The Ongoing Saga of 21J's YZ250FX

2015 Yamaha YZ250FX Part 8

 

The tinker couldn’t leave well enough alone, I put 81 hours on the bike in 2015. That’s 81 absolutely trouble free hours at that. Although I must say I didn’t really expect to have problems and would have been very surprised and disappointed if I had. Yamaha’s have a well deserved reputation for their toughness and longevity in the motor department. I know the old motors were considered bullet proof and the new backwards motor seems to be cut out of the same cloth. 

Late last fall I started talking with Travis at Thumper Racing about the possibility of increasing the bore, I wasn’t  really interested in making a 270 out of it I wanted at least a 290. From what I understand staying with a 270 allows the motor to rev as freely as a 250 and going bigger doesn’t allow it to be quite as free. But I’m not a rev it to the moon kind of rider in the first place so having a motor that takes a little longer to rev out but gives me more low to mid torque is ok with me. 

Turns out Travis hadn’t yet had the opportunity to have a 250 around long enough to make a kit for it, which meant he had to start from scratch. I dropped the bike off in September so they could take it apart take measurements to have the sleeve and piston made for it. They sent the cylinder and head to the manufacture so they could make molds in order to produce a piston. Some weeks later I picked the bike up and finished out the Cross Country series for the year.

When the piston arrived I took the bike back up so they could tear the motor down and bore the cylinder and send it off to be plated. That process took several more weeks and by the time they wrapped it all up it was the first of the year. While the motor was apart they took the time to check the motor and valves install new valve guides seals and the newly bored and plated piston and rings.

The plan was for me to pick it up the first of the year, but if you’ve read any of my other articles you know I basically broke my back the 2^nd of January. So here I am the end of February finally picking it up. The good news about this I’ll be able to ride it.

When I got home I couldn’t wait to try it so I did a dyno butt test in my drive way (I have a fourth gear driveway) and the results of that were very encouraging. The real proof will be out on the trails but initial feel is that it’s going to do exactly what I hoped it would do, increase low and mid range power. 

My first real ride on the new 290 at Mammoth Bar was everything I’d hoped it would be. They have a short tight MX course and before I would ride the bike between 2^nd and 3^rd today I could ride it in third period. Up top on the trails there’s a couple of sections where you can open it up (6^th gear stuff) I could feel the bikes new power pull me along from 5^th to 6^th maybe it felt like it took  longer to build the revs, maybe not, never the less it was pulling. There is a single track cross trail that goes between the two down hills, in the past that would be a 2^nd 3^rd and 4^th gear section today it was 3rd and mostly 4^th gear across that section. Being able to pull a higher gear across the section means the suspension works better. The other plus was with more low to mid range pull, the front end was much easier to lift up when you needed to. I didn’t notice any more vibration than before which was virtually none. I have very little time on it but I see absolutely no minuses. I’m not sure what the fuel consumption is going to be time will tell. 

I’ve put almost nine hours on it now most of them up on the trails out of Georgetown. It’s been very wet and slippery and being able to ride in a higher gear because of the new power helps with traction. And so far the fuel millage looks good. The pictures show a Pro moto Billet side stand I installed. My #13’s kept hitting the stock side stand so that’s the reason for it. One picture shows the electronic controller installed between two pieces of foam on top on the battery the other with the top foam removed.

I’ll write a part nine sometime down the road. 

Doug 21J

 

PS I installed a 390 cc kit in my 2011 KTM 350SFX that I owned a few years ago and used a Thumper Racing kit. I was extremely happy how that turned out so when I thought about making my 250 into a 290 I called them again. Since they didn’t have an off the shelf kit ready for the 250 I was the guinea pig so to speak. It took a lot of time and effort on their part to produce the first kit. But I couldn’t be happier with how it turned out and all the people at Thumper Racing have been great to work with. I highly recommend them.

The kit is $775.00, at present you’ll need to send your cylinder to them so they can bore it and have it coated. (Two to three week process) In the near future they will have cylinders already in stock ready for immediate exchange. The kit comes complete with piston rings and a gasket kit.

You will also need to purchase a programmable controller to set the proper air fuel mixtures, the big bore needs more fuel. They use an EJK (Electronic Jet Kit) that will come from them preset for the big bore kit. The kit is $225.00. You can contact them at ElectronicJetKit.com

You can contact Thumper Racing at info@thumperracing.net or 775-309-4956.