before getting on the brakes coming into turns.

Sell it and get a 150, it'll hang with 250f's.  It's more work to race it as fast as 250f but it'll make him a better racer. 

I would imagine the 125 can be made to run close to a 150? Will a 150 cylinder drop onto the 125 bottom end? 

I'd forget the motor upgrades.  Get it running well, keep it in top form, save the money for tires, sprockets, riding lessons, practice fees, pistons, cranks, etc!  5-10 hours a week of riding it relatively hard he'll be throwing a piston in it 6 times a year. 

 

Staying on top of maintenance and staying on the track will do more than any modifications will.  a 125 is the perfect bike to learn and improve.  If he really wants to beat the 250f's keep improving, keep practicing, use that 125 to really learn how to ride, how to work the controls, how to carry momentum, line choices all that and grab a 250sx 2 stroke in a year or two.  Keeping up with 250f's will be a quickly forgotten issue once you are 10lbs lighter and 10 hp ahead with the 250sx.  They are eligible for the 250 class as well.  It takes a talented rider to ride 2 strokes well.  The 2 years or so on the stock engine 125 will be perfect training. 

90% rider, 10% bike

 

 

How hard is he running the 125 right now?

90% rider, 10% bike

 

 

How hard is he running the 125 right now?

 

That question doesn't address the real problem though.  Everybody can "run" the 125 hard out of a corner.  Coming out of the corner at a crawl pace and then hammering it wishing for more power to clear the jump.  That's easy to ask for and what a faster bike will provide.  That's not what the goal should be though.  The goal should be to be a good enough rider to be able to carry enough speed to clear it with a 125 working it and most importantly riding with good form and technique. 

 

Im 210lbs and am riding a 125 for this season for this exact reason.  At 140lbs he's got a whole lot working to his advantage,  He can be easily 20-80lbs lighter than his competition on body weight alone, and factoring in a 125 vs. 250f or 450 he can be another 20-40lbs lighter. 

 

Work the angles, come in hot and come out hotter.  Go where the 250fs aren't.  Hit them where there lazy. 

 

Enough pep talk but that's what everyone needs to do, enough with riding these 4 strokes that do all the work for you and don't require any skill. 

With 10 hours a week you will be dropping tons of money on basic maintenance, keeping everything fresh will do more than having the potential for big hp, but not enough money left for a new piston...

Keep everything tight, clean, and lubed and just let him learn. With that much time on the bike he will figure out how to go fast without spending huge money on modifications.

.. The goal should be to be a good enough rider to be able to carry enough speed to clear it with a 125 working it and most importantly riding with good form and technique...

 

That's what I was referring to ;)

 

In other words, is he using the bike to it's full potential/ out-riding is capabilities? 

I forgot to mention that dirtrider i agreed with you but your question still left some room for them to lie to themselves about how hard they ride that 125!

You dont have these problems with your 500 lol.

Tuning should really only be used if you can keep the bike in the high RPMs for the entire lap, and race. If you put the power up top and ride it low, you'll have zero power. As standard these bikes love to rev hard.

A C rider does not need engine mods.  Like others said, just more seat time and focusing on turning is the key.  One thing I noticed when I race tight tracks is my two strokes do not like to ride the ruts from 4 strokes.  I try to square up the turn and ride across their ruts when I can.

Just keep doing laps they will need to stop for engine rebuilds before you know it! ;-) keep fighting the good fight! Lol

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