OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Self propelled Victa power plus

It doesn't look like asbestos, Chris: I'm old enough to have handled the stuff a number of times. It looks like animal hair in the picture, but that seems insane.

mmmm Yeah I was thinking heavy gauge glass fibre, I think paint shops/hardware stores or local shire councils offer a free fibre identification service.

The baffle plate seems to separate the wadding from the outlet, from my veiwing of the pics.

Thanks for the input Joe for ethanol fuel.
Chris for the paint.
FAST GRASS for the muffler.
Grumpy for the muffler theory. Thanks alot.

I tend to think of myself as a cleaner that can pull stuff apart, clean & put back together. It gets most engines working for me, I am slowly learning the engine theory, there is more than you think to small engines.

What I am trying to get my head around: is, I had a 2 stroke power torque engine that would not rev above idle due to a blocked muffler.
Once I cleaned it out, the engine then ran as expected.
So the back pressure was greater and the flow through the engine restricted, giving slow speed.

Now with this power plus engine, the head is only a small bowl compared to a normal one, so the fuel air mix would get compressed more.
This would give better detonation, therefore the baffle has to have more holes to accomodate this.

I have run 2 strokes with the muffler off. The still run fine, just alot of noise.

For some reason I thought the engine needed some back pressure to run correctly.

First, you meant combustion, not detonation. Detonation refers to the situation where petrol-air mixture burns much faster than usual. The ignition is timed so that the peak pressure normally occurs slightly after Top Dead Center, when the available space is increasing rapidly as the piston descends. Detonation brings forward the time when peak pressure occurs to approximately TDC, which results in an abnormally high peak pressure in the combustion chamber, accompanied by a knocking sound, which strangely enough is called a knock. In severe cases this sudden ultra-high pressure is prone to breaking piston crowns. Detonation is a severely destructive phenomenon, which at high speed can destroy pistons in less than 5 seconds from its onset.

Crankcase compression two stroke engines rely on a rather marginal process to scavenge the burned gas from the chamber after combustion. The momentum of moving columns of gas plays a key part, and at light throttle, when there isn't much incoming charge, scavenging is very poor and the engine will need a two or more successive attempts at scavenging before a combustible mixture will be achieved (called 4 stroking, or 6 stroking, or 8 stroking depending on how many tries it takes). One of the methods used to improve scavenging is to tune the exhaust system to produce pressure pulses timed so that there is a momentary low pressure at the exhaust port late in the exhaust process: this low pressure helps suck out the burned gas. Having a heavily baffled muffler right up against the exhaust port has the opposite effect: it messes up scavenging by causing a sustained positive pressure at the exhaust port (back pressure). This matters a lot more to a crankcase induction two stroke than it does to a four stroke. So, to get power from a crankcase induction two stroke what you would like to have is a pulse-tuned pipe (preferably tapered, megaphone-style). This is easily accommodated on a motorcycle but is a cost and packaging challenge on a lawnmower.

Any petrol engine will produce more power on less fuel if you raise the compression ratio, up to the point where detonation occurs, and provided you maintain the same volumetric efficiency. Power is also increased by improving scavenging efficiency, which in a crude baffled system can be brought about by omitting the baffle or making more or bigger holes in it. Of course this increases noise levels, which is a problem when there are laws about such things.

Thanks grumpy for taking the time to explain this, well written and detailed. I will need some time to digest this information.

Sounds like a well timed balancing act between intake, combustion, scavanging and exhaust. Pretty amazing stuff.