Breaking in a 2 stroke

Run your premix as heavy as you can and ad some marvel mystery oil in with mix (before fouling plugs). Can due it on either gas or alky (if alky run engine). Start it up and let it get some temperature in it, set idle so it will run itself at about 2000 rpm plus or minus. Run for 15min and let cool. Keep eye on temp while running and take a plug reading if necessary after first run. After cool fire it up and do it again; if temp is stable or has dropped slightly from rings seating; you can use the brake and engage clutch to put some load on the crank. Everyone has a special way of breaking engines in but this has been a common for most guys.
 
I've done it all different ways....if its built right...run it in practice, harder in heat race, beat it to death come feature time!
 
Before I built my dyno, I pretty much followed what has already been posted. But I changed things after I built the dam.

I discovered that if I just let the engine run a rich high idle, after a period of time, the engine would suddenly jump up in RPM. After that I would make three full throttle pulls. Never stuck one. The jump in RPM I assumed was when the rings seated. I could think of no other reason why the rpm would suddenly jump. Later on, at a race, I saw a guy doing the exact same thing on the kart stand. Talking with him later, he and I agreed that the rings seating cause the increase in RPM.

If you have any doubts about this method and choose to do it the other ways suggested, do a compression check. Get a good gauge, about $100, and compare. Pep Boys does not have a good compression gauge!
A good Yamaha pumps about 160 PSI, a read or Rotary, about 210 PSI.

Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley)
 
With our nicasil bore H2O motors we often change piston/rings at the track, re-fire the engine to check for leaks and put some heat in the rad, then hit the track wide open.
 
I agree with Pete. If the fit up is done correctly there shouldn't be much run-in time needed.

I think the running-in process has been dragged on from the days when materials and machining techniques weren't as good as they are now and may have required a final lapping of the parts to get the proper fit.
 
Sooo true!...my engine builder in the early days always told me to run it exactly how you intend to race it on the track. Agreed...correctly built will NOT come apart.
 
Theshadow05 said:
Sooo true!...my engine builder in the early days always told me to run it exactly how you intend to race it on the track. Agreed...correctly built will NOT come apart.
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I never thought the running-in thing was about the engine coming apart. I think it was mainly to do with seating the ring seal and getting the piston to find its own clearance. Perhaps raw materials weren't as stable with expansion rates as what they used to be. That's just a suggestion, not necessarily a fact. Just a hunch. :)
 
Yes mainly for seating in rings, but there are the other variables such as bad main bearings, rotary valve keepers, squish etc.
 
I discovered that if I just let the engine run a rich high idle, after a period of time, the engine would suddenly jump up in RPM.
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Use ash less oil like Briggs 2-stroke from a small engine repair shop. Do what Al says. Then hammer down.

By the way; I just replaced the top end in my KTM 200 and did this exact procedure. RPMs jumped just like on a kart engine.

DK
 
Don K said:
Use ash less oil like Briggs 2-stroke from a small engine repair shop. Do what Al says. Then hammer down.

By the way; I just replaced the top end in my KTM 200 and did this exact procedure. RPMs jumped just like on a kart engine.

DK
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Interesting stuff, about how long did it take for the rpm to jump?
 
arc100 said:
Interesting stuff, about how long did it take for the rpm to jump?
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on my dyno, with just a little load, about 30 minutes, plus or minus 15 minutes. That was with two cycles. And I always broke them in with the oil they were going to race with.

Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley)
 
I had some "Hemingway's Break-In Oil" for this purpose, letting the engine "idle" on stand until jump in RPMs. I suspect it was low-grade weedwacker oil that was a bit more abrasive than the synthetics... I use castor and use same premix now...
 
Get it warmed up either on a stand or the track, then drive it hard airing on the rich side. One session should be enough
 
I feal that an engine built right (so that it doesn't need break-in), isn't built right.
2-cycle GP bikes had really tight piston to cylinder clearances, because they made more power that way, like in the .0015" range.
But they absolutely needed break-in.
And many times, after intial running, the jugs were pulled, and minor high spots on the piston were addressed, and the ring was inspected for 100% contact around the circumfrance.
If that all looked good, they were ready for battle.

One can substatute effort by running a really rough hone, but then the bore size goes away quicker.
 
But then, that also depends on the quality of the materials you're dealing with (ie: Forged pistons have an entirely different expansion rate than do cast ones.) Plus, one can buy a dozen rings before finding the one(s) that won't show light all the way around when in the cylinder. Parts selection can be every bit as important as what you do WITH the parts you choose.
 
GoFast,

I've run KT100's with .0007" clearance with no break-in.

I can absolutely guarantee that if an engine is built correctly, it requires no break-in. I did it on many engines for many years.

It's simply an understanding of how a particular bore deforms, and how the piston will act (and the shape of the piston).

PM
 
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