Rebuilding a 2 stroke? | Page 4 | GTAMotorcycle.com

Rebuilding a 2 stroke?

Part Source lends them out for free so I'll get one sometime soon and redo it.
 
unL33T said:
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.
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I've been using Millennium Technologies and IHMO i think they are by far the best in North America - http://www.mt-llc.com/

I just had a YZF250 cylinder done and it was a lot, lot, lot worse than yours :) With the exchange rate your looking at somewhere between $2-300 but your cylinder is smaller with minimal damage so it will be on the low end of that. I believe they have a Canadian Shipping address so you don't have to deal with cross border fees and customs. Its definitely a good cylinder so I wouldn't junk it.

You can get the Mitaka or Pro-X piston (made by the same manufacturer) and gasket kit for about 50 pounds ($100) in the UK and since you don't have to pay the VAT that pretty much pays for the shipping. I believe the Wossner is around $150.
 
unL33T said:
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.
Click to expand...

I think it depends if the kit was packaged up on a Monday morning or Friday afternoon :)

Seriously, I bought a 290 Athena Big Bore kit for an RMZ250 once and when i measured the piston it was an A when it should have been a C. I had to get them to exchange it, but I would imagine that is a really rare occurrence.
 
bitzz said:
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
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Yes, its actually 14.7 x 12.5 (+/- 0.5) which works out best case to be ~190 PSI. I don't have a port map of the cylinder but I'll have a look on line and see if I can find one. I'm assuming the port hight is around 20mm so ~130 or 140 should be achievable with my back of the napkin estimate.

It depends on the gauge as well. If its from Princess Auto then all bets are off :)

Let me see if I can find a port map and I'll calculate what it should be mathematically, I have the Engmod2t software that lets you derive all of this. Real world experience may vary though :)
 
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bitzz said:
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
Click to expand...

Well there are a number of considerations.

Compressing a swept volume of 14.7 PSI into a combustion chamber that is X cc's is going to produce a different reading than compressing the same swept volume of 14.7 PSI into a combustion chamber of 2x cc's. P=F/A.

In layman's terms, think of it as pumping exactly the same amount of air into a tractor tire and a mountain bike tire. Which will have the higher pressure when you put the gauge on?

My sons bike was a trackbike. Took the squish down from 1.9mm to 0.7 mm and had the squish band cut properly. The head volume was taken down significantly. I cant remember exactly how much but I'm tempted to say 8cc's compressed?

I had one of Kennys pipes on it and replaced the 28MM Dellorto with a 33mm Keihin PWK and a programmable Zeel.

Nice bike, I wish I still had it
 
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To the OP. I got this from Stan Stephens some time ago as I had a RS for getting to work when i lived in the UK. It may be useful

- 80 psi - should be struggling to start / not long left to failure
- 100 psi - very low change rings / barrel
- 120 psi - rings wearing
- 140 psi - cylinder and rings in as new condition for stock machines
- 170 psi - cylinder and rings in as good condition for tuned machines

The tuned figure is obviously for one of his tunes

^^ Leakdown test is always a good idea.

It sounds like possibly a guage problem as @ 90PSI its going to run like *****.

If you are worried, go and see Willy at WASP in Ajax. It will take him a couple of minuites to check out.
 
I'm not too worried about it.

Jetting and oil pump setting hasn't been changed and it survived 7000 km of abuse (like going to Ottawa and back from Toronto at 140-160 the whole way) like that and it's probably got too much oil, if anything, since it's coming out the tail pipe.

Started up instantly yesterday morning and today. Only took a bit more effort on Monday because it had sat for a week. Seems to run great. I've not had it out on the highway yet since I got it back (bitzz suggested that wouldn't be great for it) but I easily got to over 110 passing someone the other day so if it's down on power it's not very noticeable.
 
unL33T said:
I'm not too worried about it.

Jetting and oil pump setting hasn't been changed and it survived 7000 km of abuse (like going to Ottawa and back from Toronto at 140-160 the whole way) like that and it's probably got too much oil, if anything, since it's coming out the tail pipe.

Started up instantly yesterday morning and today. Only took a bit more effort on Monday because it had sat for a week. Seems to run great. I've not had it out on the highway yet since I got it back (bitzz suggested that wouldn't be great for it) but I easily got to over 110 passing someone the other day so if it's down on power it's not very noticeable.
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Nice bike. I wish I could find another. It would be fun for a Sunday morning
 
I rebuild 2 stroke motors for stihl / husqvarna saws and others, may i make a suggestion?
Buy the top end kit complete, piston and all, and slap it on.
I'm a machinist by trade and i would rather have a nice new matched piston and cylinder set than try to mate old and new parts.
Unless that cylinder is in really good shape of course, most need some machining.
Keep the parts that are good and fix up the old top end set as a spare during the winter, or sell when rebuilt.
 
AGAVE said:
I rebuild 2 stroke motors for stihl / husqvarna saws and others, may i make a suggestion?
Buy the top end kit complete, piston and all, and slap it on.
I'm a machinist by trade and i would rather have a nice new matched piston and cylinder set than try to mate old and new parts.
Unless that cylinder is in really good shape of course, most need some machining.
Keep the parts that are good and fix up the old top end set as a spare during the winter, or sell when rebuilt.
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That's what I did. If I got the old cylinder replated I'd get a matching piston with it.
GreyGhost said:
I have a leakdown tester you can use if you want. I'm in north GTA
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How does one do a leak down test? Last time I looked into it I was told I had to custom build block off plates for the intake and exhaust, disconnect those items, and do the test. Too much effort for me.

I'm only checking compression to get a baseline so I can compare similar to the post by Ulsterman. I don't think there's anything wrong with it.
 
Leak down test is dandy and all, but a simple compression test says enough.
I won't fix a lawnmower unless it makes 125psi, same for most small engines.
Yours should be higher than that or it's too worn. rings are taking alot of the stress.

I have something on the bench right now, let me make a little video since i have some time today. brb
 
I've been doing my tests "cold" (not completely cold but not at operating temp) to reduce the chance of me burning myself. Just found out I should be doing them hot. Would that make a significant difference in my numbers?
 
Delboy said:
Well there are a number of considerations.

Compressing a swept volume of 14.7 PSI into a combustion chamber that is X cc's is going to produce a different reading than compressing the same swept volume of 14.7 PSI into a combustion chamber of 2x cc's. P=F/A.

In layman's terms, think of it as pumping exactly the same amount of air into a tractor tire and a mountain bike tire. Which will have the higher pressure when you put the gauge on?

My sons bike was a trackbike. Took the squish down from 1.9mm to 0.7 mm and had the squish band cut properly. The head volume was taken down significantly. I cant remember exactly how much but I'm tempted to say 8cc's compressed?

I had one of Kennys pipes on it and replaced the 28MM Dellorto with a 33mm Keihin PWK and a programmable Zeel.

Nice bike, I wish I still had it
Click to expand...

wow, 0.7mm squish may be asking for trouble, but it was a racebike so....i wouldn't go tighter than 0.9-1.0 mm for a street bike. check your squish 33, and your piston to cyl clearance. was the piston to cyl clearance checked during rebuild?
 
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Ulsterman said:
Very nice but its hard to justify spending 6k. You see really nice examples in the UK for between 1000-1500 pounds.
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That's because they aren't rare there. I get what you're saying, though. Unfortunately here they are rather expensive. There is a dealer in Ottawa asking $9k plus $1500 for street kit. Mine was $4500 with 5000 km on it. Almost bought one before that for $6500 with 4000 km on it. So under $6k for one with only 18 km (that's not 18000 km) on it is pretty good, in comparison.
 
boyoboy said:
wow, 0.7mm squish may be asking for trouble, but it was a racebike so....i wouldn't go tighter than 0.9-1.0 mm for a street bike. check your squish 33!
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Yes. it was a track bike. I have an RZ for VRRA/AHRMA and I run it around .8 but would agree on .9 to 1.0 for the street.

I read once that the factory RS125 Honda's ran about .55 to .6 and the piston was just kissing the head. I guess when you have a new motor every weekend you can do things like this :)
 
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