RS 125 top end rebuild failure

[monty16v]

well i have a 1999 aprillia rs 125

i bought the bike not knowing when the last time it was rebuilt

then i blew up the piston after around 1000 miles, also cracking barrell and damaging cylinder head.

so i bought a good condition 2nd hand aprilia barrel and cylinder head, and a new vertex twin ring piston kit, all new gaskets and a new small end bearing.

Rebuilt the top end on friday and everything was going fine

was running it in not going too hard for around 150miles (just the occasional burst)

Then on monday i think its melted a piston again, lost power when was just riding normally, maybe 50-60mph, clutch in, pulled in, put bike into gear and try turn engine over, it turned over but tightened up then so i just left it as i didnt want to damage another barrel


havent stripped it yet to check as it stressed me out a lot and cant even be bothered looking at it now to be honest

any ideas
cheers monty
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OffTheClockMotorSport

[MaybeGuy]

Did you re-use the old circlips?
What plug are you using? Is it all standard? If not, whats your main jet size?
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Blue_SV650S wrote: it was a sh1te wheelie, but it proves that he can get it up in 3rd and can do angles. In summery, mattsprattuk is a gobby little sh1tebag, dopehead tw4t, but sadly for all of us, he probably isn't THAT full of sh1te!!
Kickstart wrote: Hi I tend to agree with Matt. All the best Keith

[Kickstart]

Hi

Is the oil line in place? Or could it be blocked, snapped, have air bubbles?

What was the ring end gap set to?

Has it used any water? Not unknown for the water pump to leak into the crankcases on the RS and cause problems for the barrel.

Reusing circlips is a REALLY bad idea (or worse, some of the non original ones have tails that can snap off, although probably only a major issue when thrashed).

Anything that could cause it to run lean? Missing air filter, damaged inlet manifold. Worn main bearing seals?

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Frost]

So did you just buy a new cylinder and hope for the best, or did you get it properly measured / rebored?

[monty16v]

[Kickstart]

Hi

Cone filter probably leans the mixture out and so should be rejetted. However as stock an older one with the 34mm carb would be fairly safely rich as standard. If a later one fitted with the older carb then god knows how it has been jetted.

You should have a piston which matches the bore size (there are a couple of different size bores used with a tiny difference between them)

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[monty16v]

[MaybeGuy]

From that, I vote power valve/piston collision if it is in fact standard.
And to second guess Kickstart is a bad idea. (FYI a barrel can be rebored and replated)
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Blue_SV650S wrote: it was a sh1te wheelie, but it proves that he can get it up in 3rd and can do angles. In summery, mattsprattuk is a gobby little sh1tebag, dopehead tw4t, but sadly for all of us, he probably isn't THAT full of sh1te!!
Kickstart wrote: Hi I tend to agree with Matt. All the best Keith

[P.addy]

Don't skimp on parts when rebuilding a 2 stroke.

I personally wouldn't have anything that had been used.

It cost me £150 for a barrel, piston, all the bearings and rings/clips.
£25 for a full gasket set.

So £175, all new Mitaka kit, ran perfectly after that... I ran mine in a decent amount before opening it up at all though.

[Kickstart]

Hi

In theory you should have measured the bore to get the right piston (in practice I suspect most don't). Ring end gap should be checked to ensure there is enough clearance. However with a 2nd hand barrel you are never going to be certain that it hasn't been replated and that the stamp for the size is meaningful (and not unknown for cast iron liners to be fitted).

Getting a 2 stroke set up on a dyno can be difficult. Most places won't know what they are doing with a slide carb or a 2 stroke these days and plenty won't want to risk their expensive exhaust probes on a 2 stroke.

I would check the carb jetting (and given you were taking it easy, the needle jet needs checking carefully), but I would be surprised if it caused an issue in low speed / running in use.

I am assuming it wasn't over heating.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[monty16v]

[P.addy]

[MaybeGuy]

It's a common failure on RS engines. (or maybe just the 250s, not sure about the 125)
Of course they can be bored out. The rebore is then replated. Nikasil doesn't just make its own hole.
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Blue_SV650S wrote: it was a sh1te wheelie, but it proves that he can get it up in 3rd and can do angles. In summery, mattsprattuk is a gobby little sh1tebag, dopehead tw4t, but sadly for all of us, he probably isn't THAT full of sh1te!!
Kickstart wrote: Hi I tend to agree with Matt. All the best Keith

[Kickstart]

Hi

There is a positive stop for the power valve.

Although the barrels shouldn't be rebored it has happened . Not that the resulting mess will last any length of time. If replated the size stamped on the bottom is quite likely to not match the actual size.

Cast iron liners are pretty common though.

The Mitaka barrels do seem to be fairly good, and are FAR cheaper than original Rotax barrels. Not heard any reliability issues with them (although some supply them with better rings) although possibly very slightly worse performance.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[dudders7]

Just a thought, sounds exactly like one I had. Rebuilt engine, all new parts. Ran it gently occasional burst of acceleration and after about 100 miles i was doing about 45mph and the rear wheel locked up.. whilst on a bend. Managed to stop the bike without hitting anything. Put bike in neutral pushed it forward, fine not chain or sprockets, next engine, turned over fine and started up, strange I think and get on bike and carry on, 3 miles later same thing. I was ready this time, anyway wont bore with all the details, got it home, head off, no scoring or anything. Figured out in the end piston was the wrong way round. Turned it round put everything back together, worked fine, did another 2000 miles then sold it. So just check the piston is the right way round... easy mistake if there is no arrow on the piston. Good luck
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Riding in the sun in Australia on a 04 Worked ZX10R along with a fleet of other crazy toys

[monty16v]

will strip it down next day or 2 to see the damage
i think it either ran lean, oil pump failed, or else there was a fragment in the crankcase from the last time it melted the piston
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OffTheClockMotorSport

[monty16v]

well i was annoyed with the bike so havent stripped it until today

to find a big piece of circlip lying on top of piston and a few pieces of it jammed between cylinder wall and piston.

so my new piston and cylinder are going to be junk.

I imagine the cylinder is going to be quite hard to get off the piston as well, as its quite well siezed
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OffTheClockMotorSport

[P.addy]

[monty16v]

i think the cylinder is already scraped, as there is bits of circlip between it and piston, and its pretty well siezed.

i have spare cylinders anyway, and as for the piston i will probably put in another vertex item

might change main bearings and crank seals this time while i have it stripped as well
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OffTheClockMotorSport