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Seizing a Two Stroke

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Recently I heard of a guy seizing his cr 80 just by engine braking in high gear. I've been considering a kdx 200 or a wr 250 (the 90s yamaha) recently, and this worries me. I have zero experience with 2T engines, having only ridden air cooled four strokes. Are there any other ways I can seize or otherwise damage a two stroke engine besides revving too high for extended periods of time?

Thanks.

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No oil in tranny...no premix oil...overheating...coasting...poor jetting...and on and on....

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I should have been more specific. Even with the correct fluids and proper jetting are they any other ways I can seize a motor?

Edited by Nick Merrell
misspelling

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I should have been more specific. Even with the correct fluids and proper jetting are they any other ways I can seize a motor?

Revving it hard too early without allowing it to warm up first.

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Once piston and/rings go beyond their wear tolerances, it can cause damage. Not allowing the engine to warm up properly can cause a cold seizure, especially if you run a forged piston. Air leaks can cause it to run hotter and possibly seize it up.

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No oil in tranny...no premix oil...overheating...coasting...poor jetting...and on and on....

how does costing seize the engine?

I honestly dont know.

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when you are coasting the engine is spining at the same speed as it was to get there. the difference is that when you are getting to speed or holdong your speed you are on the gas. because a 2 storke gets its piston, crank, and connecting rod lubricated from the oil in the gas you can seize the engine do to lack of lubrication.

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when you are coasting the engine is spining at the same speed as it was to get there. the difference is that when you are getting to speed or holdong your speed you are on the gas. because a 2 storke gets its piston, crank, and connecting rod lubricated from the oil in the gas you can seize the engine do to lack of lubrication.

Spot on +1. Coasting = high speed decel. (throttle shut)

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when you are coasting the engine is spining at the same speed as it was to get there. the difference is that when you are getting to speed or holdong your speed you are on the gas. because a 2 storke gets its piston, crank, and connecting rod lubricated from the oil in the gas you can seize the engine do to lack of lubrication.
If you run enough oil in your mix, this will NOT happen. Coasting seizures happen when people run stupid ratios like 40-50-60:1 in a 14,000 rpm engine like an 80.

I wrote this a long time ago, and this seems like an appropriate thread in which to re-post it:

Pre-Mix 101

Looks like it's time for a little pre-mix 101. I don't usually get into ratio discussions, because mix ratios are like religions to most people, and they tend to be closed-minded and hard-headed on the subject, but I'll put in my $.02 here anyway.

Anyone that believes that spooge and plug fouling are caused by too much oil in the mix is flat out wrong. If you know how to jet, you can run any amount of oil you choose, and have absolutely zero spooge.

There is a prevailing myth that less oil is better. This simply isn't the case. While there isn't a magic "one-size-fits-all" mix ratio, and it is possible to use too much oil for your conditions, generally speaking, more oil is better, within certain limitations.

When an engine is jetted too rich, the excess fuel leeches heat from the combustion process, causing the combustion chamber temperatures to be too low to effectively burn the oil, or even completely burn all of the fuel. The result is spooge and deposits. The spooge is nothing more than unburned fuel and oil passing out the exhaust.

If you have a spooge problem, you have a jetting problem. You don't get rid of the spooge by reducing the oil, you get rid of it by fixing the jetting. Correct jetting will produce an air/fuel ratio of about 14:1, which will produce combustion temperatures in the 6000F range and exhaust temperatures in the 1200F range. This will provide sufficient heat to consume the premix oil.

The same goes for plug fouling. Rich jetting does two things. First, it promotes incomplete combustion of the fuel and the oil due to reduced combustion temperatures. The incomplete combustion of the fuel and oil promotes deposit formation inside the engine. Second, rich jetting reduces the combustion temperatures, which in turn reduces the engines ability to burn off deposits. Combine increased deposit formation with reduced ability to burn off those deposits, and what do you get? Spooge and plug fouling.

You don't choose a mix ratio based on "spooge" or plug fouling, you choose the ratio based on the amount of oil your engine needs to provide sufficient protection and adequate ring seal. The common misconception is that mix ratios are "one-size-fits-all", when in fact nothing could be farther from the truth.The amount of oil that is correct for one rider on his bike may not be enough oil for another rider/bike, or it may be too much oil. It all depends on engine displacement, riding style, and how hard you push the engine. A trail rider on a 500 that never reams the bike out is probably fine on a diet of 50:1, where a super-fast up-and-coming young future pro that screams an 85 'till the dogs howl the entire time he's on the track might not get a full day of racing out of an engine on less than 30:1. Your engine's oil needs are determined by displacement, rev range, and the loads you put on your engine.

When you shut your engine down and let it sit, much of the oil drains down into the crankcase and forms a puddle in the bottom. The depth of this puddle is your indicator of whether you are running the correct amount of oil for your engine's needs. Ideally, you want this puddle to be between 1/8 and 1/4 inch. If it's less, you need more oil in your mix. If it's more, you are running more oil than you need for your conditions.

With that said, to have that amount of residual oil in the crankcase at 50:1 (a ratio made popular by magazines and oil bottles), you can't be riding very hard, or your bike is jetted richer than necessary simply to deliver enough oil. I arrived at 32:1 for my bike with my riding style because that is the amount that gives me the proper amount of residual build-up. Small-bore engines require greater oil concentrations than larger engines to achieve the proper amount of residual build-up, because they rev higher and have higher intake velocities. Along the same lines, someone that pushes the engine harder, and keeps the revs higher, also needs to use higher oil concentrations to achieve the proper residual build-up.

When I was much younger and a lot faster, 32:1 wasn't enough oil for my conditions. I needed 26:1 to have enough oil. And I have run as much as 18:1 with no spooge or plug fouling issues.

To understand why the mix ratio is so important, you have to understand what happens to the oil in your fuel when it goes into the engine. While the oil is still suspended in the liquid gasoline, it can not lubricate anything. It has about as much lubricity at that point as straight gasoline. When the gasoline enters the engine, it evaporates, dropping the oil out of suspension. Now that the oil is free, it can lubricate the engine. The oil mist is distributed throughout the engine by the spinning crankshaft and the moving air currents to coat all the internal surfaces.

People believe that the oil just rushes right through a two-stroke along with the fuel, but that just isn't so. It can take 90 minutes or more for the oil migration through a two-stroke to result in a complete oil exchange on a slow trail ride, and even as much as 5 minutes for a full-throttle 20 minute moto.

The oil eventually makes it into the combustion chamber, where it is either burned, or passes out the exhaust. If the combustion chamber temps are too low, such as in an engine that is jetted too rich, the oil doesn't burn completely. Instead, some of it hardens into deposits in the combustion chamber, on the piston, and on the power valve assembly. The rest becomes the dreaded "spooge". The key to all of this working in harmony is to jet the bike lean enough to achieve a high enough combustion chamber temperature to burn the oil, but also still be able to supply enough oil to protect the engine. If you use enough oil, you can jet the bike at it's optimum without starving the engine of oil, and have excellent power, with minimal deposits and spooge. At 50:1 in a small-bore engine, you simply can't jet very lean without risking a seized engine due to oil starvation.

One small point. No one ever broke an engine by using too much oil.

Now we come to the issue of ring seal. Simply put, the rings alone can not effectively seal the cylinder. They also need oil to provide a complete seal against the bore surface. And up to a point, more oil will provide a better seal.

I have run Dyno tests on this subject, as a school project in Tech School. We used a Dynojet dynamometer, and used a fresh, broken in top-end for each test. We used specially calibrated jets to ensure the fuel flow was identical with each different ratio, and warmed the engine at 3000 rpm for 3 minutes before each run. Our tests were performed in the rpm range of 2500 to 9000 rpm, with the power peak of our test bike (an '86 YZ 250) occuring at 8750 rpm. We tested at 76 degrees F, at 65% relative humidity. We started at 10:1, and went to 100:1. Our results showed that a two-stroke engine makes its best power at 18:1. Any more oil than that, and the engine ran poorly, because we didn't have any jets rich enough to compensate for that much oil in the fuel. The power loss from 18:1 to 32:1 was approximately 2 percent. The loss from 18:1 to 50:1 was nearly 9 percent. On a modern 250, that can be as much as 4 horsepower. The loss from 18:1 to 100:1 was nearly 18 percent. The reason for the difference in output is simple. More oil provides a better seal between the ring and the cylinder wall.

Now, I realize that 18:1 is impractical unless you ride your engine all-out, keeping it pinned at all times. But running reasonable ratios no less than 32:1 will produce more power, and give your engine better protection, thus making it perform better for longer.

The bottom line? Choose a mix ratio that is adequate for your needs, and jet accordingly. You don't fix plug fouling and spooge by adjusting your mix ratio.

This is an interesting read that also supports my "more oil is better" claim.

http://www.bridgestonemotorcycle.com...oilpremix6.pdf

And this is a good article as well:

http://www.maximausa.com/technical/l...summer2001.pdf

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All very good reading but to my mind one thing is missing !

If you set the tickover screw so the bike very nearly ticks over even with a shut throttle the engine still gets a little fuel/oil. I personaly have seen engines seize solid at the end of a long start straight in the past and the only thing that could be found was the slide was fully shut, even Dave Thorpes works honda seized at the end of Weston beach race here in the UK many years ago due to the self same thing so its worth bearing in mind :thumbsup:

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I have 5 2 strokes in the house hold between mine and the kids. We run 32:1 on all but the KTM 65 which recommends 65:1. I am not really set on the 65:1 as when the summer was over, we had to replace, crank, cyl. rehoned, new pistons and rings in as little as 50-70 hrs. Good info on oil ratio as I have been seriously thinking that I should go with more oil in the KTM 65. I also will tell the kids about the coasting thing, good info. The two others we rebuilt this year got a quick rehone and could have got away with just rings, but I changed pistions while I was in there. I am thinking that the 65:1 ratio may have been a direct relationship with the amount of rebuilding we had to do on the engine.

The Bug

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Having wrung the snot out of 2 strokes for decades I must be the luckiest fellow alive, because I have never had the coast sieze syndrome. I guess if you are worried hit the clutch or kill switch and wring the throttle a couple times.

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In that 65 I would be running 20:1

I always ran 32:1 in my old rm250 and that's what I plan on using for my 500, I would rather have a little too much oil in the mix than not enough.

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Having wrung the snot out of 2 strokes for decades I must be the luckiest fellow alive, because I have never had the coast sieze syndrome. I guess if you are worried hit the clutch or kill switch and wring the throttle a couple times.
I have never seized a piston in any bike I've owned in forty+ years. I don't think it's luck, however. It's proper care, good maintenance, building the engines properly, using good fuel, and running enough oil.

I have had two engine failures in my riding career. One was an upper rod bearing failure in a 1969 Honda CB100, and one was a thrust bearing failure in a '92 KDX250. I'm convinced to this day that the new premix oil I had just switched to in the KDX caused that failure.

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I always go 32:1 ratios for my pingers.

Don't worry about seizing the engine, they aren't hard to grasp onto the concept of them after a bit of ride time on them. I always pull in the clutch when decelerating. After all, what are brakes for?

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In that 65 I would be running 20:1

I always ran 32:1 in my old rm250 and that's what I plan on using for my 500, I would rather have a little too much oil in the mix than not enough.

20:1? You'd be fouling plugs like crazy. That thing already fouls plugs pretty bad at 65:1. I run all my bikes between 40:1 and 50:1 and have never seized a piston. I've broken chunks out of pistons and they still spun completely free. In modern bikes, if you seize a piston in under 60 hours, your doing something wrong.

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20:1? You'd be fouling plugs like crazy. That thing already fouls plugs pretty bad at 65:1. I run all my bikes between 40:1 and 50:1 and have never seized a piston. I've broken chunks out of pistons and they still spun completely free. In modern bikes, if you seize a piston in under 60 hours, your doing something wrong.

it depends on how you ride it.....rev it mercilessly and 20:1 will be fine, putt around like a trolling motor and it will likely foul plugs at 40:1

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20:1? You'd be fouling plugs like crazy. That thing already fouls plugs pretty bad at 65:1. I run all my bikes between 40:1 and 50:1 and have never seized a piston. I've broken chunks out of pistons and they still spun completely free. In modern bikes, if you seize a piston in under 60 hours, your doing something wrong.

Did you just happened to glaze over when you came to Chokey's post? Oil ratio is not what fouls plugs.

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Great write up! I run 32:1 in my cr250 and there is a little puddle of oil that drips onto the floor from a hose that seems to come from the crankcase. Is that the 1/8 - 1/4 in. puddle you were talking about? Also, If im splooging, i have to lean out the bike right?

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