I know this doesn't belong in the marketplace and I know the moderators will delete it shortly but I thought I would post it here as well as in the technical documents section in the hopes that more people would see it.

I have been getting a lot of inquiries lately about engines backfiring or "Popping" on shutdown.

Here is some information regarding that issue that I hope will enlighten us all.


Backfiring occurs when the fuel mixture is too lean or too rich and a misfire occurs caused by incomplete combustion in the cylinder thereby dumping an unburned air/fuel mixture into the exhaust system where it is ignited causing the backfire.


Backfiring in the exhaust system can damage an engine if it happens when the exhaust valve is open.......it can destroy an engine if it is severe enough.

Big singles are prone to backfiring and REVERSING DIRECTION on shutdown (when run too rich or lean) because they don't have one or more another pistons whose inertia helps keep the crank rotating in the proper direction.

Big twins like old Harleys were also prone to this problem due to their crankshaft phasing (I won't explain why crankshaft phasing makes a difference here because that will take too long).

When this backfire happens and the exhaust valve is open the backfire in the exhaust system enters the combustion chamber Via the open exhaust valve and pushes the piston back down in the cylinder in the WRONG DIRECTION !

On these big Suzuki singles the starter cannot rotate backwards but when the engine backfires on shutdown and rotates in reverse it rips the starter from it's mounting bolts because there is a solid connection between the starter gear and the flywheel gear Via the solid idler or intermediate gear.

This situation can also damage the flywheel gear among other horrifying things like destroying the flywheel/magneto cover and/or the engine case.

This is why Suzuki put the "Starter Limiter Gear" clutch mechanism in the 1995 and later engines thus allowing the engine to reverse direction when backfiring on shutdown but disengaging the starter gear from the flywheel gear by way of this "Clutch" mechanism.

This mechanism is simply several friction discs separated by metal plates riding on a shaft with a one way cam mechanism that allows full clutch engagement on one direction but slips in the other (reverse) thus preventing the engine from trying to reverse the starter when it "Pops" backward.

The older 1986-88 engines can be retrofitted with the starter safety mechanism by changing the solid idler gear for the "Starter Clutch mechanism" and changing to the 1995 and later flywheel/magneto cover.

The shaft for the "Starter Clutch mechanism" is longer in the 1995 and later engines and the shaft support in the cover is shorter in the earlier 1986-88 covers so you need the 1995 and later cover when using the"Starter Clutch mechanism" .

Here is a quote from the web site http://cyrilhuzeblog.com/ which will help understand backfire on shutdown process.


""A backfire is caused by the buildup of un-burned gasoline in the exhaust header and pipe system. Once the gas has built up to the right mixture with air, it explodes, making a loud bang. A visible flame may momentarily shoot out of the exhaust pipe.

It can happen when you start your motorcycle, for example if you stopped the bike with high rpm’s and extra gas was left in the carburetor, when you accelerate or let off the throttle. Typically backfiring results from various malfunctions related to the air to fuel ratio.

A backfire can occur in carbureted engines that are running lean where the air-fuel mixture has insufficient fuel and whenever the timing is too advanced. As the engine runs leaner or if there is less time for the fuel to burn in the combustion chamber, there is a tendency for incomplete combustion.

The condition that causes this is a misfire. The result of a misfire or incomplete combustion is that unburned fuel is delivered to the exhaust where it may ignite unpredictably.

Another backfire situation occurs when the engine is running rich, meaning with excess fuel, and there is incomplete combustion, with similar result. Backfire is rare with fuel-injection and computer-controlled fuel mixtures"".

- See more at: http://cyrilhuzeblog.com/2010/06/12/glad-that-you-asked-why-do-motorcycles-ba...


FYI.....anybody who has even tried to start a big single or twin back when they had kick starters and had the engine "POP" and kick back against your leg will appreciate why almost all modern engines now don't have them......just imagine this.....you push the kick start lever all the way down and your leg is fully extended and your knee is locked straight and all your weight is on that leg and then the engine backfires or "Kicks back"............OUCH!!!!

I don't agree with everything you stated.....and I believe you have not separated an "afterfire" that occurs in the muffler from a "kickback" that occurs in the cylinder.

The afterfire that occurs in the muffler at shut down occurs when you turn the ignition off, and the engine continues to rotate and pump the fuel/air mixture into the exhaust system.....where the hot header/muffler elevates the temperature and ignites the fuel/air mixture.  The resulting explosion is either a "poof" or 'pow" - depending upon the type of muffler and fuel/air mixture.  Bikes that are running a lean idle tend to make a louder explosion than those running one that is more rich.  My bike seldom makes this afterfire, and it if does it is only a small "woof".  The only time it makes the noise occurs when the weather is hot, and I have been riding at speed for a while.

The "kickback" that breaks the starter gears doesn't always involve an actual explosion - coming up against the compression stroke on shutdown can cause the engine to suddenly rotate backwards on shutdown......I can often hear this happen on my bike when the ignition is shut off, and the engine is coasting down to a stop....sometimes I can hear a little whine as the piston pushes the starter backwards (the torque limiter only works when the reverse rotation makes enough force to overcome the friction in the plates).

The popping and banging that occurs when you close the throttle completely when slowing down, or between shifts is a result of the air/fuel mixture going lean when the throttle is closed.  When you are shifting or coasting....the engine has a high vacuum, and the needle jet and main jet are not flowing fuel - only the pilot jet circuit is providing fuel to the engine....and the mixture can go too lean to be ignited by the spark plug and burn in the cylinder - the result is that the fuel/air mixture can be ignited in the hot header/muffler system and make the popping/baging sound that occurs on deceleration.

Super Thumper wrote on 05/08/16 at 12:24:50:

At this point I'll have to stop you and ask why?

If the exhaust is open, it is at BDC to TDC of the exhaust stroke.  Yes there is some overlap with the intake stroke, but both valves might be viewed as not really open, but just taking up the slack or laying it down.
Two, what fuel that's in the exhaust is burned out and a backfire originates in the muffler.  The pressure pulse may actually reflect on the piston, but it's got to go thru a barely open valve (at best, the valves on a savage are barely open).

I can't see a backfire (or more correctly, afterfire) doing any thing but sending everyone into a duck and cover.