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09 Engine build

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Hey guys, 

 

I'm rebuilding a 09 250F. The motor was blown and most everything was trashed. Still waiting to hear back if the cams and cam journals can be saved. 

 

I'm considering putting different cams other than stock back in it. I already have the 270 kit and I am wondering which cams would complement the increase in displacement. The head will be stock so no porting/polish with stock valves and springs. It still has the stock exhaust for now and the air boot is stock other than the screen removed.

 

It will be for MX and Im just a vet rider. I do like having more bottom end coming out of turns since I'm 185 lbs but I also don't like a low to mid only setup and like to be able to rev it out when I'm hanging on for dear life in the whoops and can't manage a shift.

 

I figured the big bore would add some power down low and was wondering if the stage 2 hot cam would allow it to rev like I want without negating the gain in bottom too much. If anyone has done the 270 kit and played around with cam setups can chime in about what it was like that would help me out. I have never ridden this bike so I don't know how the power output was before so your experience will be appreciated. 

 

Thanks in advance 

 

John

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A 270 kit will sign off earlier than the stock bore. But it will make more power down low.

Would suggest some port work to help the higher displacement breathe, maybe just take out any imperfections, and polish the exhaust. When you remove the valves you will see where the valve seats don't quite match the ports so clean that up a bit.

 

As for cams, I had done hotcams and was not to my liking. They tended to give me a weird low end lag after deacceleration. I messed with my carb settings for a monthbefore throwing the stock cams back in , the stock ones eliminated the weird "flat spot"

 

What compression are you gong to run?

 

You may get a big bump from that larger piston, be aware of detonation, I would suggest a bit of race fuel till you figure where your at in temperature inside the engine.

 

Al

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A 270 kit will sign off earlier than the stock bore. But it will make more power down low.

Would suggest some port work to help the higher displacement breathe, maybe just take out any imperfections, and polish the exhaust. When you remove the valves you will see where the valve seats don't quite match the ports so clean that up a bit.

 

As for cams, I had done hotcams and was not to my liking. They tended to give me a weird low end lag after deacceleration. I messed with my carb settings for a monthbefore throwing the stock cams back in , the stock ones eliminated the weird "flat spot"

 

What compression are you gong to run?

 

You may get a big bump from that larger piston, be aware of detonation, I would suggest a bit of race fuel till you figure where your at in temperature inside the engine.

 

Al

 

I am getting a new head now so I will have to look at trying to smooth out the ports a little before I assemble it. I decided to go with the stage 2 hot cams after talking with Brent Kirk at Fast heads today. He said most people go with stock cams or a stage 2 with the big bore kits when used for MX. I had good results from a stage 2 set up with a 450 engine in the past but that engine was pretty much low to mid only before.

I will be running the supplied piston and gaskets supplied with the cylinder works kit so I am assuming it is at least close to stock compression. The jetting that was already in the carb was what was recommended by MXA with a 180 main and 42 pilot, stock needle and clip position. What direction do you normally go with re jetting with the big bore. I'm assuming it should be ok to start it with what Ive got and make changes based on sound, performance and plug readings. I have some race gas that I can use that is around for my RMZ that is running higher compression. 

 

one thing I found out today from my conversation with Fastheads was that they could fix the head by welding the cams up and re-machining the journal surface to be oversized and then overbore the cam journals on the head. He said that it wasn't possible to weld up the journals on the head itself to re-machine to stock size because of the placement bolts. I thought it was cool that they could fix a head that was that badly damaged but in the end I decided to go with a new head so that I am not stuck with only the OEM cams with out having to send the head back with new cams to get fitted to the oversized cam journals. Just FYI I guess if you are in a rebuild predicament later and are set on which cams you want you can save your head and probably save a few bucks. 

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on the jetting, you will be surprised. You ,ay have to go smaller on the main due to the volume of air that will flow now thru the engine. I would check the temperature after a brief ride to make sure your ok but normally it will go rich.

 

Your head guys sounds lagit. I was involved in the early 2000's with drag racing and a head company called Brodix, they showed an example of their work by welding a head that blew up in 1/2 and making it work. Amazing stuff. But for budget reasons I would go with a new casting.

 

On the cams, I had heard you can use a combination of stag one and stage two that works pretty well. I think stage 2 intake and stage 1 exhaust. I only had a set of stage 1's so I experimented with stock cam placement / aftermarket.

 

Hope your build goes well.

Keep us informed.

Al

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Well today I finally got it all together and fired it up. I took it out for a light ride around the neighborhood after the first heat cycle. Feels like it's going to rip. I had to fight the urge as I was keeping it short because I want to check the oil and cams/ valve clearance again before I beat on it. Besides that my neighborhood is gay and people like to call the cops. I get dirty looks even when I go slower than people in cars speeding through. Crazy.

It does pop a lot during deceleration so tomorrow I'll be checking the plug and trying to fine tune the carb. Electricity is out right now so I'm out of luck.

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Decel pop is usually one of two things, The copper washer between the head pipe and head is leaking or cracked, or your piloj jet needs either to be cleaned or changed to a larger one.

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I took this bike to the track today to try out some of my jetting changes. I changed the pilot from a 42 to a 45 and that definitely made it run better. I started with the fuel screw out at 1 1/2 turns and ended up out 2 1/4. I think that pilot will be good but may try a 48 just to see how it does. The main jet I went from a 180 to the stock 182 since the plug looked really white. It does feel a little fat at times on the high side so I may go back to the 180. 

 

From what Ive heard from others that did the big bore change, they usually go leaner with fuel settings but so far I have gone richer and got pretty good results. I have to say I am very impressed with the 270 / stage 2 cams combo. Ton of power and it still makes power up top. I can feel it pulling pretty good up near the top without falling flat some some engines do. And on the opposite end of the power band this thing is still a lot more impressive than a standard 250F. It is a midrange beast compared to my 250F and the bottom is more than adequate. The only other modification I did was cut out the back fire screen from the air box and I was thinking of cutting the extra couple of inches of plastic from the air box vent hole in the side plates. Wondering if anyone has done this and what the results were? Should only take a few minutes with a box cutter or dremel tool. 

 

Overall I am very happy with this engine and it should be a lot of fun to ride once I set the rest of the bike up.

 

Now for the bad stuff.

 

Brakes:

Brakes? What Brakes? Seriously, these things are a joke as they sit right now. Hit a downhill section and I thought I was going to launch it off the berm at the end into the unknown because I hit it at the same speed I normally do. I knew they were bad from previous short test runs but really underestimated how bad. They will get a complete fluid change next and ill try to break any glazing with some light grit sand paper on the pads and rotor and that will probably help a little. Still plenty of pad life left. I see an oversized rotor and steel braid lines in this bikes future though.

 

Front forks:

Ouch!! these things are atrocious. I found that really surprising since I have the same forks on my RMZ. These things are going to need a little work to say the least. When landing from a medium sized jump, high speed small jump, or a slow slap down landing they have a severe metallic clunking sound which resonates into the hands and actually hurts the wrists. Pretty weird since this was happening on relatively small jumps. I checked all the clickers and adjusted them back to factory specs (the compression was at 4 clicks out and rebound at 3 on one leg and 12 on the other so they were pretty messed up). I bled the air from them and loosened then re-torqued the pinch bolts to 20nm but it would still do it. I checked the fork guards and they are clear of the tube. I don't think that this is from bottoming and it seems to be more of a mid stoke hang up of something which then causes a severely harsh rebound. I couldn't tell if it was one fork or both. Any tips would be appreciated. I am going to disassemble the forks later and check the bushings and do a fluid change. The forks don't look like there has been any fluid leaks and there are no nicks or abrasions on the lower tube. They actually look like there has been very little riding on them.

 

So far its been a fun project. I just replaced the rear wheel bearings and linkage bearings. All of the other chassis bearings checked out so far and were re-lubed. I have a new chain and sprocket set on standby. I ordered a 49 rear and that is geared 1 tooth down but after riding it today with the 48 I think I will return the 49 for a 48. This thing is already a wheelie machine as it sits. Doesn't need anymore help with lower gearing. I also have new graphics on the way so this bike will look awesome when I get done with it. I'll try to get pictures of it but it might take a while. Going out of town for work for a month. 

 

Just thought I'd update with what was going on with this thing. Like I said before, any input or tuning tips are appreciated. 

 

John.

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we built an 11 KX270 earlier this year. It ran no different than when it was a 250 just had more mid range power.

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we built an 11 KX270 earlier this year. It ran no different than when it was a 250 just had more mid range power.

What did you change with your fuel specs? Did you run stock maps or add an adjustable fuel tuner?

I'm comparing this 09 kxf with a 270 to my 11rmz 250 with HC piston, full porting, stage 1 cams, vortex ignition, full exhaust. There really was a big difference.

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