First dirt bike - '86 Yamaha IT200. Piston removed, what next?

[Jack Hanks]

Bike was running when I got it although losing power when it hit the power band. Everything I read said no matter what check the cylinder and piston before you start riding. So I've got the piston out and wondering what to check/look for. 

The manual lists a bunch of procedures but not sure what's most important and where to start. I have paperwork indicating this bike was bored out to 240 back in 2012. 

Any help is greatly appreciated! I'm completely new to tearing down and rebuilding small engines. Looking forward to learning from you all. 

[Singletrack_rider]

First off you need to know the brand of the big bore kit installed. Once that is known you can contact the seller and find out what the specs should be., Then you need to have the cylinder measured to see if its in spec. If it is get a new piston and set of rings if not then we will  go from there. 

Did you test the compression first before tearing down? 

Check out this forum also . Most of the guy are in Australia but you can get some good info.http://www.yamahait.com.au/forum/

[Jack Hanks]

Did not get the compression tested before.  Too eager to break into it.  Do you think it's possible for a visual inspection of the cylinder to be enough to say it's in good shape?  Would hate to order new piston and all that only to have bad compression in the cylinder.

Luckily I do have paperwork for the big bore kit.  I'll try and find a bike/machine shop around me to get the cylinder measured.  

Joined the IT forum earlier today!  

Thanks!

[Singletrack_rider]

Take a picture of the piston and post it up.

[76xtdrvr]

19 hours ago, Jack Hanks said:

Bike was running when I got it although losing power when it hit the power band. Everything I read said no matter what check the cylinder and piston before you start riding. So I've got the piston out and wondering what to check/look for. 

The manual lists a bunch of procedures but not sure what's most important and where to start. I have paperwork indicating this bike was bored out to 240 back in 2012. 

To get the stock bottom end to accept a piston for 240cc, the cylinder would have to be bored out over 1/4". I don't think that's possible for that engine. Before buying any replacement parts you need to get some measurements taken to determine the true displacement, especially the cylinder bore size and piston diameter.

This link might help with your inspection.

http://motocrosshideout.com/2014/01/27/5-signs-that-your-dirt-bike-needs-a-new-top-end-2-stroke/

Here's a link to a Yamaha IT forum. http://www.yamahait.com.au/forum/index.php?board=8.0

Edited September 5, 2017 by 76xtdrvr

[Jack Hanks]

Awesome link, thanks. Plan to post a bunch of info and pics this week and hopefully get it to a machine shop for measurements this week also. 

[Jack Hanks]

In the meantime here's the paperwork for the over bore. 

[76xtdrvr]

20 minutes ago, Jack Hanks said:

Awesome link, thanks. Plan to post a bunch of info and pics this week and hopefully get it to a machine shop for measurements this week also. 

If the cylinder and piston look reusable, have them check the piston to cylinder clearance. The piston needs to be mic'd 90 degrees from the wrist pin bore, and just above the bottom of the skirt. 

[Jack Hanks]

Sounds good, will bring the piston along. Just did some research and the reason they were able to bore to 240 is because he had the cylinder sleeved back in 2012.

[76xtdrvr]

That bore s/b around 73mm.

[Jack Hanks]

Alright here's a ton of pics of the piston and cylinder. Going to a machine shop Saturday.

[Singletrack_rider]

Looks  like you have some blow by on the rings. Piston not scored up so thats good. The rings may have never seated properly. Definitely a new piston and rings. You may get lucky and can get away with a hone on the cylinder but i got a feeling its going to need a rebore. I can see why you had the power loss.

[Jack Hanks]

Crossing my fingers. The guy I'm taking it to has really good recommendations, so we'll see what he has to say. What's the difference between hone and rebore?

[Singletrack_rider]

A hone will put the cross hatching back so that the rings will seat. It will not true a cylinder round or take out scratches. It will remove some material. A rebore will remove a minimum of .25mm, make the cylinder round and remove deep scratches, and will also get honed. A larger piston and ring set will be needed with a rebore.

[Truman Sparks]

Hone cleans it up and new cross hatching. Bore is to the next size piston.

 

240cc nice that will give you plenty of power.

Just run the factory oil to gas mix ratio, when you get it ready to fire up.

[76xtdrvr]

Just my 2 cents if it was mine based on your pic's: If the cylinder is round and consistent within .001" throughout the length, and the piston to cylinder clearance is no more than .004", I'd only do a very light hone to crosshatch the cylinder, Scotchbrite the piston, replace the rings, piston pin, circlips, and top end bearing. Anything beyond these tolerances would mean re-sleeving and a new piston, or rebore/hone with oversize piston.

[Jack Hanks]

Just my 2 cents if it was mine based on your pic's: If the cylinder is round and consistent within .001" throughout the length, and the piston to cylinder clearance is no more than .004", I'd only do a very light hone to crosshatch the cylinder, Scotchbrite the piston, replace the rings, piston pin, circlips, and top end bearing. Anything beyond these tolerances would mean re-sleeving and a new piston, or rebore/hone with oversize piston.

I'm hoping that's the case. Nothing seems visually bad, but we'll see how it measures out. Thanks.

[Jack Hanks]

While I'm waiting on that, have some other questions. First, the manual calls for cleaning and oiling the sponge filter deal, but doesn't mention the filter in the attached picture. I'm guessing I should clean it the same way? Does it need oil?

Also I have the carb in two pieces. I don't see carbon buildup anywhere. Do I need to further disassemble and clean? Should I reset it to factory settings plus altitude adjustment? I'll be going from 5,000 to potentially, 12,000 ft above sea level. PO says it's jetted for 6,000 I think.

[76xtdrvr]

If you're investing any $ in the top end, that air filter s/b replaced. They're still available and need to be oiled with a good air filter oil. You won't see any carbon in the carb, but make sure all the jets are clear. You can remove the main jet and pilot jet. Then make sure you can see light thru them. If not, soak in Solvent or Pinesol until clear. No need to rejet if the p.o. says it's set near the altitude you ride. Ride it first to determine what jetting changes might be needed.

https://thumpertalk.com/shop/oem.php?category=5&manufacturer=7&year=1986&model=29374

Edited September 7, 2017 by 76xtdrvr

[Jack Hanks]

Back from the shop. Piston was cracked, so lucky it didn't let some pieces fly. Cylinder needs a bore, if possible. He's going to give the machine shop the measurements and ask what they think. Hope to hear back early next week.

He had me check the piston rod for up and down play and I didn't feel any. Plenty of side to side play and some bluing on the crank shaft? Picture attached.

Also attached a picture of the top of the airbox. Looks like I should have a cover up top? But what you see is all that's there.

If I need a new cylinder, is there anywhere to find out what makes and models/years I can use for this bike? Or is it limited to IT200 only?