Why Air Cooled Two Strokes are The Perfect Motor

The BMW bearing thread got me thinking about this.

So for years I've joked with my friend that an air cooled two stroke is perfect:

1. There is no need to do an oil change ever. 

2. There is no need to worry about oil pressure.

3. It eliminates all those cooling sytem woes; thermostat, radiator, water pump, freeze plugs, hoses and all that other clutter.

4. You can rebuild them in an hour and don't even have to pull the motor out.

5. You know when it's time to rebuild them because they no longer pull maximum RPM.

6. If you blow it up (you jetted it wrong) the parts are absurdly cheap and see number 4. 

7. You can rejet the carb in the time it takes most folks to plug a laptop into a port and type on the keyboard.

I love the concept, but they tend to be limited on size and power output.  Even with air-cooled Porches and VWs, the problem was that as hp increases, the BTUs created from combustion and friction overcome the cooling potential, and they often had oil coolers to make up the difference.  Two strokes wouldn't have that benefit.

In reply to Curtis73 (Forum Supporter) :

First I am not truly serious but I do love them.

In a light car, 1200lbs, an air cooled triple would be really cool.

 I grew up with two stroke singles and twin and just love them.

I've read too many stories of stuck pistons to believe in cheapness.  And, while not air cooled, I found myself working on a Mercury Marine two stroke V6, marveling at how tiny it was for an apparent 200hp (so I was told), and started to plot and plan right up to the point where I found out how much anything at all for those little things costs. 

SAAB had an overrunning clutch on their 2 stroke cars so the engine could not be driven by the car.  Closed throttle means no air means no fuel means no oil and no oil at high speeds means engine death.  I suppose they could have gone with an oil injection scheme, which would also eliminate the possibility of getting your mix ratio wrong, but that also flies in the face of the idea of minimal parts.

One thing that I do kinda like about 2 strokes is that blowby gets a second chance to get burned, I think.  Unless it blows out the exhaust.  I'd almost prefer to have a blower scavenged engine with exhaust valves, like a Big Diesel.

Used to rebuild the heads for a local company that had a fleet of tugs.  The valves must have weighed in the range of pounds.  Cylinders had individual heads, each head had four gigantic valves and probably weighed 100lb each, bare.

What is the rebuild interval on an air cooled 2stroke?

buzzboy said:

What is the rebuild interval on an air cooled 2stroke?

Varies wildly depending on use. 

Motocross bikes went about 18hrs before reringing them. My 125 road racer (Elsinore 125 based) went about 10hrs before needing rings but it was turning 13,000 rpm.

Some of the road bikes would do 40-50K.

In reply to Pete. (l33t FS) :

Stuck pistons are almost exclusively down to jetting. There was a saying about "lean is mean". People would go down one more jet size looking for that last 2-3hp and then seize the engine shortly thereafter.

But, they stink and sound terrible. 

In reply to Pete. (l33t FS) :

"SAAB had an overrunning clutch on their 2 stroke cars so the engine could not be driven by the car.  Closed throttle means no air means no fuel means no oil and no oil at high speeds means engine death."

To your point, if I have to ride a long downhill on my 125 2 stroke, I pull the clutch in and blip the throttle occasionally on the way down, to make sure the engine gets fuel and therefore oil. My neighbor told me a story once about riding his Kawi triple 2 stroke to the beach one year, he spent hours on the interstate drafting a semi truck, so the rpm he was faster than what it normally would be for his throttle opening. By the time he got to the beach, the bike didn't make enough compression to start. He ended up pushing it into his hotel room, lining the floor with newspapers and throwing a set of rings in it. Said it was as good as new and drove it home the following week.

ShawnG said:

But, they stink and sound terrible. 

You beat me to it. They sound awesome and the beautiful smell of two smoke is a bonus.

I was using my two stroke air cooled generator a few days ago and trying to figure how many hours I've put on it. I couldn't, but it's a lot.

I have air and water cooled 2 strokes (dirt bikes). I would 100% say water cooling is a worthwhile complexity addition. Not sure why, but 2 strokes in particular seem to be very sensitive to operating temperatures. Maybe because of incomplete oil combustion, because of crankcase heating the charge, etc, not really sure. My water cooled bikes run more consistently, make more power, last dramatically longer between top ends, and (knock on wood) I've not actually had any cooling system failures. 

I like the way the 4 strokes run better, but I keep the 2 strokes around for all the reasons you mention. The only reason I still have an air cooled one is because I like vintage stuff. If I could easily add a water jacket, I would...

In reply to gearheadE30 :

My 76 YZ125 that I ran from 1992 until 2018 would run like crap for the first 3-5 minutes of riding. After that it was great.

In contrast my 01 YZ125 ran great with in 45 seconds of start up. 

I know have a Beta 520 four stroke; It makes gobs of power but it's heavy.

I love the simplicity of 2 strokes.

In reply to dannyp84 :

I never  worried about blipping the throttle on downhills on either one of my YZ 125s. Premix is still going through the engine.........I also don't rely on compression braking on them as they have almost none............I use the actual brakes if I need to slow the bike.

I have seen some very long-lived 2-strokes.  The Merc 70 on one of our boats was bought new in 1988.  Since then it has been used about 3 times a day, every day between May and September.  Salt, fresh, brackish, you name it.  Trolling at idle, WOT, pulling skiers, that thing has been through the ringer.  I wish we had put an hour meter on it.  It just won't die.  I have a small pile of 9.9s, 30s, an 18, and even a 10-horse Evinrude from 1961.  The evinrude needed a new flywheel because the pull start had worn into the aluminum keyway badly enough that it wouldn't engage.  I can't even begin to think of how many hours are on it.

In reply to Tom1200 :

This is totally getting off in the weeds, but I like bikes so I'm going for it anyway 

My experience with the air cooled bikes is if I go for a B rider pace, they work great, fairly consistent, no real issues. If I go for an A rider pace, they are good for 30 minutes and the last hour or hour and a half of the race I'm riding around detonation, rich bog, lean bog, or some combination of the three somewhere in the rev range. Usually plus some clutch fade. Modern clutches are so good, it's easy to forget where we came from. I'm an enduro/harescramble guy though, I bet with the airflow of MX and 20-30 minute motos they would be much better off.

the only air cooled bike I have left is a 1985 TY350 trials bike. It's great if you use it as a trials bike and it gets to cool off some. If you ride a loop of an event without stopping, like you might if you had a problem and are now running up against the time cutoff, it runs like absolute garbage by the end due to heat. And that's with the cylinder head/squish fixed, race gas, and a modern carb on it, on a fresh engine. 

I do love my 300XC though. Just over 200 hours on it, 3 top  ends but original bottom, I will probably race one more season on it and then be thinking really hard about if I want another 300 or a 350F. 

I almost always pull the clutch in if it's a long decel or rolling situation on the 300. I race with another guy roughly the same speed who lets it roll in gear. We get basically the same life out of a top end, both on 2018 KTM 300XCs. After learning that, I've stopped being so careful about it.

In reply to gearheadE30 :

I could see woods riding being an issue; revving the snot out of it in 1st through 3rd gear with not a lot of airflow could make for an unhappy mix...............then throw a bunch of mud in the fins. 

We have the opposite problem here in the desert. I did an hour GP on my 76 YZ the course had a mile long section on a gravel road but the rest of the course was deep sand.  On the straight section I would hit the kill switch every 100yds or so to gas cool the motor.

As long as you are moving or the motor has some type of fan (like snowmobiles of old) the motors are fine.