CR 125 bogging on low-end

[Forgiven96]

Hello everybody, I'm new to 2 strokes (and to this forum) and I need some help. I recently rebuilt the top end of my 96 CR 125 (piston and cylinder) and just started it up yesterday. As were doing the break in procedure that the manual (Clymer) calls for, we noticed that the bike had awesome top-end power, but would bog down and stall out when we let the rpms drop. It is also semi-hard to start. I used an OEM honda cylinder, OEM replacement Wiseco piston and it has a new spark plug and air filter. I don't know much about jetting or the carb, so any basic knowledge and help will be beneficial, thanks.

[Henry]

1. Welcome to !

2. its probably Jetting, Faded is the "go to guy" on jetting, if he doesn't jump on here shoot him a PM im sure he can help.

[psycho393]

I think it's the just the way that CRs are.

[char393]

Maybe your piston is in backwards?

[monkeyonahill]

I had the same problem with my '01 CR 125 when i first got it, try your pilot jet or mainjet, nd welcome to ATM! and if its hard to start check ur spark plug, if its like a caramel brown then its good, black rich, white lean (BAD) Good luck and have fun!

[Florida 393]

For a 125 2T try these until Faded comes in:
1. set the float height per the Honda specs (get a Honda manual from their website)
2. Check the reeds for cracks or chips. Replace if necessary
3. Spend at least a day and some brass jetting the bike until it is running crisp.

Write what you do down and each jet change is noted along with the results. Then you have a base line to go by.

Bill

[Premixed]

Jetting guide:
[Only registered and activated users can see links. ]

Faded's Float settign guide
[Only registered and activated users can see links. ]

Carb Diagram:
[Only registered and activated users can see links. ]

Other useful information

[Only registered and activated users can see links. ]

The problem is most likely jetting. Does the bike smoke alot, or drip oil from the exhaust?

[NYMXer]

How long have you had this 125? Did you have it long enough to wear out the rings or did you buy it that way? Jetting doesn't go bad overnight. Many times people are quick to change jetting when there are other issues. I've seen guys jet up so their bikes will start when they should have checked the compression and found it low, hense hard starting. Richer jetting masks that but then they put in the new top end and try to run the old jetting..............doesn't work well.

Maybe you put the piston in backwards (arrow facing to exhaust), maybe you have an air leak or exhaust leak?

Give us a little history of the bike to help us help you.

[Forgiven96]

Ok, here's the short history of the bike since I have owned it:
I purchased it used in September only to have it blow a head gasket a month later, scarring the piston and the cylinder (my own stupidity). I replaced the cylinder with an OEM honda, and the piston with a stock bore Wiseco. We just recently got it all put together (on Nov. 16th) because I have been waiting on parts (and I'm also low on funds). We just started it up on Thursday and the only riding we have done is for the break-in.

I'm 99% sure that we put the piston in with the arrow facing the intake, I checked the plug and it was gapped wrong, so I regapped it, but it didnt seem to help much. Also, the plug looked good, not being white or black. The bike smokes until it gets up to normal running temperature then it is pretty clean. I also haven't noticed any oil dripping from the exhaust. I havent messed with the jetting or the carb at all, so I appreciate the advice because I get easily lost messing around with that stuff, haha.

Thanks for the quick responses and everyones' efforts to help.

[OLDMXRACER 393]

Originally Posted by Forgiven96

I'm 99% sure that we put the piston in with the arrow facing the intake

I'm not the "go to guy" on engine work, but I think that arrow should be pointing towards the front on the bike.......Welcome aboard!

[FFRacing11x]

Arrow to the front/exhaust...Tdub

[Forgiven96]

Originally Posted by FFRacing11x

Arrow to the front/exhaust...Tdub

Hmmm....now I'm confusing myself, haha. I had a friend help me that has alot of rebuilding experience, so I doubt he would have made that mistake, but I am going to double check just to be sure. Thanks guys.

[Forgiven96]

Also, what are the potential hazards of running a piston backwards? Thanks again.

[Premixed]

When we say arrow toward the exhaust we're effering to the exhaust port(towards the front wheel).

[2000kx250]

Originally Posted by Forgiven96

Also, what are the potential hazards of running a piston backwards? Thanks again.

Well, theres a window cutout in the side skirt of the piston so that when the piston is at the top of its stroke, the gas/oil mixture flows into the bottom end. Withought this window facing the intake, your bottom end gets no lubrication so it would be toast pretty soon; you should probably check into that before you think about taking it for a ride, or even starting it for that matter. I am almost positive that the arrow should be facing away from the window in the piston and towards the exhaust side.

[MGCR125r]

According to Wiseco site. The piston MUST be installed with the arrow pointing towards the EXHAUST side of the cylinder.

One thing to check also is that you have adequate spark at low speeds. Pull the plug and groud it to the cylinder head. Put in in a place where you could see the spark jump across the gap. Gice it a kick. You should see a nice blue and consistent spark. I worked on a bike that had a bad ignition system that would only kick in if you push started it. It wouldnt fire if you kick started it.. So it too didnt idle, but it also had very poor and eratic performance at high speeds.....

[04cr125r]

how is the compression?

[ALEXANDERBELL_]

Cant believe Everybody going on about your piston, They insult your inteligence and have forgotten your original question. if your bike bogs then simply your jetting is too rich. does it bog as you slowly roll throttle or as you crack it open from low revs. if the first your pilot wrong or you air screw needs turning out a little. if it bogs cracking wide open from low revs then your main is too rich, go down jet sizes one at a time till its right. put some masking tape around your throtle tube, mark increments of 1/4 and the 1/2 these again mark as positions 1>8 then ride bike feel how each position responds. then youll have good idea which jet area needs looking at. Remember if its boggy (bwwoaaar) then its rich, if its lean it goes irratic as though its running out of fuel. if your not sure what I mean run your bike with petrol tap off till it runs out, youll know then.

[divy]

the same thing is happening with my bike but i just learned to deal with it but last week i got my bike fully rebuilt an re-jetted (i thought it was the jets)
but it still did the same thing the guy who fixed my bike told me the bike was geared wrong an also he jetted it to run a lil rich to improve the life a lil bit
an thats whats making it bog down in first so i changed the gearing an its a hell of alot better now so yuo might what to check that
oh it was happening to my YZ125 2000

[yz rider125]

my 02 yz125 bogs when i have the bike is at idle and i crack it wide open. I got a leaner pilot jet and it improved some but hasent gone away.