[LightWeaver]

Hey y'all:

My CBR is in the shop right now. I've been taking my roommate's 01 YZF600R out for some spins. Being considerate, I fill the tank with 89 Octane -- the same fuel I use in the 97 CBR600f3. Brought it back 15 miles sedately (it isn't mine after all), no incidents.

He's waiting at home so HE can go ride, takes the keys and goes. A half hour later he shows up, pissed, and rashed. Seems that the bike was backfiring slightly at high revs, so he slowed down and took it to town away from the highway. One giant backfire during a left hander spun the rear wheel, threw him against the curb, and over the handlebars when the front wheel caught.

He says that he's been using 93 only in the ThunderCat. WTF!!

What do YOU use? I stayed away from 93 b/c I heard that it could gum up the carbs. Never had an issue. So did the fuel cause the backfire? Or would it be something else?

Oooooooooooops. :angry

 

[Anti-Hero]

Octane is simply put: The gas's ability to NOT detonate. 89 in most bikes is fine. I highly doubt the gas was the cause.

 

[nig]

Yep, fuel. Most Sportbikes require the higher octane. Go beg for forgiveness

 

[Neverlose]

i was always told follow what the owners manual says, and most manuals i have read say 87 is fine. With that said, I altenate 1 tank w/ 87 the next with 89, and so on.

 

[nig]

I had a YZF, I'm pretty sure the manual said 91+. Otherwise I wasted a whole lot of money over the years. It should really come as no surprise given the compression ratios of late model 600's.

 

[stoner51]

my manual says 92 for the RC51. so thats what I use

 

[Anti-Hero]

Higher octane gas, especially race fuel is meant for VERY high compression engines. It wasn't the fuel, unless the gas he bought was very low quality.

 

[Mikael]

An octane difference from 89-91 will not cause a backfire such as the one described. his bike should be just fine and dandy on 89 unless he had the ecm flashed. If anything, there would be a light detonation at very high rpms and full throttle- not a backfire.

 

[Stang Man]

My Boss's 03 ZX6R requires 91+ octane, theres even a sticker on the gas tank...

 

[freeride]

He told you a backfire spun the rear wheel?

Never let this man work on your motorcycle. I would suggest that you maybe take EVERYTHING this guy says about bikes with a grain of salt.

 

[brentsR6]

i seriously doubt that you putting 89 in his tank caused the incident

 

[Grashopr]

A bike doesn't all the sudden build horsepower when it backfires...if I ready your post right, your friend is trying to blame you for putting lower-than-required-octane-gas in his bike, which caused a backfire, which caused a surge in power around a corner, which caused the rear tire to accelerate SO fast that it lost grip, which caused him to crash the motorcycle. If that's correct..read on. If it's incorrect, please disregard the following:

A backfire (out of any motor..car, airplane, lawnmower) does not cause the motor to have a power increase. In Fact, a backfire is actually caused by the motor's feul system being too Rich, or having too much fuel. The standard amount of fuel-to-air mixture is called the Stoichiometric Ratio (Stoich.) which is 14.7 parts air to one part fuel (if I remember right). Remember, you need three things to make a "bang" with gas: Air, Fuel and a heat source hot enough to ignite the mixture. The Stoich ratio means that, given equal chance to burn..ALL of the fuel will be burnt, without any being wasted, and without any air being wasted.

The different octanes in fuel only help the gasoline keep from detonating under higher pressures (the pressure is measured by the compression ratio on a 4-stroke motor...well..on most all types of motors). The higher the compression ratio, the higher octane rating you need to keep the fuel from pre-detonating (detonating because of extreme pressure, before the spark plug has a chance to fire). Most modern day motors dont need anything more than 87 octane unless they are running above 10.125:1 compression ratio (if I remember right). Above that ratio, the need for higher octane gasoline becomes NECESSARY. Many manufacturers recommend that higher grade gasoline is used for "insurance" against damaging pre-detonation, but some also recommend higher grade gasolines for the pre-detonation uses in most higher-grade gasolines (cleaners for the fuel injection systems and better filtered fuel to keep the environment happy). Octane ratings have VERY little to do with a motorcycle backfiring, especially a carbeurated motorcycle. The BRAND of gasoline can sometimes contribute to a motorcycle (or any engine) backfiring and acting strange because of inert materials that should not be in the fuel, but are put in as "filler" for lower-cost distributors. These fillers can cause many strange reactions including backfiring, pre-detonation, and multiple different carbeuration problems over time.

What causes backfiring is if your motor has TOO much fuel, there isn't enough air to combine with all of that fuel, so the extra fuel will be released out of the exhaust without being burnt. Once the fuel gets into the exhaust pipe..at some point, it's going to find more air outside (either a pinhole in the exhaust, or as it exits the end of the exhaust pipe/can). If there is a suitable heat source (a hot exhaust pipe), and enough fuel left over to get CLOSE to Stoich., the extra fuel will ignite, making a wonderful "BANG"...however, this explosion is no where NEAR the piston, so it cannot help push the piston down...which is where you get power from.

So the claim that his motorcycle all the sudden JUMPED out from under him as it backfired is pretty tough to believe. There is as good of a chance of the backfire lighting the rear tire on fire and him sliding out of control on the molten rubber....also nearly impossible. I'd say give your buddy enough money to refill the tank with "his" octane of gasoline and call it even.

The 'hopper

 

[Anti-Hero]

Grashopr - fantastic explination! A+

Lightweaver - You're "friend" is trying to blame you for his f--kup. And that is BS.

 

[dennisred69]

my yzf says 91+

 

[fierospeeder]

too lean will cause backfire and a few other things

 

[freeride]

too lean will cause backfire and a few other things

As I think has come to be expected of you...wrong.

Too rich. As 'hopper said, more fuel than can be used is pushed through...unburnt fuel reaches exhaust pipe...mixes with air...BOOM. Nowhere near the piston...

Read his explanation in the middle of the first page, it's very simple. You're completely ACROSS TOWN from right.

 

[chesus]

i used to run 91 in my r6 and switched to 89 w/out any problems.

what i bet happened, was it started to backfire and hesitate and he gave it more throttle so when it stopped back firing and had its normal power he was giving it too much and caused a spin

i have had backfiring happen in the middle of a turn twice and i don't know why (when i used to run 91 too) , it is pretty scary tho

 

[bartzx9r]

i have always used 93 octane in my bike with no problems

 

[Grashopr]

Lean??


From SRO Automotive Dictionary
backfire: [1] Passage of unburned fuel mixture into the exhaust system where it is ignited and causes an explosion (backfire) prematurely. [2] Sometimes ignition takes place in the intake manifold by a flame from a cylinder because the intake valve leaks. Burning of the fuel mixture in the intake manifold may be caused by faulty timing, crossed plug wires, leaky intake valve, etc. [3] A welding term referring to a short "pop" of the torch flame followed by extinguishing of the flame or continued burning of the gasses

lean:
[1] A weak air/fuel mixture that has less fuel to air ratio. The opposite is rich.

rich mixture:
A mixture of air and fuel in which there is less air and more fuel. See air-fuel ratio. The opposite is lean mixture. Also compare stoichiometric ratio. A slightly rich mixture generates the maximum achievable engine power, an overly rich mixture causes spark plug fouling, loss of engine power, and excessive fuel consumption. Any rich mixture increases exhaust emissions.

From AutoGlossary.com
Backfiring:
1. The pre-explosion of an air/fuel mixture so that the explosion passes back around the opened intake valve, through the intake manifold, and through the carburetor. 2. The loud explosion of over-rich exhaust gas in the exhaust manifold that exits through the muffler and tailpipe with a loud popping noise.

Lean does nothing but generate more heat, possibly melting the piston's face, allowing it to heat to the point where it becomes brittle, where a hole can be blown through. Lean can also heat a valve's head to where it can distort, not seating correctly when cold, causing "smoking" and poor compression (if I remember right).


The 'hopper

 

[Grashopr]

i have always used 93 octane in my bike with no problems

Running a higher octane in your bike can do very little (to a certain point..dont go out buying some "Red" VP 106 octane) to harm any motor, it can only help. You are paying the higher price for the better resistance to pre-detonation, but you are also paying for cleaners and additives that arent' found in the cheaper fuels that, although aren't NECESSARY, always help. Now, if you go overboard and start running 125 octane super-duper fuel, and you have a newer bike that uses an oxygen sensor, or has a catalytic converter (I know..sounds dumb, but some newer Hondas actually have them, and I think most CA spec bikes do), the extra additives in the 'go fast' fuels can coat an O2 sensor and cause it to read incorrect, which will,of course cause your bike to run either richer or leaner (I haven't the brain power right now to think that hard), causing even MORE problems. The fuels can also plug a catalytic converter QUICK. This is why you dont run AG-fuel in your late model Mustang. It's JUS'NO'GOOD-MAN!!. Anywho...run as high of an octane as they'll sell you at the pump without worries. The extra money your spending is merely insurance against fuel-injection-system problems down the road.


The 'hopper