CR125, bottom end? how to tell?

[CheM]

My 1987 CR125, had always had a loud ticking noise, some people had suggested to me it was piston slap..... the previous owner had supplied a receipt of the top end being resently rebuilt (about 10 hours ago).

Well the top end let go the other day (i am not sure why, it most likely was not because of the ticking but the piston seemed to have Seized.

I got everything i need to replace the top end, but now i am thinking the problem might be the bottom end, how can i check? what should i check?


What leads me to believe the bottom end is messed up, is the way the piston looked, it looked chewed up from the top end. I was reading this web page....
[Only registered and activated users can see links. ] (look at it midway) and it states when the piston is chewed up, the bottom end needs work.

This is how my piston look btw.


http://s90263782.onlinehome.us/chem/piston.jpg


This is what the web site showed it would look like if the bottom end needed work, pretty similar i think

http://www.dansmc.com/pistonchew.jpg


Any truth to what i read? how can i check? what can i do? i dont want to assemble the top end if something is going to be wrong.

thanks in advance

-Victor

[Vintage Dave393]

You have a heat related seizure. Your piston is melted on the exhaust side which indicates that your jetting is lean. The ticking that you have been hearing is probably detonation caused by your jetting being lean which is probably caused by your ignition crankshaft seal being bad.

When the bike seized, were you at full throttle or were you braking? Have you compared your existing jetting to the factory recommended jetting. Have you adjusted your jetting for the cooler weather? Cooler weather will make the bike run lean. If you were already on the ragged edge with your jetting this could have been the cause of the seizure.

BTW, what premix ratio and what 2 stroke oil are you running?

[CheM]

Originally Posted by Vintage Dave393

You have a heat related seizure. Your piston is melted on the exhaust side which indicates that your jetting is lean. The ticking that you have been hearing is probably detonation caused by your jetting being lean which is probably caused by your ignition crankshaft seal being bad.

When the bike seized, were you at full throttle or were you braking? Have you compared your existing jetting to the factory recommended jetting. Have you adjusted your jetting for the cooler weather? Cooler weather will make the bike run lean. If you were already on the ragged edge with your jetting this could have been the cause of the seizure.

BTW, what premix ratio and what 2 stroke oil are you running?

Engine was at only about 1/2 trottle when this occured, i have not looked at the jetting yet but will for sure now.

I was running Yamalube 32:1

thanks

[Vintage Dave393]

Thats a good premix ratio and oil.

You are probably lean on the needle position and the main jet. I would pull the flywheel and see if the seal behind it is drooling or wet. I am betting that you have an air leak.

Search the board for a link to Faded's Air Leak Tester.

[chavez27]

my little brothers piston looked like that when we siezed it on his 99 cr 125, but it wasnt his top end that was jacked, it was the bottom end. i dont know why it would do that to the top but it just did. we re-did the bottom end with a new connecting rod and some bearings too and the top end too cause it looked like crap and the bike is runing like new. im willing to bet that knocking noise is your bottom end.

[Michael Parker]

That website is wrong; I pulled out a piston that looked just like that out of my bike, replaced just the piston kit and the bike runs fine. I know it wasn't the bottom end because a Hot Rods kit was just installed the rebuild before that.
It looks to me that something hard was inside of your cylinder and beating the piston. perhaps a screw from your reed cage.

[Aus_Rider_22]

Originally Posted by Michael Parker

That website is wrong; I pulled out a piston that looked just like that out of my bike, replaced just the piston kit and the bike runs fine. I know it wasn't the bottom end because a Hot Rods kit was just installed the rebuild before that.
It looks to me that something hard was inside of your cylinder and beating the piston. perhaps a screw from your reed cage.

That's interesting as the same thing happened to me.

I pulled out a piston that had small indentations like yours, except not quite as bad as above, but it was the same idea.

The dealer and mechanic said nothing about the bottom-end when I asked them about the markings.

Anyway, my bike needed re-plating a few hour later. So we got it re-plated and threw new piston kit in and it has ran great.

Since that piston with indentations came out I have since put 2 new pistons in and the ones that have came haven't showed any sign of the marks. The top has been perfect except for usualy carbon crowning.

I hope it isn't a sign of bottom-end fatigue as I have yet to put new bottom-end in my bike in 2 years.

[ossagp]

The picture on the website is not wrong. The one you show for your piston is not the same from what I can see. As VinD says, yours has just been too hot. And as he says, most likely from the seal being bad on the magneto side of the crank (the "dry" side). You might be wise to redo the crank or at least check both the rod and main bearings for play (any play in either one means it is shot, and I am talking up and down play, not side to side). NOTE: Just because an engine has a new rod or crank does not mean the rod bearing isnt coming apart. The race can be cracked from being dropped or an installation gone bad.

The piston that was shown on the website has a pattern of pock marks. In the case of bearing particles etc, the squish area and possibly other areas of the head will have similar markings.

[superrooster]

Originally Posted by Vintage Dave393

You have a heat related seizure. Your piston is melted on the exhaust side which indicates that your jetting is lean. The ticking that you have been hearing is probably detonation caused by your jetting being lean which is probably caused by your ignition crankshaft seal being bad.

When the bike seized, were you at full throttle or were you braking? Have you compared your existing jetting to the factory recommended jetting. Have you adjusted your jetting for the cooler weather? Cooler weather will make the bike run lean. If you were already on the ragged edge with your jetting this could have been the cause of the seizure.

BTW, what premix ratio and what 2 stroke oil are you running?

Its either what he said or the guy you bought the piston from could have taken the new piston that he showed you the recipt for and put the old one back and saved the new one so he could sell it on ebay.....it happens.

[CheM]

well guys just as a follow up....


I took the bike to a shop (well just the engine), they found the crank to be in poor shape (actually the bearings), lots of play on the rod.

The bottom end of the bike was rebuilt (they gave me the old bearings, which did not spin all that great), new seals were installed, and a new top end was put together. This weekend, i ran the bike for the first time, ran great... no strange noises at all. I rode it for 1.5 tanks softly, and after that got on it some, and it ran great!! All in all i ran about 2 tanks on it, and i am happy. 500 dollars poorer, but everything seems great now.

[YzRacer#53]

DAMN... Your piston melted a lot worse then mine I feel special now

[CheM]

yeah lol, i think it was a jetting issue...

I didnt check the jets when i bought it, but all jets were incorrect.

[YzRacer#53]

Do you have a manual? What is your stock jet sizes? What jets are in there now? What performance mods do you currently have if any?

[hunter77ah]

If the piston and head have the markings like the piston above, does that mean that the squish off? Or if bearings break do they always embed regardless of the bad crank? Or do they go hand in hand?

[bbrmotosports]

I have blown quite a few cranks in my day and all my pistons always turn out this way. Rod bearing blows and the needle bearings go right up scavaging ports ,they go on top of the piston and done!

[hunter77ah]

Originally Posted by bbrmotosports

I have blown quite a few cranks in my day and all my pistons always turn out this way. Rod bearing blows and the needle bearings go right up scavaging ports ,they go on top of the piston and done!

lol this is the best and worst thing that I've heard all day!

Its good that I now know what happened to my bike, but now I have to replace my crank.... lol