cr 125 trouble!!

L

lake

Member
Oct 30, 2008
45
0
I was at the starting gates of a local race the other weekend on my 2001 cr 125, when the five second card went up i held it, as soon as the gate dropped and i poped the clutch my bike died. I took it home and did a compression test. It was only getting 60psi, so i tore the thing apart to find that their were pittings all over the top of the piston and the head, the ring was stuck in the piston as well.I got my cylinder checked out at a local shop which they said looked fine and a new piston,ring and head on the way(the old head had already been plained by the previous owner.) My question is what would just make my bike all of the sudden run lean like this? I haven't touched the timeing ot jetting since the winter of 2008/2009 and my plug looked good.
 
L

lake

Member
Oct 30, 2008
45
0
Well i just put new reed gaskets in when i rebuilt the top end in march, so to check the left side seal would i have to do a leak down test? or is there another way to check?
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
A leak down test is the best/cheapest. Can you get a pic of the piston up? I do not believe a leaky left crank seal would pit your piston. Detonation would though. I think the leaky seal would erode the exhaust side of the deck off. Either way it seizes the ring. Vintage Bob
 
F

FNG

Member
May 2, 2008
97
0
X2 on the detonation.

As much as it pains me to agree with "Forged Bob" LOL!
 
L

lake

Member
Oct 30, 2008
45
0
But what would be causing the detonation? Could it have something to do with the head being plained? I only found out from the buddy i bought it from that the head was plained and i was only running premium gas. Should i have been running race gas? ( I'll try and see if i can get some pictures up)
 
Last edited:
M

Matt90GT

Member
May 3, 2002
1,517
1
do a google search on detonation. Bottom line run 50/50 with race gas.

Also do a search on this bike you have and you will find out the mikuni carbs are very finicky on jetting.

http://twostrokemotocross.com/2009/05/budget-racer-2001-honda-cr125/

Give you some good reading.

FYI just cause you put new gaskets does not mean then are sealing! Be sure to inspect that with some like carb cleaner with the bike running. See if it idles up or dies. Not cleaning a gasket surface would be a good reason for a new gasket not sealing.
 
F

FNG

Member
May 2, 2008
97
0


Yes, "planing" the head, I.E. cutting material will increase compression and tighten your squish. Both are a recipe for detonation without the proper fuel.


Like matt said, run a 50-50 mix with 110 and premium or something similiar.
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
If someone planed the head, you need to check the squish band to see if it will even work properly with higher octane fuel. The piston hitting the head is a bummer! How much it got planed, will tell if it can get fixed back to pump gas, or throw it in the scrap pile. Running higher compression shortens the engine rebuild times also. You are either a pro, or have an endless supply of money? Vintage Bob
 
L

lake

Member
Oct 30, 2008
45
0
I've already decided to chuck the old head and get a new one as well as doing a bottom end rebuild, because of chunks missing out of the rod bearing washer or race whatever you'd call it. Which is making the rod rub against the side of the crank.
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
If it was me, and the cylinder has any issues? I would send the top end to Forward Motion, ask Eric to set you up with a Wiseco complete engine kit, and fit the piston and rings. He can get heads also. Priceless, but compare? Unless you know where to get parts cheaper than retail. He may even be able to salvage the head! Vintage Bob
 
F

FNG

Member
May 2, 2008
97
0
You can re-cut the squish to match up with the corresponding cut in the head. Of course if you do not know what was cut out then a person like Eric @ Forward motion may be able to answer this and save you some $$ instead of buying an new head.

This will still increase your compression (which was the original intent) requiring better fuel but by restoring the squish your motor will no longer be a time bomb.

This is one of them lessons learned when modding. Some motors you can do a head cut without cutting the squish, others well here you are:(
 
L

lake

Member
Oct 30, 2008
45
0
I think i'm just going to chuck my old head and get a new one seeing i can get one for only $100. I'm not to concerned about going back to just a stock head, i have a hard enough time affording pump gas. I didn't get the head plained myself i just found out that the previous owner had done it before he sold it to me.

I ran this bike all last summer on pump gas with a plained head and pump gas, my question is could it have gone that long with out wrecking it's self, or is there another cause for it detonating?
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
Need a clear pic of the piston, and head, to make even a remote decision on that. 2 thumbs up on making it stock again. Problem, what else could have been erroneously modded? Fools with tools like making the ports pretty, and if they cut the head, the base of the cylinder is no biggie. Even the carb bore can get hacked! On another note, some heads can be salvaged, the dome would need to be reshaped to bring the volume of the head back into spec. Generally on race bikes, cutting the squish is just 1 part, the 2nd easiest, planing it is way to easy. The dome requires some real knowledge. Vintage Bob
 
U

Uchytil

LIFETIME SPONSOR
Jun 29, 2003
814
9
I'll throw in my two cents. The lower cage is what caused the pitting and the rest is fallout from that I'll bet. How many times have we seen that - too many to count on my fingers. The head shaving - are you sure? Compare the old one to a new one. Or did the previous owner just lap it on a glass palte with carborundum? The absolute good news is that 2T engines are easy, and fun, to work, and play, on.
 
U

Uchytil

LIFETIME SPONSOR
Jun 29, 2003
814
9
Oh yea, I forgot - I second Forward Motion and while you are at it have Eric throw in a 144 kit and give me a ride report. :)
 
L

lake

Member
Oct 30, 2008
45
0

The head was shaved for sure. I gave the previous owner a call and he said he had gotten it done. I'm going to see if i can get a hold of him again to see if there are any other motor mod the he had done to it.

Where is forward motion at? I live in the ass end of Canada in Nova Scotia, so sending it some wheres in the states seems a little out of my price range :think:
 
L

lake

Member
Oct 30, 2008
45
0
I uploaded some pictures to photobucket, i'm hoping they are going to work. They arn't the greatest pictures but let me know what you think if my cylinder. I've been told it will be alright to use it the way it is.
 
Last edited:

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
The big end of the rods bearings went, at some point. Not detonation, and all the race gas on the planet would not have likely helped.
 
L

lake

Member
Oct 30, 2008
45
0
Alright, thanks whenforksruled. I'm in the process of getting a new connecting rod and bearings.

I know it might be hard to tell by just looking at the pictures but what did you think of the cylinder, did it appear to look alright enough to use it the way it is? I can run my finger nail along the cylinder wall and it doesn't catch on any rought spots.
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
You would need to clean all the flashing off the cylinder. Then use a bore gauge or inside micrometer to check the bore, and that its still round. Vintage Bob
 
L

lake

Member
Oct 30, 2008
45
0
I might sound a little dumb, but what exactly do you mean by the flashing? I used some emery paper to just go around the cylinder and make sure there weren't any edges or groves.
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
Rubber gloves, green scotch brite pad, oven cleaner and some water. Do not get the stuff with lemon juice. Spray it on the bore, the aluminum should break free. Then clean up matching the cross hatch direction. The aluminum piston rubs off, into the plating cross hatch. it looks smooth, or scoured? The plating is pretty durable. I have heard argue the ball hone and what not, those remove a lot of material. The green scotch brite is not too bad, the purple can remove material like emery cloth. Check the bore near the top and bottom of where the piston actually rides. Measure side ways, then check front to back. .002" variance is all that is allowed! Replating is the best alternative, and going big bore costs no more, usually. Send the top end to Forward Motion. If the bore is good, Eric knows. Vintage Bob
 
L

lake

Member
Oct 30, 2008
45
0
I had the bore checked out by a local shop today, the variance was .001. So i'm good to go that way. I'm going to pick up some oven cleaner and what ever i need tomorrow and try to clean it up a bit.
 
You must log in or register to reply here.

Similar Topics

L
Cr 125 help
Replies
5
Views
999
noj
N
C
Getting a bike!
Replies
5
Views
2K
2strokerfun
2
D
Can I bore a 01' RM 125 cylinder?
Replies
5
Views
13K
_JOE_
J
M
RS 125 > CR 125
Replies
4
Views
3K
MXFastGuy
M
J
New top end no compression?
Replies
11
Views
11K
Jbgabriel
J
Cookies are required to use this site. You must accept them to continue using the site. Learn more…