2 Stroke Premix Help (300 Evo)

[copemech]

WOw!! My apologies. .......ive just never heard of this happening before.

What happens is fairly simple. You putt about possibly for endless hours on a trials bike, with the occasional blips and such. The oil migrates from the motor and lays up in the exhaust, sure you may have seen some ooze a dribble out at some point.

Then you take the bike out for a real ride in an open area with a real loop, or a bit of roadwork! Constant load and throttle then gets a lot of heat into the muff to light off the excess oil if there is too buch that has not been blown out. It will fire off and create a lot of heat!

I have some old pics somewhere if a stand a friend made to clean out muffs. It used an electric heat gun to get things going. Once it lit off it was blowing flames our the rear until the entire end of the ali muff melted off! And this was after turning the heat off, just blowing in air to feed the flame! Too mush oil in that one I suppose!

[slicktop]

Dad brought up a good point about fire not being possible inside the exhaust,

One: there is not enough oxygen to sustain combustion unless your engine is misfiring

Two: your exhaust is an incredibly violent place with shock waves and high velocity gasses, a flame has no chance

It is however possible to get the exhaust pipe temp to a point above the burning point of whatever combustion goop we have in the pipe, it will cook off and eventually become a carbon chunk to be spit out the silencer, but there could not be a flame in the pipe when the engine is running

I should have clarified that there was no flame shooting out the exhaust. But I can hardly explain what occured with out using the word 'fire'.

A realistic example of the ferocity would be a pan of bacon grease burning so hot that it starts smoking black. The sounds of the oil cooking inside the muffler was loud crackles and pops and metal expanding rapidly. It took about five minutes to calm down and I was quite concerned at the time.

My theory of why this happens in a trials bike vs. not being common in an enduro is the shape of the exspansion chamber.

[dan williams]

Dad brought up a good point about fire not being possible inside the exhaust,

One: there is not enough oxygen to sustain combustion unless your engine is misfiring

Two: your exhaust is an incredibly violent place with shock waves and high velocity gasses, a flame has no chance

I think you're confusing the lovely flame of a candle to the smoldering burn that flares up once the engine is off. Where my father grew up in Pennsylvania there is a mine fire smoldering. Been burning for years. Probably no open flame but just enough oxygen leaking in to sustain low level combustion. Why don't they open a hole to put it out? Because the introduction of a fresh oxygen supply will just accelerate the process. Two strokes are known for their passing of unburned fuel out of the exhaust. Even four strokes aren't 100% efficient which is why we have catalytic converters. It may be a catalytic process but it is still technically burning. It's certainly exothermic as anybody who's seen a catalytic converter glowing cherry red from a too rich mixture can tell you. Get anything hot enough and it will burn. even diamond. The point is nothing in the exhaust system should be combustible with normal exhaust gas temperatures until you coat it with oil at which point the rise in exhaust gas temperature of an engine under load passing a high volume of gas is sufficient to ignite the waste oil.

[suzuki250]

I’ve always run on 50:1 on modern LC bikes & 40:1 on air cooled

Never had a problem with smoke of plugs fouling

(I use a about 30:1 on my mx bike without a problem)

[mokwepa]

Debating ratios is like debating religion, its what works for you. As riding styles, machines and terrain differ, so should oil ratios. How to work out which ratio to use, I have absolutely no idea. Average it out I guess???

[dan williams]

Heh heh it IS religion.

[lineaway]

Dad brought up a good point about fire not being possible inside the exhaust,

One: there is not enough oxygen to sustain combustion unless your engine is misfiring

Two: your exhaust is an incredibly violent place with shock waves and high velocity gasses, a flame has no chance

It is however possible to get the exhaust pipe temp to a point above the burning point of whatever combustion goop we have in the pipe, it will cook off and eventually become a carbon chunk to be spit out the silencer, but there could not be a flame in the pipe when the engine is running

After scoring at the Nationals in New Mexico all day Saturday I was quite bored. At dusk I grabbed my son`s Raga for a little wheelie session up the ski slopes. Great fun this is after watching the riding all day. I rode nationals for years and scoring does me no justice. It was only 15-20 minute ride. (At 8400-9500 ft elevation) I ended up pushing the bike back to my camp. Thought the worst as I had packed the S/arrestor full of aluminum.I thought piston, but I had melted the inside of the mid-pipe. Not much air at 9000ft. Lot`s of heat. Yes. By the way I run 80:1.

[dan williams]

Well yeah but it's a GasGas so it's made out of cardboard. :-)

[dadof2]

Most exhaust "fires" are not inside the exhaust. All that is in there is a very hot flammable mixture. As soon as it touches fresh air which pulsates in and out of the last section of the exhaust it can catch fire.

A standard 2 stroke carburettor if correctly jetted for normal use, will run weak when wide open at high revs leaving excess oxygen in the exhaust. That is one of the reasons why fixed gear karts, which run at full throttle and high rpm much of the time use pump type carburettors.