CR250 Big Bore Kit or just Porting for low end power?

Forgot to mention the Keihin Air Striker carb on the list of 'what the bike has'.

Try a flywheel weight first.

Guess I could have spelled out flywheel weight instead of FWW, but yes it has one.

Oh my fault didn’t catch that... Personaly I would port it... I think a correctly ported 250 would be easier to ride in the woods than a 300...

Talk to Terry Varner about doing some port work. He is reasonably priced & does some good work.

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.

Forgot to ask - Does the bike have a fresh topend ? What is the current jetting & conditions ?

Don't know how fresh the top end is, although it does have real good compression. Guy I got it from didn't know and I've had no reason to take it apart yet.

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?

KX500 wrote:

Don't know how fresh the top end is, although it does have real good compression. Guy I got it from didn't know and I've had no reason to take it apart yet.

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?

You can try buying a used Gnarly through your local CL or Ebay to try 1st before doing any port work. If you are in the 70-90 deg. weather a 45 PJ would be better depending on the needle taper. Do you know what needle is in there ?

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.

That gen CR responded really well to cylinder work.
-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.

Matt Fisher wrote:

That gen CR responded really well to cylinder work.
-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.

Matt - I highly advise people against milling the base .030" as it kills the topend power. At least with a Gnarly it's an easy fix & all you need to do is change the pipe if you don't like the power characteristic.

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.

I talked to Terry Back in 2004 after reading this article on doing mods for an 03 model & went with a different tuner due to him milling the base. If you read the article it talks about the only negative was the topend power. I talked to him a few months ago & he has changed how he mods these engines.

https://motocross.transworld.net/features/unleashed-fury-realizing-the-2004-honda-cr250rs-true-potential/

H4L wrote:

Matt - I highly advise people against milling the base .030" as it kills the topend power. At least with a Gnarly it's an easy fix & all you need to do is change the pipe if you don't like the power characteristic.

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.

Port timing makes all the difference. Just .030" off and yes, you will lose over rev. I have always taken .040" off the early reed valve motor mods I did and with transfer and exhaust height changes, over rev actually is increased.

slipdog wrote:

Port timing makes all the difference. Just .030" off and yes, you will lose over rev. I have always taken .040" off the early reed valve motor mods I did and with transfer and exhaust height changes, over rev actually is increased.

Yes. The exhaust port would need to be raised to compensate. I'd imagine at .040" the cyl. head would need extensive work depending on the design. That could be an issue with the piston to head clearance.

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.

Thanks for all the replies & advice. I knew the bike had an FMF exhaust pipe, but it hadn't occurred to me until these posts about the fatty vrs. gnarly, that I might have the wrong FMF pipe. And reading more about pipes, it sounds like the fatty didn't really impress anybody while the gnarly was noted as being pretty much what I'd want.

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.

The 05+ base gasket is .010" thick vs. the 02-04 that is .020".
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.

ok, if you want a significant increase in low end power, you need a big bore kit. period. it is that simple. there is no porting or any pipe or milling of the bottom of the cylinder etc that will make a large difference irregardless of what anyone tells you.

barnett468 wrote:

ok, if you want a significant increase in low end power, you need a big bore kit. period. it is that simple. there is no porting or any pipe or milling of the bottom of the cylinder etc that will make a large difference irregardless of what anyone tells you.

BS,

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.

barnett468 wrote:

ok, if you want a significant increase in low end power, you need a big bore kit. period. it is that simple. there is no porting or any pipe or milling of the bottom of the cylinder etc that will make a large difference irregardless of what anyone tells you.

r18b wrote:

BS,

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.

ummm...if you ever gain enough experience to to make well informed intelligent suggestions you should do so at that time, but since you obviously have no idea what you are talking about at the moment, your comments are pointless.

"if you want a significant increase in low end power, you need a big bore kit. period"

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.

KX500 wrote:

"if you want a significant increase in low end power, you need a big bore kit. period"

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.

ahhh, ok then. they have bigger bore kits for some other 250 bikes, but yeah, if you are only gaining 15 cc's and are just boring your cylinder out to get it you might put it on the bottom of your list. porting for low end power is not going to make a significant change irregardless of what anyone else says. the single biggest gain in low/mid power would be the gnarly pipe. if you don't like how it runs after that, i don't see any point in doing anything else because it seems to me that you are looking for a BIG improvement in low end, plus you stated that you are not a "revver", and this is why i said that you will not get a big change unless you go to a big bore kit.

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.

"the single biggest gain in low/mid power would be the gnarly pipe".

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.

This is an 02 CR2 AF 500 I use to own a few yrs. ago & ended up selling it. Reason being a well tuned 3rd gen CR2 is not far off being just as fast when the engine tuner does it right. On the plus side it doesn't tire you out as quick.

H4L wrote:

"the single biggest gain in low/mid power would be the gnarly pipe".

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.

you really need to get a life and quit wasting your time with your feeble and laughable attempts to discredit my comments with irrelevant drivel as it unnecessarily clutters up the thread.

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.

barnett468 wrote:

you really need to get a life and quit wasting your time with your feeble and laughable attempts to discredit my comments with irrelevant drivel as it unnecessarily clutters up the thread.

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.

I have a life.. No need to start insulting if you can't have an intelligent conversation..

H4L wrote:

"the single biggest gain in low/mid power would be the gnarly pipe".

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.

barnett468 wrote:

you really need to get a life and quit wasting your time with your feeble and laughable attempts to discredit my comments with irrelevant drivel as it unnecessarily clutters up the thread.

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.

H4L wrote:

I have a life.. No need to start insulting if you can't have an intelligent conversation..

There was little attempt on your part to have any sort of intelligent conversation". Some of it appeared to be simple grandstanding on your part and that part trying to nit pick my post apart and did little to benefit the op.


"...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.
.

barnett468 wrote:

There was little attempt on your part to have any sort of intelligent conversation". Some of it appeared to be simple grandstanding on your part and that part trying to nit pick my post apart and did little to benefit the op.


"...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.
.

You need to reread your posts..

Porting will help, any good tuner can get you where you want it to be, you can always start on the cheap side with a different pipe, you can always by a used head and get it cut with a tighter squish and bigger chamber to get rid of having to run good fuel, I just bought a different head for my bike, I increased squish .020" and tried it out, didn't like it was very soft on the bottom, I recut it and took. 020" off, night and day difference in bottom end, you can call any tuner and see what they suggest, good luck

OP - Here is a race of a guy on an 03 CR250r racing against a KX500. These bikes are very competitive when tuned right. My guess would be the guy on the 500 got tired at the end of the race.

https://www.vitalmx.com/videos/member/Onboard-Ryan-Surratt-Mammoth-Motocross-2-Stroke-Class,13964/GD2,43126