Is extra premix need for new top end break-in

[yz250flash]

It has been a long time since I have done a top end on a two stroke. I know that they require an extra rich ratio (15 to 1 or so) when breaking in new, do I need that for a top end rebuild break-in also?

 

[Green Hornet]

It has been a long time since I have done a top end on a two stroke. I know that they require an extra rich ratio (15 to 1 or so) when breaking in new, do I need that for a top end rebuild break-in also?

NO

 

[razrbakcrzy]

I have never done it, just fuel up and go easy for the first hour or so then let her rip!

 

[Woodsquest]

If you change your pre-mix from, say, 32:1 down to 15:1, then the same volume of fuel mix contains less gas, it is a thicker viscosity, and less of this mix will be sucked through your jetting. Meaning: You just leaned out your bike. Not a good thing at all.

If you are concerned about the jetting during break-in, then bump the main jet up a couple of sizes while keeping the standard fuel mix. It will supply more oil without getting lean. It will spooge most likely but you are sure to be safe and not cause harm.

There is a theory that the best way to break in a motor is to ride it just as hard as you can without babying it at all. I'm not bold enough to do that. I'd say run the regular fuel / oil mix you always have found to get the job done and stay with that. Stop often to let the motor cool if it's getting hot as well.

 

[Rhodester]

In theory 15:1 could lean seize your engine if, let's say, you were jetted crisp for a premix ratio of 50:1. Adding oil reduces the amount of fuel in the mixture and makes your engine run leaner. You'll be running hotter than normal and could counteract what you're trying to accomplish with the extra lubricant. I understand what you're trying to do, but be prepared to immediately do another top end job unless you are very lucky.

 

[Green Hornet]

YYYZZZ-Like a new remote, just add batteries and ride(the couch :ride: ). Don't go experimenting

 

[yz250flash]

I understand what you are saying and I understand all the physics involved with adding more oil...rich, lean, etc. I was just recalling what it said in my Yamaha manual. Upon break-in of my YZ250 (when new) it said to run a ratio of 15:1 for the break in period.

Maybe they state that because they know that from the factory the bike is jetted rich enough to handle it?

I wasn't planning on experimenting...figured it was better to ask first since memory failed me. I do believe that I added extra oil when I did my YZ125 though. Maybe I just got lucky on it and didn't have any problems. It ran like a scalded dog afterwards.

Anyway, thanks for all the help. Maybe I will have time to break it in this weekend since it looks as though Hurricane Dennis is going to spare Florida this time (at least my area of Florida anyway).

 

[BadgerMan]

I have never changed ratios during break in. I am a 40:1 person (MX2T).

You could drop down to 32:1 from a normal of 40:1 or 50:1 but 15:1 sounds pretty extreme. If you are worried about a lean condition, you could raise the needle a notch.

 

[19Brendan81]

I always run a rebuilt two stroke with more oil than I normally use, I do this on chainsaws, mowers and dirtbikes. It is important to do this to allow the rings to wear in properly with the cylinder wall, and when done properly will give you longer service life between top end service jobs. This is why the manufacturers recommend doing it. Obviously more oil in the premix means less fuel hence the engine running leaner, but even going from 40:1 to 15:1 doesn’t have as dramatic effect as it would seem.

Ideally two strokes are tuned to run 12.5 parts air to each part of premix this is known as the “Stoichiometric ratio”. Straight gasolines stoichiometric ratio is 14.7:1, increasing past this causes lean out. Two strokes run a richer stoichiometric ratio because scavenging losses lean it out a bit, Canadian daves site says ideal is 12.5:1. SO a two stroke running 40:1 premix contains 97.5% fuel and 2.5% oil, or .975 parts fuel to each 12.5 parts air OR its Stoichiometric ratio is 1 part fuel per 12.82 parts air. Chang the fuel:oil ratio to 15:1 and the figures change to 93.4% fuel and 6.6% oil, or .934 parts fuel to each 12.5 parts air OR 1 part fuel to 13.38 parts air. So, 15:1 is half a part leaner in stoichiometric ratio than 40:1…and that’s not enough to cause lean out in your engine.

 

[Woodsquest]

Nice post Brendan. But what about the viscosity of the fuel being changed? I've been told that the extra oil not only leans out the mix but also increases the fuel's viscosity and that can additionally affect the ratio to air. I know I've seen the effect myself when experimenting with it for a quick "rejet" by adding more oil to lean out the bike.
One other thing I've noticed is that the float height makes a difference as well. A lower float level requires more vacume to pull the fuel up and into the air stream. A thicker fuel with the additional oil, along with a low float level, things can change pretty quick. I set my KDX carb floats pretty low to prevent the "pee" effect when leaned over. That's when I first discovered that the float level will affect the jetting.

 

[ellandoh]

:yikes: this guy is busting out the chinese arithmetic..sheesh :rotfl: my head hurts


lol j/k

 

[Green Hornet]

:yikes: this guy is busting out the chinese arithmetic..sheesh :rotfl: my head hurts


lol j/k

You can tell the Australian School System teaches better math. Us YANKS have spinning heads after reading that ;)

 

[cyber104]

Wow!

Now that was a very nice write up and he is completely right! Bottom line: More oil means better lubrication. Doesn't mean a better burn though.

 

[nmilne]

Interesting thing from two points of view ... firstly, when I had my KR-1S barrel replated by electrosil they explicitly recommended not going to a richer mix than normal ... the rings need to bed in, and part of that is the necessary friction with the honing on the cylinder wall to effectively grind the rings and cylinder walls into a perfect fit ... there must be some friction required, as going directly to a fully synth oil in a 4-stroke break-in usually results in glazed bores and poor ring sealing. Less important in a 2-stroke, but better ring sealing in a 4-stroke definately helps long term life as it causes less oil pollution and acid build-up from unwanted blow-by.

However, on the other side of the coin, you're probably generally best off running a premix ratio on the richer side that still gives good carburettion and doesn't cause fouling problems. A lot of 2-stroke road racing guys went back to running more oil (below 30:1) because they found the much better ring sealing properties outweighed the petrol displaced by the oil. Now on a road race bike you can (and probably should) run a lot more oil in your premix because they're on WOT for much of the time, but the theory is likely the same, as long as you can avoid fouling problems.

So what to do? I'd say just run your standard premix ratio, go easy on it for the first hour (as previously mentioned) and then treat as normal. On my KR-1S around Phillip Island I normally give it one easy 20 minute session on the track and then just hammer it and that seems to do the trick, running Motul 800 premix at 32:1. And that's on a track where you're WOT for a long, long time...

 

[Jim Crenca]

With four stroke engines, some manufacturers recommend break in with conventional oil as synthetics may not allow for proper ring seating. I'm not sure if this is true but would like to hear educated opinions. The new Corvettes come with synthetic oil from the factory but perhaps the cylinder wall finish or ring material makes a difference.

 

[19Brendan81]

When you first rebuild a new motor there is an enormous amount of extra friction between the rings and the bore, and no amount of oil will prevent this entirely, but the importance of the extra oil is to carry away the microscpocic metal shavings that occur when you first run a fresh engine. New engine = more shavings than normal, hence the need for more oil than normal to help get rid of this extra material, which if left to build up will eventually go downstairs to the bearings and thats where it will cause real damage. Its the same with four strokes, hence the first oil change on a rebuilt motor is done after 1 tank of fuel, then 1000kms, then 5000kms and thats generally where you remain changing the oil (every 5000kms)

 

[bader3245]

I always run a rebuilt two stroke with more oil than I normally use, I do this on chainsaws, mowers and dirtbikes. It is important to do this to allow the rings to wear in properly with the cylinder wall, and when done properly will give you longer service life between top end service jobs. This is why the manufacturers recommend doing it. Obviously more oil in the premix means less fuel hence the engine running leaner, but even going from 40:1 to 15:1 doesn’t have as dramatic effect as it would seem.

Ideally two strokes are tuned to run 12.5 parts air to each part of premix this is known as the “Stoichiometric ratio”. Straight gasolines stoichiometric ratio is 14.7:1, increasing past this causes lean out. Two strokes run a richer stoichiometric ratio because scavenging losses lean it out a bit, Canadian daves site says ideal is 12.5:1. SO a two stroke running 40:1 premix contains 97.5% fuel and 2.5% oil, or .975 parts fuel to each 12.5 parts air OR its Stoichiometric ratio is 1 part fuel per 12.82 parts air. Chang the fuel:oil ratio to 15:1 and the figures change to 93.4% fuel and 6.6% oil, or .934 parts fuel to each 12.5 parts air OR 1 part fuel to 13.38 parts air. So, 15:1 is half a part leaner in stoichiometric ratio than 40:1…and that’s not enough to cause lean out in your engine.

I actually disagree a whole lot about that....to seat the rings, it involves actual waer of the rings to the cylinder wall....too much lube will actually deter a good seat.....use it like you normally do, let it warm up good after a rebuild and go rip...my frineds are constatly amazed how i can get my modded 125 to hold up so good with the nasty compression that i run......