Will a yz125 /250 topend fit on my dtr125 1990 model

When fitting a new top end you may have broken the piston rings. On a Two Stroke engine the rings have a gap which must not enter any ports. To prevent this the slot for the rings has a metal peg to hold the ring gap in one place.
On Four Stroke engines there is no ring peg as there are no ports.
If a ring breaks whilst being fitted the broken parts will enter a port and then be dragged up and down the barrel causing damage to piston and barrel. The broken parts one having made their way to the top of the piston will be crushed between the cylinder head and the piston damaging both. The cyl head can be re used but the piston is most likely ruined.

Pete

It sounds as if you have rebuilt the engine without torqueing the head bolts. Although, it could be that the cylinder base gasket has been torn during the rebuild. Try to re-tighten the head bolts to the correct settings first. Good luck.

you could have a cracked cylinder head or cylinder, did they do a dye check on the head when they did all that work to it ?

G'day. The torque specs for the 8mm head bolts is 18ft/lbs.the jug?(cylinder or barrell) is just tightened down even & firm, as there is no acess for a torque wrench.
OK. other things to note.
Clean the cylinder first in solvent(kero or fuel).Then in warm soapy water in a bucket. Dry-(compressed air is best for this-wear eye protection).once dry lubricate all power valves with oil.
Using a clean white rag-soak the end in your 2 stroke oil & wipe the inside of the cylinder surface untill the rag comes back clean(you will need to do it a couple of times-turn the rag over & do it again).Lube the new piston just before assembly-Also pour a little oil down the main bearing oil holes & over the big end too.
Other things to be sure of are-
Piston on the right way round-arrow to the exhaust.
Rings right way up. Some 2stroke dirtbike rings will only go one way up.Look CLOSELY at the ring ends-on the upper or lower surface there may be an etched dot or "T" or "N" or other marking. the mark goes facing up(on both rings unless single ring piston).
rings positioned with end gaps over the locating pins(the ring grooves have a pin in them to stop the ring turning)
Piston pin clips in correctly.
Powervalve moves freely.
Again-watch when you fit the cylinder-it can be tricky to keep the ring end gaps over the locating pins as you feed the cylinder over the piston.
Hope this all helps
Regards(done this a thousand times)Andrew Porrelli

I'd bet you 10 to 1 it's not the coil. It sounds like it is starving for fuel and needs a carburetor adjustment. When you rebuilt the top end, did you put a bigger jug and piston? If so, your problem most definitely is the carburetor needs to be re-jetted.

What caused you to rebuild it? Did it seize, or quit running? Was it losing power before the rebuild?

That's where my money is but if you still believe it is the coil, you can check it with a spark checker. This is a handy little tool you can find at AutoZone, Harbor Freight, etc. It's a one piece tool that connects between the plug and the coil. The plug stays in but you remove the wire and put the checker on, one side connects to the plug, the other side you connect the plug wire. There is a light in the middle that will flicker when you crank the engine if it has a good coil.

Put the checker on and start the bike and see if you are losing spark...if the light stops flickering at top end, then you do have a coil issue...but like I said before, having seen this problem hundreds of times, my money will be on re-jetting the carburetor.

the only way to tell is to match them up with out knowing exactly what year they both are its hard to tell but the change every couple of years

if you have any friends that weld. ive had this problem myself and have found it cheaper and less stressful to fabricate yourself than to search any further

You have a 50/50 chance that they will work okay. All the mounting holes need to line up and the cooling holes as well. Get a head gasket for the '98 and see if all lines up. Will the exhaust pipe fit the '95?

Remove plug then reinsert top cap only let plug hang free now kick over starter wile somone watches the plug for spark (the darker the better!)