Rebuilding a 2 stroke? | Page 3 | GTAMotorcycle.com

Rebuilding a 2 stroke?

Here's the piston.
FSFyMNo.jpg

FUsLVVq.jpg


And another of the cylinder.
rMfuWUd.jpg


Also need to order a new power valve as I took a chunk out of that, too.
 
Last edited:
im afraid to ask...what does the powervalve cost?

your pics clarity is excellent..any of the pv?

It's about $150 USD from AF1 racing. Haven't checked my Canadian sources yet (thanks for the reminder!). I didn't get a pic and bitzz told me he smoothed it out before reinstall so it's probably hard to see what the damage was but I'll take a pic when I swap it for the new one. I think he also said it was showing wear, anyway.
 
$160 CAD, in Canada. Cheaper than AF1 if you consider exchange.
 
9000 RPM plug chop:
fkvMVee.jpg


Plug after riding it to and from work a few times:
mSoBZ17.jpg
 
Compression test for a baseline:
DwG0fVA.jpg


Seems low but what do I know.

In case anyone's interested still.
 
90 PSI is waaaay low for a 12.5:1 engine. I assume you're testing with the throttle wide open?
 
Did it again with WOT and still got 90 PSI. The tester is a pretty old one I borrowed from a coworker, though.

350 km exactly from when it was dropped off to when I filled up today about 6 km away.
 
Yeah, it's probably the tester that's wrong. Bike would run like **** and possibly not start @ 90 PSI. (Unless there's another 2-stroke idiosyncrasy I'm not familiar with.)
 
As Matt points out 90 is way too low.

When I rebuilt my son’s RS the first time I got a B oversize (0.01mm) piston had Vance at Precision Cycle hone it to fit. Pistons for plated cylinders come in very small increments so when the plating wears you can go to the next size up with a hone.

I have a note in my manual here that when it was rebuilt the static compression was around 146 PSI. The second time I rebuilt it I had the cylinder re-plated by Millennium and used a Wossner kit. I also had Precision Cycle cut the squish and dome properly. After that the static comp was around 170 psi according to the note I have here.

According to the manual the piston to cylinder clearance should be between 0.02mm->0.035mm clearance so I would check that.

Is it an OEM base gasket? If not is it too thick?

I have also noticed that on the aftermarket Athena barrels the machining is not great. When I did my RZ421 the first squish measurement was 1.9mm. I can’t comment on the Mitaka barrel as I’ve never used them and it does look like a nice piece of kit, but what was the squish after you rebuilt it?

Are you still using the OEM head or aftermarket?
 
The Mitaka pistons come in a range of sizes with very minor increments of 0.01mm and have to be selected properly to suit the bore as the plating and cylinder diameter varies.

https://www.pjme.co.uk/acatalog/Aprilia_RS125_Engine_Top_End_.html

Was this done? I've seen this on MX bikes where even though the barrel and piston are brand new, the clearance is way out of spec as the piston is too small.

IMHO, and you will get a lot of opinions about this, I prefer to buy the smallest size piston and send it with the old barrel to a good plating shop. That way they can build up the plating and hone it exactly to fit.

Your original barrel didnt look too bad, you want to see some of the MX cylinders I've had refinished. I would definitly keep it as a spare and you can get it plated if anything happens to this one
 
The piston and cylinder came together as a kit.

Where would I get the cylinder replated? I was planning on having that done, assuming it doesn't cost as much as just getting a new cylinder.
 
The piston and cylinder came together as a kit.

Where would I get the cylinder replated? I was planning on having that done, assuming it doesn't cost as much as just getting a new cylinder.

CV Tech in Quebec

If you're going that route I suggest you send them the trashed cylinder to fix and replate... that way you have a spare cylinder. I think about $500 with a new piston and top end gasket kit.

Is the bike running hot? If you were getting "blow by" the motor would run hot.
 
CV Tech in Quebec

If you're going that route I suggest you send them the trashed cylinder to fix and replate... that way you have a spare cylinder. I think about $500 with a new piston and top end gasket kit.

Is the bike running hot? If you were getting "blow by" the motor would run hot.
That's about the same price as the kit from AF1.

It doesn't seem to be running hot. Normal 3 bars on the temperature display. Did go up to 4 a couple times this week when stuck in traffic but that's not unusual. I'll see if I can find another compression tester. Anyone have one I can borrow?
 
Hmmmmmmmmmm

1) 12.5 x atmospheric pressure (about 14PSI) is 175 PSI... so to get 175 PSI on a compression test you would need 100% efficiency.
Yaa want tell me how to get 100% efficiency? at the couple of hundred RPM the starter gives us? Two stroke, four stroke or a system designed by you... it ain't gonna happen.

2) That 12.5 compression ratio (I actually lowered it to about 11.7 or so) is the swept volume of the cylinder divided by the volume of the head. Now; look at the picture of the cylinder that unL33t posted. Half of the swept volume HAS AN OPEN EXHAUST PORT... you know where the compressed gases escape from.... yeah yeah THAT OPEN PORT.
Think about that for a moment.

Sticking a pressure gauge in the sparkplug hole of a two stroke DOES NOT tell us what the compression of the motor while running. There are just too many variables
 
How does one test the compression of a 2-stroke, then?

These engines sound like a major PITA.
 
I asked AF1, since I got the kit from them. Told them I put the tester in the hole, WOT, hit the starter. Here's the response:

If you are sure you have a good seal at the spark plug hole and a good compression tester. The 90psi figure could be correct.

If you’ve been running bad jetting specs for 375 km, that’s more than enough time to have severe loss of compression.

Brand new engines should show 115psi minimum, average is closer to 125psi.

I'm going to try a different tester, if I can find one.
 

Back
Top Bottom