Seize by too lean jetting

[pyromaniac]

I've been thinking what actually make a engine seize by too lean jetting. As i understand it, the engine get too little fuel in the airmixture. But is it because it gets too little oil or does the fuel itself lubricate the engine. Im only talking 2-stroke ofcourse.

The only way i would see it is because it get too little oil but thats not it according to many comments about jetting for example "dont add more oil it can cause the engine fun too lean and seize". But i cant understand really why it would seize from too little fuel. I think something like this. The less fuel the engine gets the less heat it produces so with enought oil in the mixture it shouldnt seize even if i gets too little fuel. I mean a very rich running engine will seize if it doesnt have any oil in the fuel right?

I've been as i have said many times before riding my 250 with the same jetting year-round with a 50 degree celsious temperature change. The bike should have been running lean and rich at that time. However it have always worked great and never seized. The reason i think i have never seized a engine in the 10+ years i've been riding is because i have always used 3 or 4% oil. Lets say you run 4% oil should you most likely to be able to run the bike leaner without seizure than a bike with only 2% oil. I have hard to believe you can seize a bike running say 15% oil (yes that bike will run).

Does 4-stokes seize of too lean jetting?

What i want to know: how much does the actual fuel lubricate comparing to the oil. Is it like 90% oil and 10% fuel or what can it be?

 

[Can Can Kev]

i cant help u with the 2 stroke questions but a 4 stroke sizes if there is no oil in the cases, not from lack of fuel....

 

[coleman]

I think bikes sieze when they're lean because they run hotter.

I also think its okay to not change your jetting all year. Last season all I ever did was adjust the air screw once in a while.

 

[atc3434`]

PYRO... you seem to have it pretty much straight, a 2 stoke doesn't seize up because of a lean on fuel condition, per say... but this could cause major overheating. A really lean mixture will burn very hot, and really heat up the motor. Having the proper amount of fuel helps to cool the motor between power stokes. This is true of 4 strokes too. A super lean mixture in a 4 stroke will heat the motor up too much, and could even lead to a seizure. So even if the bike was receiving the proper amount of oil, if it was too lean, you could overheat it and melt it down. By mixing more oil in your gas, you are leaning out the fuel mixture, but your increasing the amount of oil going into the motor, so this wouldn't cause overheating, and possible seizure.

About jetting, some bikes don't really seem to care about temperature, others are real picky. My RM125 hates the cold, and won't pull strong at all, even warmed up. It's spot on about 65 F though. On the other hand, my buddies (Soon to by mine) 01 YZ125 runs great at 40F to 100F Maybe a tad rich at very warm temps, but not much. Guess it depends on your bikes personality.

 

[pyromaniac]

So i am right about that its the oil that does most of the lubrication. You say the bike can overheat if too lean, does that mean the cooling water will boil or just internal overheat?

Actually i dont understand really why it would overheat. That does not agree with my sence, as i see it the less fuel the engine have to burn the less heat will be produced. I do understand the fuel itself help cooling the engine but very very little. As i see it most cooling of the engine would be the air itself then the cooling system then fuel. Maybe the fuel making 1% of the total cooling.

 

[Danman]

Yes, in a 2 stroke the oil mixed with the gas lubes the top end. The bottom end uses the same oil as the transmission.

 

[dthoms]

No, the bottom end is lubricated by the same oil as the top end, the oil in the premix or injectors. On a two stroke the transmission oil is only for the transmission and the clutch.
Excessive heat cause more than just lubrication problems, it also swells the piston and causes more friction on the rings.
I've seized my share of bike when I was younger and every time it was always the rings to the cylinder.
I'm not saying you couldn't seize a rod bearing or crank bearing, I've just never seen one.
In the cases where I have seen bad rod or crank bearing the motor either got very loud, and or took something else out so it wouldn't run. Like the crank seals, or piston, or let the piston hit the spark plug.
I don't think if you raised the percentage of oil in the mixture that you would be able to run the bike leaner and keep it from harming the cylinder.
The roller and ball bearings in the crank and the rod could last a lot longer with out oil than the cylinder/piston rings.

 

[rickyd]

during a seizure, the piston expands due too the heat brought on by lean condition.. THe bottom end (crankshaft) is lubricated by the fuel, the gearbox is seperate..
RIck

 

[atc3434`]

Less fuel burning definatly does not mean less heat. Think about this example. When you light up a blow torch, you initially have that big yellow flame, which is nice and pretty, but not super hot. To get that nice hot blue flame, you back down the gas till its burning really crisp. The difference in heat is quite a bit. Same holds true within an engine. You back off the fuel to air, it generates more heat.
So not even being concerned with oil mixutre, look at a 4 stroke. If you run it way too lean, you can really heat it up, possibly killing it. Especailly if not liquid cooled. Same is true for a 2 stroke.

 

[derekk87]

The fuel is what actually cools the piston and head area, the cooling system(radiators) can only work after the heat has been passed through the cylinder walls.

 

[Moto Squid]

When it's lean that gas just burns so dang well...the bike will run really good too, then sometimes it goes "pop". I usually run mine on the lean side because of the better performance, but I'm playing russion rullete. Haven't lost....yet :think:

 

[ML536]

That was an interesting analogy, ATC. Thanks.

 

[whitesands26]

Going rich or lean needs to be clearly defined to what context and is usually a term used when discussing jetting which is the AIR to FUEL ratio......



And as many have already pointed out MORE AIR and less fuel = a leaner condition = more heat...In a two stroke mixed at 32:1 more fuel and less air= richer condition = cooler running bike and more oil to lubricate moving parts...

Your fuel oil ratio needs to be accounted for in your jetting....When you add more oil to the gas you effectively LEAN out your FUEL TO AIR RATIO (less gasoline and more oil to the amount of air) Therefore you end up with a warmer running engine and more splooge....I don't care how rich your FUEL/OIL ratio is, too much heat is not a good thing....

Now if you richen your fuel/oil ratio(more gas to the amount of oil per given volume of premix) then you richen your air to fuel ratio(more fuel to a given volume of air). At the same time you have LESS oil per given volume of fuel which is not a good thing either....

The best thing to do is to find an IDEAL premix ratio(usually 32:1 but 40:1 on a 250 won't hurt) and jet your bike according to atmospheric conditions...For motor longevity it's always better to be a little too rich than too lean. You want to find the right balance of power and motor protection...