Posts
283
Joined
8/30/2018
Location
Chester, IL
US
Edited Date/Time
9/22/2018 7:45pm
My name is Marc & I (might be) a Craigslist junkie. I'm always looking for old dirt bikes to part out or rebuild/rehab & sell. My most recent project is a 2002 CR250. Got it super cheap and all it really needed was radiators, PV cables and a wiring harness. I figured I was a 4 stroke guy, having had a couple 450s. So I figured I'd just sell the 250 for a nice little profit. But of course I had to ride it some, just in case I wanted to keep it.
It didn't take me long to realize that this bike is a keeper. Surprisingly, I simply enjoy riding this bike more than the 450s I had. Really the only thing I don't like about the 250 is the weak low end power. I suppose it has enough low end, but more is better, right. And maybe I've been spoiled by all the bikes I've had that had great low ends (YZ250s, KX500, CRF450s etc).
Anyway, between going with a big bore kit or just having the cylinder ported, what is the best way to go about getting some really good low end power out of this beast?
Bike already has FMF exhaust, FWW and 13/51 gearing.
And I'm an old fart who only rides in the woods. I'm really not much of a revver. Guess I've gotten used to bikes that you don't have to rev out to go fast, and like it that way.
It didn't take me long to realize that this bike is a keeper. Surprisingly, I simply enjoy riding this bike more than the 450s I had. Really the only thing I don't like about the 250 is the weak low end power. I suppose it has enough low end, but more is better, right. And maybe I've been spoiled by all the bikes I've had that had great low ends (YZ250s, KX500, CRF450s etc).
Anyway, between going with a big bore kit or just having the cylinder ported, what is the best way to go about getting some really good low end power out of this beast?
Bike already has FMF exhaust, FWW and 13/51 gearing.
And I'm an old fart who only rides in the woods. I'm really not much of a revver. Guess I've gotten used to bikes that you don't have to rev out to go fast, and like it that way.
The Shop
The FMF Fatty is the worst pipe I tried on these bikes. It takes away from the top & bottom to add in the midrange vs. the stocker. The Gnarly will add to the bottom, but will sacrifice some top. It's a trade off if you're a woods type rider that would be a better option if you don't rev the bike as you mentioned.
Carb has 175 main, needle in the middle clip & 48 pilot, probably always be in the 500-1000 foot elevation range. Just got an assortment of pilots in the mail - have yet to try any others.
Just found 'fatty' on the pipe - so I guess that could be better, eh? Would just changing to the Gnarly pipe get me to where I want to be?
I've ridden a couple of bikes with a Gnarly pipe & felt it will lower the rpm range (powerband) on a 2t. The engine will feel more like a 4t with torque off the bottom with a smoother mid transition. It won't feel as explosive in the midrange with that pipe & seems to slow the revs down. Kind of like adding a FWW.
I'd suggest taking a comp. test so it can give you an idea of the condition of the topend. I believe in stock form with a fresh piston / rings it will be around 220 psi.. Anything lower than 190 #'s I'd personally do a topend on it.
-The cheapest way to increase bottom end is to run a 05+ base gasket. That lowers the ports, decreases squish and increases compression. All of which increase throttle response and low/midrange power. Bonus- easier to jet too.
-Next would be to have the cylinder base milled by .030, and then have the head machined to fix compression and squish. Same results as the gasket, just more of those results.
-Last would be complete porting. Highest cost, biggest results.
The Gnarly would increase low-end at the expense of top-end.
Have you done this mod ? I rode an 06 model a few yrs. ago with .025" milled off the base. Felt it was one of the worst mods you can do to these bikes. Varner use to do the .030" mod when these bikes 1st came out & has changed it in the last couple of years. He now does .010 milled off the cyl base unless the owner requests more.
Agree about the CR case reed engines responding really well to port work by a reputable engine tuner. It's been the best bang for your buck in my experience in about 4 decades of riding.
https://motocross.transworld.net/features/unleashed-fury-realizing-the-…
I believe the 06 CR2 with .025" milled off the cyl. also had the exhaust port raised 1mm to compensate, but it still lacked up top in comparison to one milled only .010".
Port timing / head reshape is crucial as well as a good exhaust design. An aftermarket ign. with a map to compliment the engine mods is icing on the cake.
So I think I'll start by changing to the gnarly pipe. Heck, I can probably get $100 or so for the used fatty, so it won't cost too much.
Maybe try the gasket change this winter, if the pipe change isn't enough. And go from there.
If you're changing the base gasket that is essentially lowering the port timing by .010". I'd also suggest putting a fresh topend when doing the base gasket change.
I was fortunate to be able ride a local hare scramble pros 07 model (in 2010) that had a gnarly & lower comp. head. The engine had a smooth off idle toque to a meaty mid range with a decent topend considering it had the gnarly. The lower comp. head allowed the engine to rev a little higher. It had a 4t like power band that was easy to ride.
riding with a 51 sprocket. base gasket off 0.8 and a dpr pipe. It's starts from the bottem en keeps going. Compared to a 450 no matter what brand if they are in third gear, I'm in second. If they are in fourth gear i'm in third gear next to them.
No need for a big bore or milling the base off or all that bs. You have to keep the revs more up compared to a yz or a sx or a 4stroke. If you don't want that and your lazy get a 4 stroke and stay away from a cr or get a 01 engine.
Pit Row
I figured this was probably the only way to go, until I realized that with this engine, Big bore is 265 cc - a gain of 15cc. Does that even count as Big bore? I mean with an engine like this, how much do you really gain by increasing displacement by 6%? Honest question as I really don't know.
So then I had to wonder if maybe most of the gains were due to the porting that these shops do in addition to increasing cylinder size. I mean if 80% of big bore gain comes from porting and 20% comes from the bigger bore, I'd probably skip the bigger bore - unless of course my cylinder had damage that need bored away.
I wasn't really considering a sleeve or anything else that really bumps up displacement.
in general, reducing compression as one person suggested will REDUCE power throughout the entire power band and that defeats your purpose. in your case you want MORE compression to increase overall torque and low end power, however, this is not practical because you can not increase it enough to make a large change, plus if you increase it, you will need to run some type of race gas which isn't practical or cheap.
if you want to soften the power band so it is not so abrupt but don't want to give away much hp doing that, you can try adding a reed cage spacer between the reed and the cylinder but nobody mentioned that yet. some companies sell them for some bikes.
if you want to modify your existing cylinder to get more low end without boring it out to a big bore size, you must mill the base. there is zero other option. as long as the exhaust port remains at the same height it currently is, you can not get much more low end power.
you can mill the base .5 mm then install a head gasket that is .5 mm thicker to compensate for the shifter cylinder and this will help keep your compression close to what it currently is.
as far as changing gearing as one person mentioned, you should certainly try it if you can't get a 300 cc big bore kit and the cheapest and easiest thing to do would be to get a countershaft sprocket that is one tooth smaller. this will make a big change but it will not change the characteristics of the engine other then it will be easier to keep on the pipe and you will have to shift more often., and far more often than you would with a much bigger displacement engine.
another option would be to install a cylinder from an enduro model if it it will fit because the port timing is much milder. this would have a little more bottom end and a little softer power band but it still won't have a ton of low end power like a 500 cc 2 stroke would.
this being said, in the end, there is simply no replacement for displacement.
A pipe is only part of the equation. The single biggest gains I've experienced have been from cyl. / head modifications & not pipes. An aftermarket pipe generally moves the powerband around & doesn't necessarily add HP..
"in general, reducing compression as one person suggested will REDUCE power throughout the entire power band and that defeats your purpose. in your case you want MORE compression to increase overall torque and low end power, however, this is not practical because you can not increase it enough to make a large change, plus if you increase it, you will need to run some type of race gas which isn't practical or cheap".
I didn't suggest reducing comp.. I gave the OP an example of an 07 model I rode that had a gnarly pipe. In case you didn't know the 05-07 CR2 models have an oem dome piston. They're already on the verge of detonation with today's pump fuels. The reason it was done on that local pro's bike was to have an easier to ride powerband that didn't provide a midrange hit. In a 2-3 hr. race I'd think more power can be a handful at the end of the race. I'm sure the pro new what he was doing.
"you can mill the base .5 mm then install a head gasket that is .5 mm thicker to compensate for the shifter cylinder and this will help keep your compression close to what it currently is".
This contradicts what you said above since you mention "you WANT more comp".
"another option would be to install a cylinder from an enduro model if it it will fit because the port timing is much milder".
It sounds like you don't know much about the 02-07 CR2 models. There is no option as far as I know for an enduro cyl. on these models.
i did not say you suggested reducing compression, and in fact, nobody's name appears in my post.
no it does not contradict what i stated about compression but you obviously have poor reading comprehension skills or simply lack the intelligence to fully understand my very simple explanation about compression and you selectively only posted part of my comment about it, therefore, it is out of context.
i certainly don't know every model of every year of every manufacturer, however, that is completely irrelevant.
"...you want MORE compression to increase overall torque and low end power, however, this is not practical because you can not increase it enough to make a large change, plus if you increase it, you will need to run some type of race gas which isn't practical or cheap"."
Yes, more compression is better, but as my post clearly states, it is not practical, therefore, it is not what i would suggest. I was merely trying g to explain a little to the op what affect increasing compression will have.
I think that if he installs the gnarly pipe and it is still not nearly enough low end for him that he shoudl not throw any more money at it if he keeps it at 250 cc's.
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https://www.vitalmx.com/videos/member/Onboard-Ryan-Surratt-Mammoth-Moto…
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