Rebuilding a 2 stroke? | GTAMotorcycle.com

Rebuilding a 2 stroke?

U

unL33T

Well-known member
So my RS125 finally decided it was tired of my abuse and lost all compression. I am kind of considering rebuilding it myself as I hear it's not that hard but I have some questions.

How do I know if I need the whole piston kit, whole top end kit, or just some rings?

How do I know if the bottom end bearings should be done as well?

Some information:
I commute on this thing and hit redline pretty much constantly. I've taken it to Ottawa and back at or near redline the whole trip. Today I took it North Bay and 3 hrs in is when it died. Seemed a touch low on power but it was still able to get me into HTA172 territory so I didn't think too much about it. My low fuel light had been on for quite a few KM so I got off the highway at Trout Lake to look for gas. As I hit the ramp I pulled the clutch and released the throttle to downshift, engine stalled. Was still rolling so I dropped the clutch and engine fired up again but was very "sputtery" and had almost no power. Limped through Trout Lake and ended up on the other side (there are no gas stations in Trout Lake). Went to make a u turn back to the highway and when I slowed it died again. Wouldn't start. Assumed I was out of gas. Called CAA and they sent a flatbed with some gas. Filled it up and still wouldn't start, just backfires. Got it towed (and the tower insisted on towing with the stand down even though I know that's bad for it). Called a few shops in North Bay on the way back. North Bay Cycle & Sports just told me they had never seen one and didn't want to even look at it. Carlson Sports and RV's response was "a 2 stroke is a 2 stroke so we'll at least diagnose it". First thing they checked was air filter, looked ok. Next was spark, looked crummy so they replaced it, no luck. Finally checked compression and found it was below 30 PSI (I've heard it should be over 130 PSI). They also noted that the carb boot looked a bit cracked. So the mechanic's conclusion was that the cracked boot allowed dirt into the engine which likely scored the cylinder walls causing the compression to go. I suspect it's from my abuse but what do I know. Worth noting that the last shop I brought it to said the boot cracks were just superficial and nothing to be overly concerned about until rebuild it due but that shop screwed me over pretty bad with everything else so they were probably wrong.

Soooo.... tips? Any special tools I might need? Too much to tackle myself?

Thanks.
 
The mechanic's hypothesis is a bit out there IMO. I don't know anything about your bike but you could just have a tight or stuck valve.

How many kms on the bike?
 
Just over 12000 if I remember correctly. Rebuild schedule is every 16k as far as I am aware but I'd have to check again. Previous owner said it seized on him and was rebuilt around 5k so there really should only be about 7k on this piston. He claimed it was because they came too lean for our altitude from the factory. Talked to the owner of Corsa Mechanica who claims he imported most of them to Ontario and he said they seize at low km because people ride them in cold weather but I rode mine at around 0 and never had any trouble. Temperature always stayed at operating once warmed up but he claimed when it's cold if you hit a valley in the road the temp will drop really low on the bike I should put cardboard over the rad for that. Seemed a bit out there since on mine it doesn't vary except when I get stuck in traffic on a hot day it'll go up a notch on the gauge.
 
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Did the top end on my KDX fairly recently. The actual piston and ring swap is very easy and didn't require any special tools, although a ring compressor might be nice to have. The hardest job was just disassembling everything to drop the engine far enough to remove the cylinder. Follow the shop manual and it is straightforward. The whole process took me most of a weekend, but most of that time was just from neurotically cleaning the carbon off of everything. A pro could probably do it in a couple of hours.

Can't comment on the bottom end (bearing) work, but that shouldn't have anything to do with your low compression.

Edit: from what I can tell, you only really know if you need a piston once you get it all taken apart and can measure it to see if it is still within spec. The cylinder itself might also need to be replated, resleeved or replaced if it is out of spec, especially if it is actually scored.
 
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I have a power valve if that makes a difference. Maybe it's better if I just have someone else look at it. I know it's not seized because it will turn over all day (until the battery goes dead, at least).
 
Matt Rain said:
The mechanic's hypothesis is a bit out there IMO. I don't know anything about your bike but you could just have a tight or stuck valve.

How many kms on the bike?
Click to expand...

Sorry, an aprillia RS125 is a two stroke... so no valves, it has ports... that don't work very well at slow speeds so you'll NEVER get 130psi on a compression test on a two stroke.

For unL33T:
First pull the plug and see if there is any aluminium on it. Look down the hole and see if the top of the piston is burnt.
Next take the reed block off and inspect the intake side of the piston
Then remove the pipe and inspect the exhaust side

If you don't know what you're looking at, post pictures.

... or just tear it apart... I'd be worried about the crank
There are few shops in the GTA that know ANYTHING about two strokes, Pro6 (at least they useta) and WillyWasp come to mind.
I'm up by Lindsay, I'll look at it for free.
 
unL33T said:
I know it's not seized because it will turn over all day
Click to expand...

Doesn't mean that the rings aren't stuck.It's not that tough to pull the top end and check the rings,piston and bore.I don't have any experience with that bike,but some with 2t.
 
Stop turning it over.
You're not doing the cylinder liner any favours.
The expensive cylinder liner.
 
Thanks for the suggestions. Oh yeah I should've caught the comment about valves. Z1 Cycletech was highly recommended to me by a fellow RS125 owner.

Bitzz, what hours are you available and how long do you think you'd need to look at it? I might be able to swing by for a couple of hours on my way back from North Bay.
 
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bitzz said:
two stroke... so no valves,
Click to expand...

I knew that.
wah.gif
 
bitzz said:
Stop turning it over.
You're not doing the cylinder liner any favours.
The expensive cylinder liner.
Click to expand...

+1. Should be a Nikasil coating on the cylinder.
Sounds like bitzz should be able to help you out.

If you can't hook up with him, Pro6 has done work on my engine in the past and I've hear good things about WillyWasp.
Gord Bush may be able to help you out if you need a crank rebuild.

AF1 for parts and resources

The alternative is get yourself a manual and tear it down yourself.
Lots of help on the Ape 125 forum, here's a rebuild thread
http://www.apriliaforum.com/forums/...al-Engine-Rebuild-Thread-Gearbox-whine-in-5th

Here's a couple of videos in case you're wondering how difficult it is :
[video=youtube;UQyUsGP5YmY]http://www.youtube.com/watch?v=UQyUsGP5YmY[/video]

Others :
rs125 top end
 
Yeah it's simple when all the parts are nice and new and clean and the correct shape.

It gets a little more challenging when the piston has melted, and smeared aluminium down the bore and coated the crank bearings.



That's the bottom ring, the top ring went out the port and got stuck in the stinger.
 
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bitzz said:
Sorry, an aprillia RS125 is a two stroke... so no valves, it has ports... that don't work very well at slow speeds so you'll NEVER get 130psi on a compression test on a two stroke.
Click to expand...

I dunno... I have seen over 130. Lots of gasgas and KTM dirty bikes have compression that's higher than that, quite a bit higher... my modified RZ is about 130. My old gasgas 300 had over 130 with flaking nikasil. I would have guessed the RS had more than 130 stock. I fresh stock RZ350 is ~120.


To the OP, probably better to get someone who knows what they are doing to rebuild it for you. Someone with a bore gauge, unless the bore looks nice.. 30psi sounds BAD.

EDIT - not saying bitzz doesn't know. Just saying the OP should not do it himself, at this point...
 
If the bore needs to be redone (next size rebore or replate nikasil) you could send the cylinder with the new piston to http://www.cvtech-aab.com/en/.They will fit it properly with everything ready to bolt up.Ports chamfered and spotlessly clean.they did my Ossa's cylinder fitted to the new piston i sent to them for $300 to my door.
 
Bore does apparently need to be redone. I can get a piston and cylinder kit for about $500 CAD from AF1. Just waiting to see if Two Wheel can get the same kit and for what price. They usually meet or beat AF1 for parts prices, in my experience so far.

Crank seems fine, I'm told.

I'm not doing it myself.
 
cracked intake rubber sucking air will lean out the mixture causing the engine to overheat and....ya know

it wasnt dirt.

my guess, for 30psi, messed up rings/piston, but probally hole in top of piston (because it still turns over)-but its pooched..do not need to open

gord bush, Z1 , are good rep
get a pro to do your engine properly. dont take it apart and then bring it in - not cool. head off for a look, no more.

and yeh, you need to replace intake rubber. if you find that the intake rubber wasn't sucking air, going up to a one size larger main jet would be prudent-if it runs as well as before leave the one step larger main jet it in...its cheap insurance. if its too rich and the bike doesn't run as well or better than before you can always change back to the original - no harm done. one step richer on the mains isn't a big change. and zippin to n.bay wfo -yeh, you were on the mains. check the rs forums?

on cold days, because the air is denser, and having more oxygen = leaner mixture.
likewise, on hot days, because the air is less dense, and having less oxygen = richer mixture

ambient air temp can really make a difference to a 2 stroke. often people will notice an little extra pull from their bikes on cold days (more oxygen).

and be sure to ask the engine builder to optimize the squish band. this tightening of the squish will slightly raise compression(and midrange power -slightly) -most importantly though, optimizing the squish also aids in the cooling of the cylinder head and piston! its a win win situation for very little additional cost. and aids reliability

and on the intake rubber, absolutely you must replace it before running your new engine - it is far too common to have 2 strokes seize or melt down because of air being sucked in at a cracked intake rubber, another common place where people overlook is where the carb goes into the rubber boot-be sure the carb is properly seated all the way around. dont forget this when you put the motor back in the bike. likewise check the carb to airbox for perfect seating w no leaks. helpful hints to protect your new engine.

and i think you have the perfect bike for your riding style. how else, or where else, do you get to wind out every ounce of power from your bike and still have your drivers licence....
 
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bitzz said:
Yeah it's simple when all the parts are nice and new and clean and the correct shape.

It gets a little more challenging when the piston has melted, and smeared aluminium down the bore and coated the crank bearings.



That's the bottom ring, the top ring went out the port and got stuck in the stinger.
Click to expand...

is this the rs125 in question?
 
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