I have a Yamaha dt125 R 2004 I've just had the engine rebuilt, re-bore with new crank and piston etc. I've just had my bike out for a run about after about a month without being started. It started on the fourth attempt with the choke on and I let it sit for about ten minutes to warm up before I went out on it. About five minutes in to my ride the engine ran out of power and cut out. I pulled in the clutch and pulled in at the side of the road. Tried for about five minutes to start it and it eventually started. It kept doing this all the way home, stopped and started about four times till I got it home. It starts fine with the choke and will sit and tick over for as long as you like but it only runs for about a minute in gear and then cuts out. I've checked the engine oil, the two stroke and the fuel and they all appear fine. I'm pressuming the plug is fine as the bike ticks over. Any help you guys could give me would be very much appreciated.

Are you able to measure the bore and stroke? Cubic centimeters displacement would be pi times radius squared times height.? The dealer can take those numbers off the engine and do the job,why not? Match up the old piston if you can find it

cant work on these rate for solution

you have to buy a new bore kit which u fetch u around 5k

in adition to the above coment, which i must say is spot on, the posability of a used barel and piston if a new piston is not available could be worth considering, other things you may wish to check on that particular engine is camshaft seizure, in a uk bike it runs directly in the machined surface of the cylinder head, oil failure due to blocked oil filter has severe consequenses, located on the l/h side of the engine under a 22 or 24mm spaner head cap just below the gear change lever, this is a washable filter retained by a cap and spring, correct replacment is inperitive. are you located in UK? HOPE THIS HELPS YOU

Did you just have the motor rebuilt by a reputable shop? If so take it back and explain the problem and they should take care of it.
If not you are kinda on your own. What kinda tick? Could be leaking head gasket, base gasket, bad valve guides, incorrect bore, piston, rings. Incorrect ring gap, leaking exhaust gasket, out of time, etc, etc, etc, etc.

yea, for most bikes they only come in .020 increments now. if your looking to freshen up the top end, then youll have to go to a 56.00mm piston. there will be almost no change in displacement or jetting from 55 to 56mm. if your replacing the piston, its best to have your cylinder bored anyways. since the worn cylinder walls may have adverse effects on the new piston rings, and they may wear a bit prematurely. a bore only costs about 30-40 bucks most places, and its cheap insurance. plus you will have the peace of mind knowing that your bore is matched specifically for your piston. I would definitely recommend a fresh bore.

It is a simple job to rebuild a dt125r as it is 8 or maybe 9 bolts holding the head on, drain the coolant into a container so you con use again, undo the 5 bolts holding the cylinder head on(after taking the spark plug and coolant pipes off) after this undo the cylinder bolts should be 5 bolts to undo(12mm socket or spanner)t take the cylinder off the undo the gudgeon pin holding the piston on, at this point all you should see on the engine is the engine block with a con rod coming out of it. After you have the engine dismantled measure the bore diameter and then go to B&Q or any hardware store and get a circular sanding head for a drill that is maybe 5mm-10mm bigger than the bore, put the sander inside the bore on full power and hold it there for atleast 1minute and 30seconds, this should get rid of all of the score marks on the bore do the same to the piston(go round all score marks to make it smooth), get som gasket paper(or a engine set for dt 125r but gasket paper is only £1.09 from most shops) blue tak it to the top of the bore them go aroud it with a hammer very lightly so u have a gasket to fit the head on both ends, use a line of gasket glue(big line about 5mm in depth) all the way around the head and assemble the engine, this will only be temporary untill you have to do agen but it is cheaper than spending £150-£190 on new bore and piston.

The idle problem is a lean fuel mix caused by an air leak in the intake system. Bad gaskets /seals on the intake manifold, bad valves, bad piston rings, loose spark plugs, and bad head gasket are all things that can lean the fuel/air mix. The smoke indicates a need for new rings. I would have the cylinders bored then install new pistons and rings. Now finish the top end work by getting a valve job. You should get a liquid fuel tank liner to eliminate any rust problems in the future. Also, install an in-line fuel filter. Google “ kreem fuel tank liner “ or go to www.http://www.4secondsflat.com/Fuel_Tank_Sealer.html


My complements on the well cleaned carbs. Do the work mentioned above an you will effectively have a new engine.

A “very helpful” rating for this answer? Thanks!

The piston seize destroys the piston, rings and cylinder wall. Bad enough, and repeated seizing, can get metal bits into the crank rod bearing and crank end bearings and seals. This is serious stuff. Sorry to be the bearer of bad news.

Seizing is caused by friction. Not enough lubrication means friction that creates heat. The heat builds and the piston and rings "weld" to the cylinder. A faulty cooling system can also cause heat. Any oil will break down and loose it's lubrication ability when the heat gets too high. A water pump malfunction or plugged coolant lines, leaking coolant hoses, plugged radiator and bad gaskets can create heat.

Other sources are > timing is off, air leak causing the fuel to be too lean, incorrect spark plug heat range, carbon buildup on piston and head causing detonation, motor oil used in gas instead of 2 stroke engine oil and over revs on the engine. I have probably left a few things out but these are the usual causes. Be aware that combinations of small problems can add up to a seize.

The engine needs to be rebuilt. Normally I would just rebuild the top end but since you have had multiple seizes the bottom end is probably contaminated. New crank bearings and seals should be installed. If the cylinder can't be bored then you will need a new cylinder. New piston and rings are also required. Bear in mind, you must find the cause or causes of the heat and fix those problems also. If you don't, the new piston will quickly seize up.

Top rating for this answer! Thanks Kev...

The bike will still run even at 7:1 or a little lower, but the specifications call for about 10:1.