250 vs 300 2 strokes

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6/24/2018 8:45 AM

I had a chance to ride a new TM 300 MX the other day, and was very impressed with the engine. It had abundant torque with a very linear pull and fantastic throttle response. At one point, I slowed right down and shifted up a few gears just to see if it would pull it and it did, with surprising ease. If it was my bike, I would just gear it a bit taller to stretch out each gear a bit and you could ride it like a 450 - a very light and zippy 450.

Then I went to my own bike to actually race - a 2016 250 sx - thinking "wow, that was fantastic, I think I need a 300 kit" and ended up feeling really torn about it. No, I don't have that low end grunt to the extent of the TM 300, but my engine felt playful and fun in comparison (if that makes any sense?). The only KTM 300 I've ridden was an xc, and that engine felt almost boring - the TM felt a bit more aggressive, like you'd expect from an mx engine while the xc felt rather tame. Is the 300 sx kit much different than the 300xc? I'd hate to put a 300 kit on my bike only to find it boring. If it felt that that TM 300 I'd be happy (which is probably a useless comparison for most people since most people will never have ridden one). If it felt like the 300 xc I think I'd be disappointed. I hope this makes some sense - it can be difficult to articulate just what you felt when riding, and what you'd like.

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Current rides: 2016 KTM 250SXF, 2017 KTM 350SXF

6/24/2018 9:22 AM

The 300xc is an offroad bike. It's not meant to be ridden MX. With that said, they're highly tuanble and can be very good on a MX track with a few adjustments. A 300xc should not compare to a 250sx in power delivery because they are two totally different bikes meant for different things.

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6/24/2018 9:28 AM

Yes it makes sense. Most peolple I know prefer the 250.

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6/24/2018 9:31 AM

As far motor is concerned, if there any differences between and XC and SX, they don't matter unless you're at a level where you're doing it for a living. Suspension would be all that really matters. But I agree with the OP about being torn between the two. I sent my old yz250 cylinder off to Eric Gorr to turn into a 300 and the bottom end was fantastic, but the free revving fun was gone and it bummed me out a little. On the plus side, it really is just a piston and cylinder, fairly inexpensive and quick to swap around.

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6/24/2018 11:12 AM

That dude with the YZ big bore and the wrinkly seat cover will be by shortly to give you all the information you did and didn't think you needed to know.

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6/24/2018 12:10 PM

I wouldn't put a 300 on any KTM but a 17 and up 250. Why? The vibration on the 16 and below is unbelievable. At least with the 17 - up you have the counter balancer which is awesome.

I built a modified 2013 300sx and the torque was great. But I had it built for torque and it would rev out pretty good with the bills pipe and silencer. The vibration was KX500 level.

With my 2017, I have debated recently doing the 300 kit, PWK and STIC metering block just because, but I damn sure the minimal vibration of the 250 though.

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6/24/2018 12:21 PM
Edited Date/Time: 6/24/2018 12:23 PM

Thank you for the helpful replies so far. I'll give it some thought. It really isn't a life or death decision because I'm pretty sure than if I install a 300 kit and don't like it I can sell it and put the 250 jug back on. But it's nice to hear some other opinions. In fact, I just talked to someone the other day who took his 300 kit off because he didn't like the vibration (his bike is a 2015, so like mine pre counterbalance). Maybe I can buy his. But that's another issue - more vibration. I'm not too concerned about that though because I've never really noticed the vibration on 300s I've ridden except when sitting still revving them up.

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Current rides: 2016 KTM 250SXF, 2017 KTM 350SXF

6/24/2018 1:56 PM

We raced TM300 SMX's for the last 3 seasons.. and i have a2012 MX bike.. IMO the 300 is less frantic than a 250, nicer delivery and easier to ride

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6/25/2018 8:09 AM

Frodad78 wrote:

That dude with the YZ big bore and the wrinkly seat cover will be by shortly to give you all the information you did and didn't think you needed to know.

Dude this is the funniest shit I've seen on here in a long time. Thank you for the laugh

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"A link is only as long as your longest strong chain"

6/25/2018 8:26 AM

Love my 300 on trail and slower speeds. Has the low end torque 4 stroke grunt and power on top like a 250.

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6/25/2018 9:31 AM

As many have mentioned here, the 300 (or similar) is absolutely amazing off-road. I started out with the 310 kit (74mm piston) from Eric Gorr on an older KX I had and it was a torque monster, but it definitely flattened out earlier. Could carry taller gears for moto, but it wasn't a power band that translated well to moto. Played around with retarding the ignition timing and that helped a bit, but the 250 config was better for moto. With that said, I could easily see a 300 working great for a beginner/novice moto rider. Good torque/good power that flattens out early.

Then, decided to go to the 327, which took the stroke from 72mm to 76mm. All necessary parts and pieces were sent to CrankWorks in Arizona to modify/balance the crank with the parts (piston/rod, etc.). Bike vibrated much less and had a more aggressive longer powerband. A bit more of a 250-esque hit in mid-range. Put a flywheel weight on it and relegated it to full off-road. A perfect bike for me off-road and reliable as well.

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6/25/2018 10:14 AM

Bore it for more bottom, stroke it for more top. I had a 125, made it a 144, hated it. So much better as a 125.

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6/25/2018 6:42 PM

MXA said the 300 “full race” was too much.
I’m pretty sure they prefer the EXC 300 jug on the 250SX. Tune from there.
They had some interesting results from different pipes on 300 conversions as well. - Where the power was focused.

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6/25/2018 8:55 PM

Ted722 wrote:

As many have mentioned here, the 300 (or similar) is absolutely amazing off-road. I started out with the 310 kit (74mm piston) from Eric Gorr on an older KX I had and it was a torque monster, but it definitely flattened out earlier. Could carry taller gears for moto, but it wasn't a power band that translated well to moto. Played around with retarding the ignition timing and that helped a bit, but the 250 config was better for moto. With that said, I could easily see a 300 working great for a beginner/novice moto rider. Good torque/good power that flattens out early.

Then, decided to go to the 327, which took the stroke from 72mm to 76mm. All necessary parts and pieces were sent to CrankWorks in Arizona to modify/balance the crank with the parts (piston/rod, etc.). Bike vibrated much less and had a more aggressive longer powerband. A bit more of a 250-esque hit in mid-range. Put a flywheel weight on it and relegated it to full off-road. A perfect bike for me off-road and reliable as well.

Beginner/novice describes me quite well - 46 years old, 220 pounds, back into the sport for a few years, mostly riding a sand track. The extra grunt may suit me well.

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Current rides: 2016 KTM 250SXF, 2017 KTM 350SXF

6/25/2018 9:27 PM
Edited Date/Time: 6/25/2018 9:30 PM

MPJC wrote:

Beginner/novice describes me quite well - 46 years old, 220 pounds, back into the sport for a few years, mostly riding a sand track. The extra grunt may suit me well.

Yes, 300's are awesome. At the very least you'll have the knowledge to help someone with their buying decision someday! wink

If you're looking for a little more TM like delivery, it sounds like the SX would suit your needs. Anything can be de-tuned / smoothed out with a gearing change, flywheel weight, ignition timing, or power valve adjustment. The XC would be mellower from the get go.

Not sure if KTM still offers this same kit with ignition kit etc., but MXA did a comparo a few years ago.
https://motocrossactionmag.com/two-stroke-test-we-turn-our-ktm-250sx-into-a-ktm-300sx-big-bore/

Cylinder: The 293cc cylinders are the same for the enduro (EXC-W), cross-country (XC) and motocross (SX) models, and they have had the same porting and specifications since 2009.

Cylinder head: There are two different cylinder heads; a slightly lower compression XC head and a high-compression SX head, which still runs on pump gas.

Piston: Like the cylinder, the flat-top pistons have been the same since 2009. Some tuners feel that the flat-top piston is better at controlling detonation and works much more efficiently with pump gas.

Black box: The XC and SX kits have different maps. The ignition for the SX kit is more aggressive (as you would expect).

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6/25/2018 9:33 PM

Related question: Does the TM 300 have its own engine or is it as the others, a 250 engine with a bigger piston and cylinder? I don't know why but in my head I feel like the TM behaves differently to other 300s. I have no reasonable fact to back this up though silly

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6/25/2018 10:00 PM
Edited Date/Time: 6/25/2018 10:12 PM

A 17+ 300 with a PC pipe and factory sound silencer, high comp head, Keihin PWK carb and a STIC metering block will have the bottom end of a 300 on crack, and the mid to top with awesome overrev of a 250 on crack. Its pretty much the perfect two stroke setup. Its lightning fast.

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6/26/2018 6:38 AM

Ray_MXS wrote:

Related question: Does the TM 300 have its own engine or is it as the others, a 250 engine with a bigger piston and cylinder? I don't know why but in my head I feel like the TM behaves differently to other 300s. I have no reasonable fact to back this up though silly

Just the basics, it's a 72mm by 72mm using a bigger cylinder/piston just like the KTM300. Same crank across all models of the 250/300 2-strokes. Based on their 2016 manuals, it looks similar to the differences between the KTM250 XC/SX 300XC and 300SX kit (partially).

TM changed the head, cylinder, and power valve assembly for the 300. Doesn't look like there's a different MX specific head for the 300MX, but each model on the TM has its own CDI. EN250/300 and MX250/300...so 4 different CDI's spec'd for each model exclusively.

Other differences look like internal and final drive gear ratio changes between the Off-road models and MX models.

Could have overlooked a part or two, but here's the source. http://tmracingmotorcycles.com/support/parts/.

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6/26/2018 6:51 AM

Frodad78 wrote:

That dude with the YZ big bore and the wrinkly seat cover will be by shortly to give you all the information you did and didn't think you needed to know.

Stuntman949 wrote:

Dude this is the funniest shit I've seen on here in a long time. Thank you for the laugh

Haha..Like clockwork right?

Although I think he's called my bluff!!

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6/26/2018 7:35 AM

I've got a 250sx and have spent considerable time on a big bore YZ with the Athena kit. I much prefer the 250. The big bore feels hectic and doesn't have enough top end. Also, the bottom end power isn't really that useable.

That being said, I'm 5 foot 7 inches / 150lbs so I just don't need more power than the 250 delivers. My buddy who owns the big bore is 6ft 3 and built like a brick shit house. He loves the big bore.

So there you go.


p.s. We had a hell of a time getting that big bore dialed in. Ultimately the it was a timing issue.

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6/26/2018 9:50 AM

I've got a 250SX and honestly have never felt the need for a 300. My buddy just got a 2017 husky 300 and my 250SX can still beat him in a race (until I run outta gear). He has about 60lbs extra weight on me though). The 300's are just more mellow, less exciting and have more low end than i honestly need.

At this point in my life, if I wanted "more" I would just buy a 450. I would want something that clearly has more everywhere. If its a two stroke, I still like the 250's.

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