View Full Version : Have I fixed it? (Two-stroke advice wanted)

6th July 2008, 17:27
I've finally replaced the busted piston in my 1996 Aprilia RS250 and got the bike going again.

The first time I ran it, the temperature shot up pretty quickly. Then I realised that it was probably because I had only put coolant in through the radiator cap, and not the hole at the top of the radiator as well, so there was probably some air in cooling system. So I've put coolant in both holes, and now it's running ok, and not overheating rapidly. I would put the fairings back on and take it for a ride, but because I'm not a very experienced mechanic I'm still worried about a few things, and I don't want to take it for a ride and have it sieze up on me:

1. - The bike doesn't seem to be drawing oil. I've connected the oil hose to a measuring cylinder filled with oil, and noted that it doesn't go down very much, if at all.

2. I can hear a tapping sound. It doesn't tap with every revolution, it's a more intermittant tap. I'm not sure exactly where this is coming from. Is it the right cylinder or left, or somewhere else?

3. When I run it a small amount of greasy stuff drops on to the floor. I'm pretty sure that I put the exhaust pipes back on good and tight though.

Apart from those things, it seems to go fine. I even revved it up to 9 or 10,000 rpm and noted that the exhaust actuator valves seemed to be working ok.

Basically, I'm not sure what, if anything, is wrong. I'd really appreciate some thoughtful advice.

Even better, if anyone who has some experience with two-stroke road motorbikes could come round to my place and have a look and point me in the right direction, that would be highly appreciated. I've done all the hard work and put it back together, I would just like someone to take a look. I live in Birkenhead on the North Shore.


6th July 2008, 17:32
You replaced both pistons?

6th July 2008, 17:36
Just on this:

If you do think I have a problem with the oil pump,
could you please tell me what ratio I should pre-mix the petrol with the oil at, so I could safely ride my bike to a mechanics shop?


6th July 2008, 17:39
No just one. Do I replaced the first one that I took the cylinder sleeve off, it was busted. The other one, I just took the head off, it looked fine from the top.

I know I've been a bit lazy by not taking off the other cylinder sleeve, but if the other piston was fucked it wouldn't go at all would it?

6th July 2008, 17:46
No but if one let go surely the other may be at a similar stage? Recommendation would be to replace both pistons, small end bearings, rings, gudgeons. Carefully check both for big end bearing play. Very very carefully check the power valves for ant sign of wear. Anything that looks worn or is out of spec - replace. then you may get another 10-20,000kms out of it before the next rebuild. You just gotta love high performance 2 strokes with suzuki engines and the extravagant cost of running one eh? Not a poor mans bike. If you're poor then sell it.

6th July 2008, 17:59
I'm not sure why it siezed up to start with, but it seemed to be an air filter foam that was totally fallen apart and letting in too much air. I've replaced the air filter now.

The power valve seemed to be working fine on the cylinder I took off. I wouldn't know where to start to replace them anyway, the bolts and screws on the outside of the cylinder were done up pretty tight, and I started to have a go but ended up burring them and gave up as valve on that cylinder seemed ok. So I wouldn't be keen to do that on the other cylinder.

I hear what your saying about the second piston and rings, and I could afford to do it, but it's still another $300 bucks that I'd rather have and at this stage I'd just like to know if it would be safe to take it for a quick spin and maybe ride it to a shop to take a look.

6th July 2008, 18:44
I wouldn't necessarily replace a good piston because the other cylinder seized. It's not like a four stroke, just cos one seized doesn't mean the other one needs replacing. If you broke one mirror would you replace both of them?

It wouldn't necessarily use much oil at no load. Is it smoking? I always feel there can't be too much wrong with a two stroke so long as there's lots of smoke.

6th July 2008, 18:49
The manual said to run it at 2,000 rp, for two minutes, and it should use between 6 and 8mls of oil. But it hardly seemed to use 1 ml.

I would like to take it into shop for full inspection, just for my peace of mind as much to check that i've fixed it ok.

What would be a good pre-mix ratio so I could take a short (less than 10km) ride to bike shop that would lubricate engine enough if there was a problem with oil pump without clogging jets?

6th July 2008, 18:52
It's not smoking that much at 2000rpm. Maybe there's not enough load at even 2,000 rpm to draw much oil. I revved it up to 10,000 rpm but not for long of course. When I did that it smoked the whole garage out.

6th July 2008, 19:04
Switching to premix isn't quite as simple as bunging some oil in the petrol. Because the oil in the petrol makes the bike run lean (oil replaces petrol, but air volume is still the same). Lean mixture = seizure. Caught a few folk that has over the years .

Have you checked the oil pump setting. They pretty reliable, so long as they're set correctly. Setting it is dead easy if you've got a manual. And, once set, apart from a broken cable, there's bugger all ever goes wrong with them.

If you do want to switch to petroil (but note the caveat above), 48:1 should be OK. Unless you're nervous , in which case 32:1. But you may have hard starting with petroil.

6th July 2008, 19:16
Oil pump setting seems to be ok. When I twist the throttle, the cable pulls the oil pump around past the mark. And it seemed, as best as I could tell, to be set right in the splitter box.

But good idea, I will check when I rev it up to 9,000 rpm that the oil pump goes past the mark.

7th July 2008, 09:00
Oh I wasn't aware the piston seized as the original post says the piston "busted" leading me to believe the piston skirt broke.

7th July 2008, 11:05
Update on this: I have read the manual again, and it turns out I previously did the two-minute oil pump test incorrectly.

The manual says to connect the oil hose to a measuring cylinder, start engine, and (presumably after it has warmed up) hold rpm at 2,000 while holding (with your finger or a screwdriver) the oil pump in the full open position for two minutes. In these conditions the engine should draw between 5ml and 7ml of oil.

The last time I (and my mate because it's pretty hard to do this test with just one person) did it I missed the part about holding the pump in the open position, and simply put the rpm up to 2,000 while wondering why the oil didn't go down in the measuring cylinder.

So, it is clear now that this test checks oil pump for capacity, that there's no blockages in the oil lines or that the pump is not rooted. If capacity is OK, you've still got to make sure that it is set in the correct position in relation to the exhaust actuator and the throttle. If capacity is ok and you've set its position right, then everything should be fine.

I haven't redone the tests yet, but it looks like it should be all good.

I was pretty sure that the busted air filter was what originally caused engine to overheat and sieze in the first place, but I wanted to check at least one other thing, and I couldn't see anything obviously wrong with cooling system like a leak or busted radiator, nor any air leaks as I've had carberattors cleaaned out and I put them back on good and tight, so I thought that I'd give the oil pump a look over, and when I first did the test incorrectly, thought I had a problem with it. It looks like that is not the case.

So the lesson is, read the manual properly.

7th July 2008, 11:14
Yes the piston skirt did break. It was more than just siezed. I should really put some photos up as I know some people get a real kick out of seeing broken engine parts. I still am not convinced that one piston breaking necessarily means that there is a any real problem with the other cylinder or piston,

I'm getting sick of this mechanical sh*t, just want to get out and ride.

Although I have found out a lot of things, and I know I'd be able to fix it a whole lot faster if it does break again. At least the top end that is, I don't really want to know about the bottom end of the engine, top end was hard enough for me.

I suppose if the leak I mentioned in the first post persists I should find out where it is coming from. Maybe I just need to re-tighten the cylinder, exhaust, and head bolts now that it's been through a heat cycle?

7th July 2008, 13:27
Oh. Yes. ALWAYS retorque after a heat up, cool down. And repeat until you get no change (ie it doesn't need tightening)

10th July 2008, 22:49
Personaly I would have done both pistons because every time i've tried to take short cuts its all blown up in my face, especialy with high performance bikes like this.

14th July 2008, 11:56

I've tested the oil pump and it was working fine, no worries there. I have also found the source of the leak I mentioned before. The leak was happening when I revved it up to 9,000 rpm and activated the exhaust valve,

It turns out that one of the bolts attaching the exhaust valve cover was loose, and was actually broken. So all I need to do is get a new exhaust valve gasket and a new bolt and it should be sweet.

I also need to put the air filter box back on again, this time making sure that the hose at the bottom is attached firmly (maybe I'll tape it on) because it came loose last time after I attached it.