Modern Vespa

What's a typical RPM for idle with these engines? I know it's supposed to be higher than a 4 stroke and I would think as low as it keeps running is good enough, but is that true? I've run across some references of being too low causing plug fouling. Just asking.

A too low idle can cause gear changing issues, and when the engine goes from WOT to very low idle. If the idle is too low it will not be enough fuel flow to cool the top end immedietly following a long wide open run.

Mine are around 1100 rpm give or take a hundred

????? Fuel flow cooling the top end? We are talking about less than a quarter of a gram of fuel and air briefly absorbing heat from a couple of hundred grams of cylinder and piston. The physics don't come anywhere near to supporting any significant cooling. It is a very widely held misconception, however.

jim

Prolonged idling can foul plugs. For a variety of reasons, combustion is less complete at idle speeds. Typically a richer mixture, lower combustion chamber temps, etc. In wet sump engines, oil pressure is a concern, and in water cooled engines, reduced coolant circulation can be an issue. Neither of these are 2T Vespa issues. Ain't gonna get into a debate about the definition of "prolonged" in minutes and seconds, but I'm reasonably confident that sitting at a long traffic light does not qualify as "prolonged".

A major consideration of idle speed in all types of piston engines is maintaining sufficient inertia to keep the engine running. Any additional load on an idling engine needs to be within the idle power output capability. In a car, an example would be the cycling of the air conditioner. For our PXs, the only real additional load is when we release the clutch to either idle in neutral or to begin moving forward. Of course, you and I are so well skilled in clutch/throttle coordination that idle speed is not a factor.

As long as your engine idles relatively smoothly and will smoothly accelerate in response to the throttle, the exact idle RPM is fairly unimportant.

To me fuel cooling the top end is one of those things that isn't really true but works in the practical world...kind of like when figured out on "paper"a bumble bee can't fly.

Anyway I'm like Vader (now THAT scares me!) usually somewhere about 1100 to 1300

~1100 seems to be a good stable lower limit for me too. When warmed up on a warm day I can get it lower, but it's not reliable.

Great discussion with interesting replies.
Thank you, all.
There's a lot of blah-blah out there about 2-strokes and I got to wondering about things like fuel economy, fouling, shifting, etc. with these peculiar engines.

Actually, the story that how a bumble bee's ability to fly cannot be figured out on paper is pure myth. Google it.

The cylinder and piston cool when you let the throttle go back to idle because the amount of heat of combustion per unit of time is significantly reduced. Rather than 6,000 combustion cycles per minute, it drops to 1,100. Thus there is significantly less heat to dissipate through the air cooling fins of the cylinder and head. Of course, if your idle mixture is "richer" than your WOT mixture, those combustion temps will be lower, but that isn't in any way the fuel "cooling" anything.

The problem with mythical answers is that they often lead someone to do something less than beneficial to their scooter. Like adding more oil to premix to run cooler to prevent a seize - rather than rejetting, which was probably the problem in the first place.

I'm an even numbers guy... mines at 1000 RPM's according to the Trailtech.

Interesting .... I thought the energy necessary to vaporize a fluid to a gas was significant - and has a significant effect at a high rpm where the cylinder is being charged for instance 100 times a second at 6000 RPM, however I'm absolutely no scientist by any means. But, if fuel vaporizing in the combustion chamber doesn't absorb heat, why does my CHT temp increase if I lean the mix out and why do cars require a catalytic converter to burn off intentional excess fuel - instead of simply running the engine leaner for a cleaner burn? Again, I don't know the answers.

I do know NHRA fuelie dragsters and funny cars run a low compression ultra rich 2-1 air fuel mix (almost at hydro-lock) to both get enough fuel to generate the thousands of horse power and (I thought) to cool the engine since so much of it is vaporized but not burnt. The flames coming out of the exhausts are hot unburnt fuel reacting to the atmosphere's oxygen. It's the only coolant they have, they have no radiator or etc of any sort. But I might have misunderstood all this.

I typically aim for an idle as low as posible. Depending on the weight of the flywheel and other factors this may be 500, 1000, or even 3000.

My smallframe is closer to 3000, my TV is well under 1000.

I should know better than to use an aviation analogy!

As you lean out the mixture, the combustion temp rises because you are approaching the stochiometric point, not because there is less fuel to cool the cylinder. The most complete combustion is at the stochiometric point, and that combustion temp is typically too hot for an engine's components. Thus, most engines run on the rich side of stochiometric, and when you "lean" it is still richer than stochiometric. The amount of heat energy released in combustion is many, many orders of magnitude greater than any "cooling" effect of vaporizing the small amount of fuel in the combustion chamber.

Yes, the fuel air mixture does absorb heat from the cylinder and piston. Figure out how many milligrams of fuel are in a roughly 14:1 air to fuel mixture with a volume of 150 to 200cc. The caloric energy required to vaporize it is miniscule in comparison to the caloric energy in the 200 to 300 grams of metal it is in contact with. And not all of that heat is from the metal. The mixture is also heated by the compression of the fuel/air mix How much would a tablespoon of water cool a small, red hot cast iron skillet?

As for the drag racers, the richer the mixture, the cooler the combustion temp. Thus the ability to run without an "normal" cooling system for short periods of time. Again, look at the mass of the fuel times its specific heat versus the mass of the cylinder/piston and their specific heat. Of course you will get more "cooling" from the fuel at 2:1 than at 13:1, but it is still minor.

Fixed that for you :P

Thanks - this is good info. More evidence of why I wasn't born an Engineer or competition engine builder !!!

Thanks, Patrick. Actually it's either/or based on the induction method. The thermodynamics, however, are the same.

The simple air cooled 2-stroke used in a Vespa is hardly peculiar.