125-200cc Decision Regarding CC Upgrade

The cross country race series that I have been running is currently on summer break. Now would be the perfect time to do a top end on my CR125 and while I’m at it why not spring for a bigger bore.

That problem is that I still can’t decide between a 144 or 165. I do 90% of my riding near sea-level and have been pretty happy with the power as it is, but could see how more power would be less tiring. Since I like the 125 I figured I should just do the 144, but then I read some posts about EG being hit or miss so I’m a little hesitant. I’ve read nothing but great things about the WB motor, but will it run OK with my stock pipe?

I’m hoping for something that will work with my bike as-is without fooling around with changing pipes and carbs and all that. At least until I destroy the stock pipe. Am I going to hate a 165 with my stock pipe? I assume I won’t be taking advantage of all the available low-mid but it’s still 40 more cc’s right? Will it sign off way to early, or just a little? I don’t mind losing a little over-rev.

Thanks in advance guys. I’m looking forward to this little project and I appreciate all the info available around this place.

I think it depends on what type of rider you are.
I really like ringing my 125 out and always looking for some rpm's,that's me.I had a chance to ride my friends new KTM300 for probably an hour on tight single tracks and you couldn't pay me to take it,too much power and the fun factor for me was gone,I was bored on it.
When I decide to upgrade my 125 I'll probably have Walt make it a screaming 144 as I think the 165 is a bit overkill for me even though all I ride are nasty single tracks in the mountains of Idaho.
Since you race you're probably more concerned about power and a 165 as you will hear from others on the board has lots of it.

144 has 125-like power, but more of it. 165 changes the power output altogether and makes the motor much more forgiving down low and in the mid.
I'm 200lbs, so I need all the power I can get from the small bore hence my decision to run the 165 and sell my OEM 144 kit.

Walt did my 144 and my 165, I personaly prefer the 165 but I am a fat lazy old man and it is just so much fun to ride.
The 144 is a load of fun and if I was 20 years younger and a few pounds lighter I would love the bike and it still pulls me around with total ease and puts a huge grin on my face every time I ride it, but a 3 hour event the 165 is my first choice evey time and it gives me face ache with all the smiles and grins.
A good friend of mine still rides a 125 (well sorted) and dosnt even want to go to the 144 let alone the 165, he also said my 165 gives him arm pump but I think thats more down to my suspension being a lot firmer than his as its setup for my fat ass not his scrawny little butt.
The 165 is not good with a stock 125 pipe but then again the stock pipe from the 125 aint in my mind a lot of good either, I had my best results with a hgs and a dep but have since tried a fmf and they work well on the 125 / 144 but again not good on the 165
Would be worth trying to get a ride on a 144 as they do work very well over the 125 but Walts 165 is just brilliant but not for eveyone

Right ON! I sent you a PM, thanks man.

I would strongly recommend you get that FMF Fatty while you're at it.

I was going to suggest trying a Fatty first (it made a huge difference on my bike) but get the 144 and Fatty and you'll LOVE the power.

Well it looks like I missed out on the 144 that was for sale. It seems this thread generated enough interest that it sold pretty fast and I missed it... No big deal. He still has the pipe. It sounds like EVERYONE seems to agree that the stock pipe is junk. Maybe I'll buy a better pipe and a new ring and put off the bigger bore for a bit longer.

No maybe. Buy the pipe.

You won't regret it.

Pipe,head work,lectron carb and gearing= Screaming 125

Which cylinder, pre or post 08, works best for the 165 conversion? This is for a 04 CR125. I know that both will work, just wondering if there is an advantage one has over the other for the conversion.

My non-scientific butt dyno says that the older style cylinder(pre-08) has better bottom/early mid. They feel pretty similar from there with the newer style having a slight advantage up top. The difference is very minimal though. The older style cylinder is a bit easier to work with but the older power valves are much tougher to work with-yields a push overall. I have converted a number of 04 CR125's and they all turned out great.

Thanks for the info. I'll probably punch out the 09 cylinder.

So I've been thinking about this some more. After searching a ton of posts around here I went out and purchased a pretty decent compression tester. I don't have a baseline but it's currently at 148psi with around 30 hours on it. I guess this means that I'm not really in a rush to do anything?

I quoted the above because I'm curious, Walt do you still do your 144 set-up for these bikes? If so then I think that would be the ideal set-up for me, based off of all the responses that I've read. Thanks again for all the great info everyone.

My RM 125 has 175 lbs of compression. Just depends what it had at 0 hours.

I would say your are do for a new top end, my 2008 cr 125 had 165 psi new and so did my sons yz 125. Only difference may be your gauge. You might want to try it on a bike that you know has fresh top end and compare. I just check my 165 after 25 hours and still at 210 psi and a lot of the hours have been off road racing very hard hours.

Thanks for the info guys. Walt contacted me to let me know he still does the 144.

I'll probably go outside this afternoon and pull the tank and give the bike a thorough scrubbing, drain the coolant, then bring it inside and start tearing it down over the next couple of days. I'll definitely get a baseline psi after the refresh to eliminate any guess work moving forward. Awesome!