Hi-rev runaway

[BoatBuoy]

I haven't experienced this but I am curious. I've heard many comments that some of these 2-strokes can "runaway" in neutral and that's the reason for the vacuum shorting switch for one set of points.<br /><br />The only way I could imagine this, is for the revs to get so high that the crankcase vacuum is able to override the throttle return spring and pull the butterfly open. At that high vacuum, it also pulls in the diaphragm on the override, shorting one set of points. Is this correct?

 

[ezeke]

Re: Hi-rev runaway

The problem is usually red hot carbon in the firing chambers so electric spark is no longer required. The motor keeps pulling gas into the cylinders and the heat keeps firing it. <br /><br />You can't stop it electrically. Sometimes you can stop it by disconnecting the fuel supply. <br /><br />Unabated it stops itself by blowing up the engine.<br /><br />BTW, it is not at all limited to 2-strokes. web page

 

[mikeyzx2]

Re: Hi-rev runaway

isn't it the term "dieseling" when it happens in a four stroke?

 

[BoatBuoy]

Re: Hi-rev runaway

My confusion lies with the fuel supply. I understand red hot carbon and such, but without fuel combustion can't occur. If the throttle is shut, how can it continue to pull fuel? If you can't stop it electrically, what's the point of the vacuum cutout on one set of points?

 

[rickdb1boat]

Re: Hi-rev runaway

I believe it's to stop it from getting to the point of no return...

 

[ezeke]

Re: Hi-rev runaway

"As the piston begins its upstroke, it closes the intake and exhaust ports and begins to compress the fuel and air mixture trapped in the cylinder. The upward travel of the piston also reduces the pressure in the crankcase compartment. The resulting suction opens the leaf valves which admit additional air and fuel from the carburetor into the crankcase, thus preparing the next cylinder charge." from OMC Theory of Operation<br /><br />The throttle valves are closed when the engine is at idle. How does it keep running?