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My Thumper Racing 300BB Kit Review for rspiker!

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I had posted this on KTMTalk. rspiker asked about top end, so I thought I'd just put the whole thing on here....

I recently put in a Thumper Racing 300 BB kit on my ’07 250SXF. There’s been a lot of interest in these type of kits recently and I understand how difficult it is to make the plunge. Almost everybody has certain expectations and fears about spending that much $$$ and making a permanent change to their bike. I’ve read many reviews where the rider is just too excited about their new toy to describe it objectively. Six months later they’re getting another bike because that last on just wasn’t right for them. So, I’ve tried my best to not exaggerate and to pay attention to actual differences and not just go by feel. But, like most opinions on the internet, feel free to take it with a grain of salt.

About me: 5'7" 165lbs 46yrs. I spent 20+ years riding off road, this bike is my first MX bike. I've been riding track ~2.5 yrs. (got tire of having to drive 2 1/2+ hours to trail ride, when I can be at the track in 20 minutes).

Skill Level: I'm no pro, not even one of the "fast" guys, but a long way from a novice too. I'm in that large group of intermediate riders that can ride the whole track, do the jumps and can turn pretty good. Of that group, I'm good enough to catch up to many of them, but not good enough to get by them. (at least before I get tired). Unfortunately this spring I've missed a lot of riding time and have been knocked down to the lower end of that intermediate group, but I'm working on getting back into things.

The Bike: FMF 4.1ti Power Bomb system, JD jet kit, Trail Tricks suspension, Scott's steering damper. I'd been riding bigger bikes off road, but wanted a 250F so I wouldn't have to fight to control the power while trying to learn MX. The bike had ~85 hours on it at the time of the conversion; I've put about 3.5-4 hours on it since.

Why the 300 kit: I wanted some mid range so I wouldn't have to shift as much and make it easier to accelerate out of the corners, but didn't want to change the overall characteristics of the motor/bike, nor spend huge amounts of money. Plus, I'm just not ready for a 450, not sure I ever will be. I figured it was time to freshen up the bike, plus just the excitement of a change.

The conversion: I had a semi-local guy do the sleeve work. It looked great and I believe he did a great job, but if I had it to do over, I’d let TR do it. It would have been a couple bucks cheaper and probably just as fast, if not faster. I did all the disassembly / reassembly. It’s not rocket science, but it’s not amateur hour either. If you’re a decent mechanic, it should not be a problem, but I have buddies that should not attempt it.

Jetting: I used the pilot and needle that TR sent and am still using their settings. I was running the JD kit with a 180 main, the TR kit came with a 178. I started with the 178, then went to the 180 and am now running a 182. On initial testing the bike didn’t seem to rev out, but the motor was still tight too, but I decided go to the 180 for my first day at the track. It seems to rev pretty good, but want to try at least one more step so I put in a 182 for may next day at the track and it seem to scream as good as ever.

The difference: Low to Mid – All I can say is wow! The change in the low end power feel does not correlate to the actual difference in displacement. Admittedly I was purposely trying to not expect a huge difference. Nobody claims that 40cc’s will turn a 250 into a 450. It’s just 15% after all. So I was expecting a slight, but noticeable change. IMHO for off-roaders on XC / XCW’s this would be a huge benefit, but for even track riders in my skill range, it really helps make the bike more forgiving in the turns if you get the rpm’s too low. Plus it greatly extends the usable rpm range which means less shifter. On my first day at the track I was going in a gear higher on some of the turns, so for the next trip, I changed the gearing from 13/48 to 14/45. For my local track this really worked well. There’s one turn in particular that became my litmus test. It’s a hard pack, slick, bowl with a step down double right at the exit. The 450’s could take the inside, give it a blip and clear it. I’d have to take the mid and scream it and usually clip the top. Now I can take the inside and make it or take the mid and over jump it.

The differences: Mid to High – Not being an expert that can keep it screaming, this one is harder for me to measure. I have only one litmus test for it. My local track has a long back straight with a slight right hand bend and a drop off type jump. There’s enough room to easily be in the upper rpm’s in 6th before the bend. I typically chop the throttle to set up for the bend, and then get back on it before the drop off. Before the change, mostly what happened was I heard the change in the exhaust note. I’m sure I was accelerating, but I can’t say I could really feel much. After the change, I could actually feel a slight acceleration force. Was it extreme? NO, it’s still not a 450, but was it noticeable? Yes!

Overview: The surprising increase in low to mid has definitely made the bike easier to ride. There is less shifting and less work for those just-out-of-the-corner jumps. It’s a way better value than an exhaust system. That said: It may be that my full exhaust system is helping more now that I have more displacement. I don’t know for sure. When I got the exhaust system I was not riding well enough to really define a change. I have an 18” wheel with a trials tire and a quiet / SA insert that I use for off-road riding. But, I’ve not had a chance to test it there yet and I’m not sure when I will. I'm not sure what my next bike will be one day, but if it is another 250F, I'd do this conversion again and not wait 2 1/2 years. Just remember; It's 40cc's, it's not a 320, 340, 350, 400 or 450!

I hope this is as clear as mud!

Regards,

P7

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I too have similar results but off-road, no track. It pulls lower in the rpm range, you can be off half a gear or even a gear and it will pull out. Ive been riding the last 2 times starting at 3500' and going to 7000'. I have lowered the pilot to the next xmaller size since it never needed the choke to start. I also turn the FS in .5 turn as I go up, mid trail. The bike is working wonderfully and is much easier to ride than my 525 which can be a handful in the tight trees and rocks. The 300 is just enuf power and has not left wanting more to get over those rocks piles or to get the front wheel up for a tree root. Im completely happy with the motor. I did get the sleeve done by TR on exchange so it went quick, just the ship time. I waited to have all the parts there before I took it apart since Ive never had a 4st apart before. I have the factory repair CD and it was quite easy to do.

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I just installed a TR300 kit on my 08 250sxf a couple of weeks ago for a cross country race, which was basically high speed grass track with several steep hills and some endurocross log sections thrown in.

I was undecided on what bike to race on, and I took my 300xcwe to the practice session also, main reason being that since the stock 250sxf is relatively hard to start, if I stalled on the logs I would be done for the day. The grass was still damp and when turning the throttle to clear the logs, the 2 stroke would just spin the rear tire while the TR 300 would grab traction and lift the front just fine, so this made the decision to run the TR300easier.

At the start of the first moto, I was not concentrating and had just put on my goggles as the gate dropped. I still managed to get to the first turn in 4th. For the second moto, I managed to pull the holeshot easily. Please note that I am still running the stock pipe. My impressions on the motor are that it is much easier to ride, since the gains at the bottom and midrange are substantial. You lose a little bit of overrev, which is what you would expect of a 6mm larger piston, but this is irrelevant to me since the bike does not need to be revved out anymore. This is a big plus for durability. I did not have a scale on hand, but it would be interesting to find out the weight difference compared to a stock piston.

Fuel consumption is about the same as before, for those concerned with range with the stock tanks. No overheating, flame outs or detonation either. And the bike starts much, much easier than before, 1 or 2 kicks most of the time and no more than 3 kicks even after stalling.

To me, it has been money well spent. I can´t figure out why the first thing people do to their new bikes is replacing the pipe when a big bore kit is much more cost effective.

As for the 300 2-stroke, I just sold it and bought a 250xcf that is being delivered next week. I have another TR300 kit waiting on the workbench for the new bike to arrive...

Regards,

Mauricio

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"need"? I doubt that. There's probably a slight difference, but I doubt enough to justify the cost. An open pipe is a high rpm improvement, the BB kit helps the top, but adds the greatest improvement in the low - mid.

If I hadn't already had the pipe, I would not have gotten it after the kit.

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On my first day at the track I was going in a gear higher on some of the turns, so for the next trip, I changed the gearing from 13/48 to 14/45.

Thanks for the write up. Looking to make the mod this winter. I do not understand the gearing. What is the difference and why go to a 14?

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there is more power everywhere and the motor can pill a higher gear ratio. Keeps you from shifting as much and wont spin as easy.

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Lowering the ratio also extends the velocity range of any gear. Let's do the math!

13 / 48 gearing equals a 3.69 / 1 ratio

14 / 45 gearing equals a 3.21 / 1 ratio

This works out to an approximatly 15% change.

Let's say for some imaginary gear using the 13/48 that at 3000 rpm you're going 10mph. At 12000 rpm you'd be going 40mph. This works out to a range of 30mph.

In the same imaginary gear using 14/45 the 3000rpm now equals 11.5mph and the 12000 rpm equals 46mph. This works our to a range of 34.5mph.

Put those two together and it equal less shifting!

But...for this to work well, the motor has to pull it well.

If the motor doesn't have enough power at 3000rpm to handle the 15% increase in load, then you have problems.

The 300 kit already extends the usable rpm range by making it pull lower, the gearing just helps it take advantage of this.

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