Re: How long at or near WOT
By seahorse - How were you measuring the cylinder head temperatures? Were you using a dash mounted gauge or an infrared temp gun?
Head temps? That won't do you much good in a liquid cooled engine. You can stick a piston due to extreme cylinder temps caused by a lean condition long before you see an increase in head temp. <br /><br />I'm talking about instantaneous cylinder exhaust temperatures, using a pyrometer. This is one of the best proven ways to monitor the combustion of fuel/air mixtures.<br /><br />My point is that no carburetor has a perfectly linear fuel delivery system. Carbureted engines run off of the Bernoulli Principle which says as the air velocity through the carburetor increases, the pressure decreases. In other words, if you had a butterfly carburetor with the butterfly wide open (WOT), the velocity of air would be great. Thus the pressure would drop causing fuel to be "sucked" from the bowl and into the metering system. With the butterfly almost closed (idle), air velocity would be minimal causing very little fuel to be metered. The pressure difference meters fuel flow proportional to the volume of air flowing into the engine. <br /><br />But small amounts of fuel can't be metered through a big jet very well, and big amounts of fuel can't be metered through a small jet very well. Trying to do so will result in rich and lean conditions, along with stalling and bogging. That's why we have carburetors with idle jet and main jet circuitry. <br /><br />So we must have transitions in metering. But there are spikes, gaps, and inconsistencies occurring where the air/fuel metering is transitioning. Whether it be a transition from the idle/pilot jet to the mid jet, or the idle jet to the main jet. Not to mention most carburetors have small castings or machining flaws which disrupt proper mixing. <br /><br />This is why some carburetors are designed to include an ammulus around the main discharge nozzle which supports the short duration between the main metering system and the idle system, when both would be inoperative. They are inoperative at transition because too much pressure drop will occur for the idle jet to handle, and not enough for the main to use. This is where the lean condition and high cylinder temperatures can happen.<br /><br />Round and flat "slide" carburetors have helped reduce the transition gaps, but they will still exist.<br /><br />The worst situation for a 2-stroke carbureted engine is where the engine is running WOT and maximum RPM, and then it's suddenly idled down. At WOT the engine is receiving maximum fuel, which is cooling the cylinder. At sudden idle, the cylinder no longer sees the fuel which was cooling it, but it's still reciprocating and trying to fire at extreme RPM. Thus you have piston seizure. This is why a lot of 2-stroke racers "burp" the throttle or "choke" the engine after a hard WOT run. They're simply saving their engine by squirting more fuel into it which causes the cylinders to stay cool.<br /><br />Most consumer carbureted outboards are jetted very liberally to avoid problems. But that is why they get worse economy and spew more emissions. <br /><br />
Now combustion temperatures will rise as power is increased, but not necessarily the cylinder head temp due to modern outboard cooling system design.
Combustion temperatures in a 2-stroke will only rise if the air/fuel mixture is leaned out. It is not necessarily RPM or power dictated. Again, many 2-strokes will run cooler combustion temps at WOT where full fuel is cooling the cylinder than at 3/4 throttle where a metering transition is occurring causing a leaner air/fuel ratio.
