An exhaust fire is not caused by running a rich oil mixture, it can happen in dry coke deposits from petrol.

For a fire to occur (some basic chemistry here) you must have oxygen, a source of ignition and fuel. A correctly running / tuned trials bike runs very slightly rich of stoichiometric and therefore if your bike mixture (fuel air ratio) is somewhere near correct there cannot be "spare" oxygen in your exhaust to start or support a fire.

I have no objection to people running their bikes at 80:1 or whatever, that is their choice, but some of the statements made in support of lean oil ratios and supposed disadvantages of rich ratios are a pile of rubbish.

FACT more oil means less wear

FACT more oil gives more power

FACT more oil improves ring sealing and piston cooling

FACT Sherco recommend 50:1 fully synthetic

FACT GasGas recommend 50:1 fully synthetic

FACT Beta recommend 25:1 on mineral or 66:1 on fully synthetic (Bardahl which is top end quality and viscosity)

FACT mix ratios as rich as 25:1 do not cause plug to oil up in normal trials use.

FACT Back in the days of much dirtier oils, villiers, Greeves, BSA etc used to run at 16:1 and plug fouling was only an occasional problem.

FACT more oil reduces chances of engine damage if, your engine takes in water, throttle sticks open or you have coolant loss.

Dude decafe.

Hi Dan, could you translate the meaning of "Dude decafe" for the benefit of us Northern English.

He means relax

The internet yelling of FACT doesn't make it so. Otherwise a sure way to more horsepower would be the addition of oil to the fuel used in a four stroke. The facts are more likely a certain amount of oil will do all those magical things you espouse but more than that builds up and carbons ring lands, fills exhaust systems with spooge and smokes out the neighbors. Much of the "factual" information comes from two stoke GP work done by Yamaha in the 70s. It was good work but it is also considerably out of date as well as being focused on 12,000 RPM road rockets. Sherco and GasGas recommend well into the cautious zone of ratios. By the same token I would bet they also have similar maintenance regimens to Beta which expects you to clean, adjust and lube damn near everything after each ride. Maybe for a factory rider with his own mechanic but it ain't gonna happen for Joe Sunday rider.

Found this interesting bit but there is more on the web; http://www.gasgasrider.org/forum/showthread.php?t=1296

I suspect this is going to be the norm in a few years much like my Subaru uses 0W-20 oil, not because it has superior lubricating properties but because it ekes out a few more miles per gallon. I have, in older bikes, had a muffler fire. I didn't try telling it there was insufficient oxygen for it to burn to see if that put it out. I'll try if it happens again and let you know the results.

There are a lot of people running 80:1 and 100:1 without problems. As for me, my bikes make plenty of power, last for years and don't get me punched in the head by the guy behind me in line. I use good quality synthetic and VP C-12 race fuel. If you run crap gas then yes a lower oil ratio is probably a good idea but in that case you're probably running "Bob's quality outboard oil for two stork motors" anyway.

I don't have to de-carbon my engines every year like I used to at 50:1, I don't have to futz with plug heat range anymore and the only major engine work I had to deal with was new main bearings on a bike that sat in storage for years while I played with a newer bike. I'm pretty sure that wasn't an oil issue as when I put it in storage I filled the tank with 50:1 race gas and occasionally started it leaving the choke on to intentionally oil the snot out of the engine. Even that smokey treatment didn't prevent moisture from bunging up the mains.

Hey heffergm if you're riding Meriden next week you can take a putt on my bike and tell me if it's down on power. Also what you think of my clutch which has been modded and uses the 6 fiber plates from an '08.

Edited May 11, 201412 yr by dan williams

We are not talking 4 stroke here. I was referring to typical 2 stroke mixes of between 100:1 and 15:1 and in that range in a correctly tuned engine more oil gives more power and this is based on far more recent research than the original Gordon Jennings / Yamaha work.

That manufacturers are working towards lean mixes for emissions reasons does not alter that.

Some reasons more oil gives more power are, less friction, higher compression due to better piston / bore sealing and less blowby which contaminates incoming mixture and reduces crankcase pumping efficiency.

I use pump gas but avoid the cheaper and unbranded stuff. The cheaper stuff contains less cleaning agents and is more prone to pinking.

That a lot of oil causes build up on the piston and ring sticking is a common myth (all to frequently quoted by those promoting lean ratios). Ring sticking and carbon build up (if oil related) is caused by poor quality oil or blowby accompanied by high piston temperatures. With plenty of oil heat is conducted out of the piston to the cylinder walls and cooling system via the oil film. With lean mixes this oil film just is not there. Plenty of oil seals the rings, and piston to bore clearance better preventing blowby. It is hot blowby gases getting down the side of the piston that cause most carbon build up and ring sticking. I have seen plenty of engines run on quality fully synthetic at bewtween 60 and 80:1. The piston sides and crankcases are full of brown gum build up. Engines run at between 25 and 30:1 even on horticultural semi synthetics and mineral 2T are frequently completely clean or only have light honey coloured deposits.

Wear - If you you use a lean mix of even the best synthetic the oil film is very thin say 2 micon. This means that particles over 2 micron cause wear when they are rubbed between piston and barrel.

Up that oil film (by using rich mix ratio) and you can increase the oil film to say 8 to 12 micon. This increase of the film thickness greatly reduces the number of particles that have the potential to cause wear whilst passing through the engine.

I use a fully synthetic oil from a leading manufacturer. The oil meets both JASO FD and ISO EGD specification. At 100cc oil to 3 litre petrol there is virtually no smoke, less smoke than from some bikes on well known oil running at 75cc oil to 5 litre petrol.

and that sir is a much better answer. Thank you.

Even though Beta specs 100:1, I run 80:1 for safety. When I removed the top end there was small oil droplets on the head. The pipe was clean and I thought all was good. A couple of weeks later I decide to open it up a little on my dirt road and I started smoking bad, like a skeeter truck. I had never experienced that before and hit the kill button thinking my motor was fried. I was in disbelief as I listened the the bike sizzle and crackle like a pan of bacon.

After it calmed down, I started pushing. For about a mile. I crossed a creek and got water in my boots, and then it was all uphill to the barn. So I decided to try to start it just to make it up the hill and it fired off and ran like a watch. It was then I learned about exhaust fires.

Even though Beta specs 100:1, I run 80:1 for safety. When I removed the top end there was small oil droplets on the head. The pipe was clean and I thought all was good. A couple of weeks later I decide to open it up a little on my dirt road and I started smoking bad, like a skeeter truck. I had never experienced that before and hit the kill button thinking my motor was fried. I was in disbelief as I listened the the bike sizzle and crackle like a pan of bacon.

After it calmed down, I started pushing. For about a mile. I crossed a creek and got water in my boots, and then it was all uphill to the barn. So I decided to try to start it just to make it up the hill and it fired off and ran like a watch. It was then I learned about exhaust fires.

I'm still lost as to where you guys are coming up with these recommended mixture values. My manual says 1.5% on full synth, 4% on mineral. That translates to 66:1 and 25:1 respectively. I'll be running 66:1 from now on.

Whatever works for you is fine. It's not a cleanest piston competition. ;-)

I've never suggested anyone recommended 100:1. I've only said that I've run that lean without problems but I also run $15/gal fuel so maybe that helps keep my motors happy. Certainly keeps the VP dealer happy.

Edited May 13, 201412 yr by dan williams

[quote name="slicktop" post="366261"

After it calmed down, I started pushing. For about a mile. I crossed a creek and got water in my boots, and then it was all uphill to the barn.

I just got this Lawrence of Arabia vision in my head of you pushing the bike for endless miles across vast wastelands while epic music swells in the background and nomadic tribesmen watch you from the distance.

I just got this Lawrence of Arabia vision in my head of you pushing the bike for endless miles across vast wastelands while epic music swells in the background and nomadic tribesmen watch you from the distance.

I had a very hollow feeling in my stomach thinking the bike was toasted. After finding out the real issue I felt kinda foolish for pushing in the first place.

Which brings up this question, if and when this occurs again does one just power on thru and burn the oil out quicker or stop and let things cool?

Good question. I'm inclined to think quenching it and cleaning at home is better than cooking anything that lives near the pipe which is damn near everything on a trials bike. It's always the long fast trail that seems to get it going. Several bikes ago I decided to clean accumulated goo out of an intermediate muffler by standing it on end and lighting a fire in it. It burned for about 30 minutes dripping goo out the bottom until I figured I'd smoked thr neighborhood out for as long as I dared.

I had a very hollow feeling in my stomach thinking the bike was toasted. After finding out the real issue I felt kinda foolish for pushing in the first place.Which brings up this question, if and when this occurs again does one just power on thru and burn the oil out quicker or stop and let things cool?

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