Maximum No-Load Engine RPM

[kllarsen]

Hello,<br /> I have read several times that you should never go over 2000 RPM with no-load on the engine. (Neutral or on muffs) Can someone tell me why this is? I have worked on automotive engines for years and did not think twice about taking my outboard up to4-5000 rpm's when running it on muffs or in neutral. <br /><br />What is the maximum no-load rpms on an outboard and why is it hard on it to rap it up in neutral?<br /><br />The motor I have right now is a 2001 Johnson 50 horse.<br /><br />Thank you<br />Keith

 

[CATransplant]

Re: Maximum No-Load Engine RPM

Two stroke engines don't do that well when revved up with no load. They can go into runaway mode where RPM's way exceed the max RPM for that engine. When that happens, you can't shut the engine down with the kill button and often not even with the choke before it self-destructs.<br /><br />It's a quick way to turn an expensive outboard into junk. It won't always happen...in fact, it's fairly rare, but it can happen. All it takes is a red-hot little piece of carbon to keep firing the engine independent of the spark plugs.<br /><br />I can't think of any reason that you'd want to rev your outboard near its design limit with no load. It doesn't help diagnose anything, since all the WOT stuff is dependent on loading the engine.<br /><br />But...it's your engine, not mine. I wouldn't do it, though.

 

[Chinewalker]

Re: Maximum No-Load Engine RPM

One reason not to rev up a motor - when slowing the engine back down it'll often do a "din-din-din-din" type of cycle as the revs slow down. This is essentially the motor spinning faster than the fuel and timing would normally allow. It effectively "slams" the reeds back on their seats as the crankcase compression overcomes the lack of air coming in due to a lower throttle setting, often leading to breakage of the reeds...<br /><br />- Scott

 

[kllarsen]

Re: Maximum No-Load Engine RPM

Thanks for the replies. I had no idea this could cause any harm. I wont be doing that again! The only time I would do it was first thing in the spring when starting it the first time after fogging it. Trying to keep it running below 2000 rpms is a major task when it is loaded up with oil from winterizing it.<br /><br />Thanks again! I learn something new every day!<br /><br />Keith