Flushing RPM's

[PD2]

I know that you are not supposed to rev the engine while flushing with muffs, but I have never known the technical reason. What is the reason for not doing this if you have water running through it and how sensitive are engines to damage from this?

 

[north40]

Re: Flushing RPM's

The danger is that without a load the engine could possibly over rev and most engines don't like this nor will you when you start buying new rods, pistons etc.

 

[dtherrien]

Re: Flushing RPM's

acually you should not go over 2000 rpms on the muffs...mainly because there is not enough water from a hose supply when you exced 2000plus rpm. You can damage the pump and overheat your engine.

 

[Paul Moir]

Re: Flushing RPM's

Some 2-stroke engines will "Run away" if they're reved without load and backpressure on the exhaust from being stuck in the water. Running away is where they spin up all by themselves all the way. In serious cases, stopping the engine with a switch is impossible and you have to pull the fuel line and wait for it to run out. Choking the motor helps. I think north40 is refering to this.<br /><br />2-stroke outboards employ roller bearings and a spinning unloaded roller bearing's rollers won't roll unless loaded. (Please pardon my wording!) Instead they'll slide and wear, leaving chatter marks on the races and flattening the rollers. I've heard this is a reason not to rev 2-strokes unloaded but I'm not sure if the forces are really light enough for this to happen. Good reason though.

 

[orca]

Re: Flushing RPM's

PD2<br />If you've ever experianced run away you will know what Paul means, I have had it happen at least 6 times.<br />One memorable time I started a 2 Cyl OMC with VRO. The revs went straight uo to 6000 RPM, Shut the throttle, no change, Turn off the key, no change. remove the plug leads, no change. Pull off the fuel line and the motor still keeps running. The motor was dieseling on the oil coming from the VRO. The only way to stop the motor was to turn on the primer choke and pump the bulb. Next move was to remove the spark plugs, try doing that at 6000RPM.<br />Now ifit happens we just ram the choke on and it dies straight away.

 

[PD2]

Re: Flushing RPM's

Thanks for all of the responses. You have scared me enough that I will not plan on testing it. I haven't ever experienced a "run away" and will definitely try to avoid it. Thanks.

 

[Silvertip]

Re: Flushing RPM's

You think run-away is bad on a two stroke, you should see what happens when a big diesel decides to run away. It's not a pretty sight.

 

[phantoms]

Re: Flushing RPM's

It's not a pretty sight.

Definitely not! About the only quick choices you have are a block of wood over the intake opening, or a CO2 Extinguisher. You can also clip the inj. lines with a bolt cutter, but you risk the motor sucking oil out of the crankcase to continue running on.

 

[andymach23]

Re: Flushing RPM's

Last time i collected my boat after service at my local authorized Yamaha dealer, I watched the mechanic servicing a mid size merc. He was running it on the hose and screaming the throttle, cycling it from low to mega revs. This has stuck in my mind and the clincher for me to do my own servicing. That and being charged 3 hours labor to change my oils and grease the propeller shaft!

 

[TxShorty]

Re: Flushing RPM's

Had a guy bring in an older 2 cyl Johnson with choke flaps in the carbs that had run away while he was flushing it. He tried to choke to kill the motor, and the choke flaps were ripped from their hinges and pulled through the motor. Wasn't worth building the old motor...