two or four strokes?

[newagetwotone]

I remember being a kid and riding our CL100 four stroke and the neighbors two stroke dirt bikes and even though they weren't much bigger they felt like their had a lot more umph. I recently ran across someone riding a two stroke yamaha street bike, a RD350 i believe.

Its been a long time what are the ups and downs to two and four strokes? I was told that two strokes are more powerful but why then don't they still make twos or for that matter bigger two strokes? No 750 four cylinder two strokes?

Just trying to understand

[The_AirHawk]

By the time time emissions-technology caught-up with 2-strokes, they were behind the "desirability curve" and on their way out. Many states in the North and Northeast are contemplating partial bans, or have banned two-stroke motors for recreational and power equipment altogether (due to poor emissions).

I think a lot of it has more to do with reputation and perception (so far as Motos go), rather than any real disadvantage two-strokes have vs. four-stroke.

[Gluge]

If you compared two engines of the same size and cylinders -

Two-stroke Positives:
1. Almost 2x as much peak power
2. Lighter
3. Cheaper
4. Very little engine braking
5. Smoother shifting
6. Easier to kick or push start
7. Quieter (generally)
8. Cheaper maintenance (though more frequent it still comes out cheaper)
9. Don't have to change the oil

Four-stroke Positives:
1. Better emissions
2. Better mpg
3. Generally rev a tad higher
4. less frequent maintenance
5. More linear power band (more mid - low end)
6. Doesn't burn oil like the 2-stroke
7. Don't have to mix oil or keep the oil tank full

I think that's pretty much everything. The main reason there are no more 2-strokes is they have a very hard time meeting EPA particulate emissions, especially at idle. They actually put out less CO2 than 4-strokes I believe but more particulates and other things.

I own 3 2-strokes and 1 4-stroke. (honda RS125, aprilia RS250, SAAB 96S, 300ZXTT) So I prefer them for the simplicity of working on them though they require work more often I find it's cheaper than a 4-stroke.

My race bike needs a new piston every 300 miles and a new crank every 1200. My street 2-stroke bike needs new pistons every 5000 miles and a new crank every 15000. My 2-stroke car well... I'm not sure really I just get it rebuilt when it seizes which is about every 5 years it seems.

[pplassm]

Wow, I think you could almost write a book on this topic. Sort of like the demise of the British motorcycle industry.

Everything already said in this thread has credence.

The current state of the art in the motorcycle world has yielded strong, lightweight, high-performing four strokes (dirt and street)that customers buy in droves. Development of a new two-stroke would have to combat that momentum, not to mention solving the technical problems of emission and fuel economy, as well as the perception problems left by previous incarnations. The perfomance advantage just wouldn't be enough to justify it.

Current COMPETITION four-strokes are extremely powerful, but it comes with a price. Valve adjustment interval on my KTM 525 is 25 hours, with a complete rebuild, including crank and valve replacement at 250. At 35 MPH average speed, that's less than 9000 miles.

Non competition bikes, OTOH, can be very reliable. My DRZ400S makes less than optimum power, and it is heavier than it needs to be, but valve adjustments are at 20,000 miles, and some bikes have lasted over 100,000 miles without rebuilding.

As far as 4 cylinder 2 strokes, Yamaha unveiled a prototype 2 stroke 4 cylinder 750 in 1972, I believe. They shelved that project and released the ill-fated TX750 twin instead.

There is also a builder in the UK who makes 5 and 6 cylinder bikes out of old Kawasaki H series 2 strokes. His work is awesome. His latest project is a V-12 based on old KZ1300's:

Home built Kawasaki 2300cc V-12 motorcycle - Feature Review - Motorcyclist Online

also, video here:

Movies

[Buzglyd]

Originally Posted by pplassm

Wow, I think you could almost write a book on this topic. Sort of like the demise of the British motorcycle industry.

Everything already said in this thread has credence.

The current state of the art in the motorcycle world has yielded strong, lightweight, high-performing four strokes (dirt and street)that customers buy in droves. Development of a new two-stroke would have to combat that momentum, not to mention solving the technical problems of emission and fuel economy, as well as the perception problems left by previous incarnations. The perfomance advantage just wouldn't be enough to justify it.

Current COMPETITION four-strokes are extremely powerful, but it comes with a price. Valve adjustment interval on my KTM 525 is 25 hours, with a complete rebuild, including crank and valve replacement at 250. At 35 MPH average speed, that's less than 9000 miles.

Non competition bikes, OTOH, can be very reliable. My DRZ400S makes less than optimum power, and it is heavier than it needs to be, but valve adjustments are at 20,000 miles, and some bikes have lasted over 100,000 miles without rebuilding.

As far as 4 cylinder 2 strokes, Yamaha unveiled a prototype 2 stroke 4 cylinder 750 in 1972, I believe. They shelved that project and released the ill-fated TX750 twin instead.

There is also a builder in the UK who makes 5 and 6 cylinder bikes out of old Kawasaki H series 2 strokes. His work is awesome. His latest project is a V-12 based on old KZ1300's:

Home built Kawasaki 2300cc V-12 motorcycle - Feature Review - Motorcyclist Online

also, video here:

Movies

Didn't Bimota crash and burn a while back trying to develop the V Due?

[pplassm]

Ooops, that Yamaha was built in 1971. Here's a photo:

1970's wotzit... - Classic and Antique Bikes - CYCLE WORLD FORUMS

[pplassm]

I believe that was the "nail in the coffin" for Bimota. The fuel injection system they developed for the V Due allowed them to pass Euro emissions standards, but it ruined the bike. They were subsequently retrofitted with carburetors and sold at a loss.

Newagetwotone: The V Due was a 500 V-twin 2-stroke that Bimota built as a 500 GP replica for the street.

Hmmmm. This one says it's still fuel-injected:

Moomba Cyclesports...bikes for sale

[Gluge]

Originally Posted by pplassm

Current COMPETITION four-strokes are extremely powerful, but it comes with a price. Valve adjustment interval on my KTM 525 is 25 hours, with a complete rebuild, including crank and valve replacement at 250. At 35 MPH average speed, that's less than 9000 miles.

I wouldn't say competition 4-strokes are extremely powerful not when talking about how they compare to 2-strokes. As a competition 2-stroke will be much more powerful.

Your KTM 525 makes about 45 hpr right? That's about as much as a 125 2-stroke race bike. A 250 makes about 90 hpr. and can run with a 600cc sport bike.

[Gluge]

Originally Posted by pplassm

There is also a builder in the UK who makes 5 and 6 cylinder bikes out of old Kawasaki H series 2 strokes. His work is awesome. His latest project is a V-12 based on old KZ1300's:

Home built Kawasaki 2300cc V-12 motorcycle - Feature Review - Motorcyclist Online

Only he didn't fit it with expansion chambers. You need one expansion chamber per cylinder which is why you don't see many 2-stroke engines with more than 4 four cylinders it gets hard to find room for more expansion chambers and with out them the 2-stroke won't make good power. In fact before the expansion chamber was invented 2-strokes and 4-strokes were on pretty equal ground power vs engine size wise.

[newagetwotone]

So is there anything wrong with buying an old RD350?

eBay Motors: Yamaha (item 110174291581 end time Sep-30-07 18:19:33 PDT)

Other than they sound like a lawn mower?