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2011 kx250f major engine mods..need advice

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I just had a couple questions concerning my 2011 kx250f. I broke my foot recently so I went off on a buying spree of new parts. As a result I now have two motors for the bike. One motor that is stock and another that consists of a CP racing 14.5:1 high compression piston, Pro Circuit Del West valves and springs with copper valve seats installed, Pro Circuit cams, port and polish by Pro Circuit, and a Rekluse Z-start clutch (the crank is still stock with 28 hrs on this motor). 

 

I was wondering what gas I should be using in this bike? I have heard 92 octane mixed with 110 race gas would be perfect. 

 

Also, will I need to get this bike remapped before I fully notice these modifications? Is it necessary for me to get it remapped with these mods?

 

I plan on keeping the stock exhaust on the bike, what are your thoughts on the stocker paired with the multiple engine mods?

 

If you have any other advice concerning my approach to this bike please feel free to share. Any help is greatly appreciated.

 

Oh, and I bought a brand new cylinder works cylinder for this motor and when I put everything together with the new piston I felt as if the piston was not sliding as easily as it should have along the cylinder walls. I made sure all the rings were seated properly and assumed that it was simply from being a new cylinder coupled with a new piston, and that it just needs to be broken in. What do you think about that hypothesis?

 

Any help is greatly appreciated 

Thanks, Cody

 

 

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Yes you will need to have it remapped as you're going to flow more with the porting modifications. Second- You should run at minimum a 50/50 mix of 110 and 93 with that piston. I dont care for the High comp pistons because it just adds more low end which the 250f doesnt really make power there anyways.  With the stock exhaust you're going to really limit the air flow that you gained with the head mods. Also, you'll see 1-2 HP alone with a stock motor with just a remap. That high comp piston is giogn to put more stress on the rod, you should really look into putting a carillo rod in there as the 11 cranks were suspect on some bikes if you're looking to make it really reliable. 

Lastly, You would of gained WAY more power just putting a 2012 dual FI setup on your 11 than you will see with all that motor work with the stock 2011 single setup. 

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Did you measure the cylinder and piston to make sure they are compatible, maybe you have an oversize piston. As above I agree the twin injector setup of the 12 would have been a better buy.

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Thanks for the info, the cylinder and piston are indeed compatible. I am looking into some exhausts, I am hoping to find a good deal on Ebay at some point. I would have gotten the stock compression piston if the high compression one hadnt come with the Pro Circuit head I purchased. I live in Tillamook, Oregon at the moment and sadly I dont think any shops around hear could do remapping. Do you guys know anyone nearby that can do remapping for my bike?  Also, the twin injector setup seems like a good move, how would I go about doing that? Finally, is it safe to run my bike with the stock mapping and all my engine mods? 

Edited by croos247

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Thanks for the info, the cylinder and piston are indeed compatible. I am looking into some exhausts, I am hoping to find a good deal on Ebay at some point. I would have gotten the stock compression piston if the high compression one hadnt come with the Pro Circuit head I purchased. I live in Tillamook, Oregon at the moment and sadly I dont think any shops around hear could do remapping. Do you guys know anyone nearby that can do remapping for my bike?  Also, the twin injector setup seems like a good move, how would I go about doing that? Finally, is it safe to run my bike with the stock mapping and all my engine mods? 

 

You can PM me because I put a whole kit together with mostly new stuff just knowing anyone with an '11 would want it. . . as they should. . . but it's still in my dining room . . . mocking me.  lol  Help, I need new safety equipment, again! probably because that Dual FI KXF is so freaking fast! I have to laugh, or I would cry.

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I just sent you a PM. Lol anybody out there know where I can get a nice header and slip on for a good price along with a good affordable remap?

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Drzadam on here does them for 50$. He did mine when I went 11 to 12 dual setup.

Just buy a stainless yoshi system it's one of the cheapest name brands if I remember correctly.

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Perfect! I'll look into that pipe but I may just keep running the OEM system. Thank you for the help everyone!

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Just wanted to clarify on my piston/cylinder concerns! The piston I am running is a CP7700 14.2:1 compression piston which has a 77 mm diameter. I am running it with a brand new cylinder works standard bore 77mm cylinder. The rings are seated correctly but the piston does not want to move up and down very easily. It will do it but not as smoothly as I had suspected it. Could it be because the bike hasnt been broke in? Is it safe to start the bike? Could it be my ring gap? What do you guys think?

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That's a good question that is hard to answer from a distance. I assume you have plenty of motor oil coating all of your parts, including the piston, cylinder, piston pin and especially the crank. You say the rings are seated properly and I assume all of the gaps are aligned properly. You know that sometimes the rings can be tricky to slide in. I have had them pop out of position on install before. Since I don't know any better, and since you have time, I would strongly recommend a conservative approach. You know what that means. Double check everything. Believe it or not, we've had people put pistons in backwards . . . that ends up badly, by the way. I am sure you wouldn't do that, but if a ring is jammed up you would want to know it, for example. Sure it could be the ring gap, as you mentioned, or it could be that things are tight because its new, but it should move smoothly with no binding at all. I know that's not much help.

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Considering that I have the time I'll just take it apart and double check everything. I was hoping that the new cylinder paired with the new piston is the culprit of the slight resistance. Maybe i'll test my stock piston in the cylinder just to make sure it works the same way. Just to make sure, how should the gaps be aligned? 

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Get a manual and a vernier and measure the piston and cylinder to check that the clearances are in spec to the manual.

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Should I open up a new threat specifically for my question regarding my piston and cylinder?

 

No need to threaten anyone on TT  :naughty: Haha, just kidding!  Actually the ring gaps shouldn't be an issue at all.  Just make sure they are aligned properly.  Here is a picture that will help.  

 

PISTON1_zps52e51d7d.jpg

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Sounds like one of the thin rings that goes above or below the oil retainer may of jumped out. Usually if that happens though it will be REALLY hard to turn over.

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I can get the piston to turn over using a socket on the crank but it is rather difficult to turn over using the kick starter. I am going to start taking my motor apart again tonight and take a look at it. This time I plan on doing everything with assembly lube!

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Did you overlap the oil ring expander? It sure sounds like it.

I was very careful to make sure the rings were properly installed when I slid the cylinder onto the piston, I am going to take the head off and check it again tho.

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It can be very easy for those lower rings to get messed up on assembly. The only thing you need assembly lube for is cam journals and I put it on the lobes and cups too. I run motor oil down the connecting rod to the big end and make sure everything else is coated with oil.

Check the piston movement before you put the head on. Just hold the cam chain so it doesn't bind. Remember to reset your tensioner and don't over torque your caps. You are using 87 inch pounds, with an inch pound wrench, right? Just checking.

Just to be clear, when you say the piston wasn't sliding easily, was that with the head installed or not?

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Reason I ask is because with the head on difficult movement might not be the piston at all. If the plug is out, so compression isn't an issue, and time is set right but it doesn't move easily, it could easily be over torqued cam caps. You would not be alone in that case. Let us know if the piston moves freely without the head on. Check back soon.

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