2 stroke engines

[goinbig]

Ok, probably a dumb question, but here goes.

I was mainly just wondering why it seems that 2 stroke engines need to be rebuilt so often? Is it something caused by the rider (like over-revving the engine, etc)??

And going along with that, how often do they normally need to be rebuilt? And what is the average cost (a rough guess-timate) usually for all the parts neccessary?

[BNaylor]

Originally Posted by goinbig

Ok, probably a dumb question, but here goes.

I was mainly just wondering why it seems that 2 stroke engines need to be rebuilt so often? Is it something caused by the rider (like over-revving the engine, etc)??

And going along with that, how often do they normally need to be rebuilt? And what is the average cost (a rough guess-timate) usually for all the parts neccessary?

Are you referring to racing Motocross 2 strokes or regular enduro off-road? It depends on the total hours on the engine and whether its raced or trail ridden. I've got a '03 Honda CR250R and didn't re-do the top end until it was 2 years old. Just put in a Wiseco Pro-Lite piston and rings ($120.00) and had it bored and cylinder nickosil re-plated ($250.00). I still have the original clutch and transmission.

The 85cc and 125cc 2 strokes are higher revving than a 250cc and may require rebuild sooner. Honda recommends re-doing the top end on a two-stroke that is habitually raced every 12 hours.

In contrast, I had a late 70's model Yahama RT enduro and rode it for 3 years and it never needed the top end or anything else rebuilt. The compression tested as good as the day I bought it.

[goinbig]

Well I asked because I recently bought a '97 YZ 250...which is my first 2-stroke bike (used to always have 4-strokes). The engine was rebuilt a couple years ago, and has only been riden a few time since then.

I almost always ride at the coast (sand dunes) with this bike and was just wondering what the average engine life would be for that type of conditions. I also like to ride real hard, but have been kind of worried.

So is it generally because the engines are run at such high rpm's, or is it lower rpm's that are hard on them?


I was also a little nervious because when I would re-build 4-stroke engines, it would almost always be over $500 in parts....and that's getting them at a discount! (but I guess there are a LOT less things to break in the 2-stroke engines).

[MagicRat]

2 strokes need more frequent rebuilding mostly because their oiling system is not very good.
The cyl and piston is only lubricated by an oily fog in the fuel. Often the oil mixture in the fuel is very lean, which only makes things worse.

But 2 stroke rebuilds are easy and cheap.
On the older air cooled bikes, I could remove and replace the cyl, rings and piston in 10 minutes easy.

[goinbig]

Oh wow, that definatly changes my opinion on things then!! Sure can't complain about a 10 minute (or so) job!

So another question is what's a good oil/fuel ration then? I know it depends a lot on the bike, but I'm just wondering. I currently run 32:1...which is what the last owner says the bike likes the best.

[BNaylor]

A premix ratio of 32:1 is standard for 250cc class motocross machines like your Yamaha YZ250. You can go as lean as 40:1. I run 32:1. I use Honda Synthetic HP2 premix because of the power valve. Yamalube is great stuff too.

As to whether it will take 10 minutes to redo the top end thats debateable. First on your YZ250 you have liquid cooling which will add to the total time to get it back on the air. Plus you'll have to remove plastic such as the rad shrouds and possibly the fuel tank, etc. There is also the power valve to consider and adjust when a cylinder is removed not to mention the reed valve. Plus mid nineties and up motocross machines have a nickasil plating in the cylinder bore stock. Unless you are in a major metropolitan area that has a specialty shop that specialize in this, normally you have to send the cylinder out to have it re-plated or bored like to companies like US Chrome, Pro Circuit, Kustom Kraft, etc. So there will be waiting time for parts to consider.

Another thing you can do with a 2 stroke just like a 4 stroke is run a compression test if you suspect cylinder, piston or ring wear.

[72_supernova]

Just a huge tip to anyone that is relitivily new to riding, if your going to be riding in sand with a 125 or 250 i would highly recomend using a padle tire. even if you have tires that are supposed to work good in sand cause' belive me if you plan on going up any big dunes and you don't wanna blow your top end then get a paddle tire.

[BNaylor]

Originally Posted by 72_supernova

Just a huge tip to anyone that is relitivily new to riding, if your going to be riding in sand with a 125 or 250 i would highly recomend using a padle tire. even if you have tires that are supposed to work good in sand cause' belive me if you plan on going up any big dunes and you don't wanna blow your top end then get a paddle tire.

I've been riding a modded '03 Honda CR250 on sand dunes for over three years and run Dunlop D756 medium-soft terrain tires. Thats includes some as high as 30 feet plus. I've never blown my top end. A 125cc class machine is a different story because you have to rev high to get up them and I'd agree a paddle tire would be beneficial.

[MagicRat]

Originally Posted by bnaylor3400

As to whether it will take 10 minutes to redo the top end thats debateable. .

Otherwise, a good post. So lets debate,

I said "older air cooled bikes", 10 minutes is all it needs. This includes such mouldie oldies as a Yamaha IT 175, Can Am 400, a bouquet of Suzukis and Hondas, and even a Kawa KE100.

All about 10 minutes. No kidding.

As for the liquid colled machines, sure its a lot more time work and money. This is why when it comes to 2 strokes, I own air cooled only.

[BNaylor]

Originally Posted by MagicRat

2 strokes need more frequent rebuilding mostly because their oiling system is not very good.
The cyl and piston is only lubricated by an oily fog in the fuel. Often the oil mixture in the fuel is very lean, which only makes things worse.

But 2 stroke rebuilds are easy and cheap.
On the older air cooled bikes, I could remove and replace the cyl, rings and piston in 10 minutes easy.

Originally Posted by MagicRat

Otherwise, a good post. So lets debate,

I said "older air cooled bikes", 10 minutes is all it needs. This includes such mouldie oldies as a Yamaha IT 175, Can Am 400, a bouquet of Suzukis and Hondas, and even a Kawa KE100.

All about 10 minutes. No kidding.

As for the liquid colled machines, sure its a lot more time work and money. This is why when it comes to 2 strokes, I own air cooled only.

There's nothing to debate. Its 2005 not 1975. Going back to 1969 I've owned a Yamaha 80, Kawasaki 90, Hodaka Super Rat 100, Yamaha DT1 250, Yamaha RT1 360 and Kawasaki 500 Mach III (all air cooled). I agree as long as you have all parts on hand a top end can be rebuilt in 10 - 15 minutes. I've lost count how many I've done over the past 35 years.

But I totally disagree with the leading part of your statement above which implied two strokes in general. They've come a long ways since the good ole days and air cooling. There is no comparison due to technology.