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Engine rebuild tips for a newbie

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I've decided to rebuild my 1991 yz 125. The problem is I'm a complete noob to the 2 stroke engine rebuilding. The top end looks pretty simple and I think I feel comfortable doing that... It's the bottom end that kind of sketches me out. I would really like to replace the top and bottom so I know when the maintenance was done and everything is new. I don't want to sink a stupid amount of money into this bike if I don't have to though. 

 

There isn't any noise coming from the bottom end but what is a good indicator that it needs replacing? Should I just be looking for unusual play? I'm just looking for the advice of the pros that have done this over the years... What's worked best for you? What's screwed you over the most? 

 

If you can help me out and save me some time making mistakes... I'd appreciate it. 

 

Thanks in advance. 

 

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Just finished a complete rebuild of my sons YZ125 04'.  Good tools and a nice work area and a service manual are great thing to have.  If you are going to split the case, having a large work area that you can stay organized is ideal.  Take pictures as you go always helps. 

 

Bottom end play on the rod bearing can be check with a dial indicator down the spark plug hole.  May give you a good idea if you need or want to do the bottom end.  If you dont want to use an indicator then you will need to pull the put end and check by feeling for up and down play on the rod. 

 

Top end pretty simple like you said.  Bottom end is not so much.  Need special tools to hold clutch and flywheel to remove nuts.  Special tool to remove flywheel.  inch pound and foot pound tourque wrenches for assembly,  some way to press in all the new bearings and remove the old ones.  

 

Is may cost a little to have all the right tools to start but it is very rewarding to work on your own bike any time you want.

 

Hope this helps.

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^Great info above

 

If there isn't any noise coming from the bottom end bearings and no play in the crank or rod I wouldn't rebuild the bottom end.

 

Now if the bike sat for years, has rust on the internals or was blown up, that is a different story.

 

What is the history on the bike? Did you buy it new? Any idea on how many hours are on it?

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Since this is my first bike, I bought off of craigslist cause I wasn't sure how many spills I would take. The only information I got from the seller was that the top end was done about 2 years ago. (Which makes it 3 years now) My guess is that the top end is shot because when i kick it over, it takes 20-25 times to start the bike. It feels like there is no resistance when I kick the bike. 

 

I picked the bike up for $600 and I'm at the deciding point of spending quite a bit of money to replace everything... or take that money and put it towards a newer bike. 

 

Thanks for the above input. I wasn't sure exactly what specialty tool I would need but I will look into those now. Are they make/model specific or universal?

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I would get a compression tester and start there.  New top end should be around 150psi.  Normal could be 125-135psi.  Anything under 100psi I would say needs new top end. 

 

Some tools are specific like the flywheel puller.  Others are universal like the clutch basket holder.

 

If compressions is good I would look at rebuilding the carb for starters and see if that fix's the starting problem.

 

Good luck.

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