F650 Backfiring FAQ
compiled & edited by Kristian #562
Mike in SC, BCWill, Flash #412, HsN, Randy #748, Lyndon, Kristian #562, 22/11/01
Please read the Disclaimer before attempting any work in this FAQ.
Last Updated: 19 March 2007, by Winter #1935
For other related FAQs:
There are quite a number of different causes for
backfiring. What this FAQ will try and do is to help you isolate the cause of
it. There are a number of solutions for the Classic F which cannot be used on
the GS/Dakar, simply because the GS/Dakar doesn't HAVE a carburettor.
So that means if you have a GS/Dakar you're lucky, because
there are fewer causes. It doesn't mean it'll be any cheaper to fix though.
What causes Backfiring
- "Deceleration Backfiring" is the most common
backfiring that occurs when you change down and throttle off. This extract
from the Virago Site explains it succinctly.
"In general, backfiring on deceleration (as opposed to acceleration) is
generally caused by a lean condition in the pilot circuit. What happens is
that the mixture leans out enough to where is fails to ignite consistently.
This, in turn allows some un-burnt fuel to get into the exhaust pipes. Then
when the engine does fire, these un-burnt gasses are ignited in the exhaust
pipe, causing the backfire. Newer Virago carbs have an 'enricher circuit'
which cuts in on deceleration to help this problem. Earlier carbs do not have
this. I would say check the pilot circuits, and set them a bit richer. It is
hard to do this right without some equipment to test where you are.
Aftermarket pipes often increase backfiring, probably because they may the
bike run leaner."
A more detailed
explanation can be found below.
- Some riders have also experienced Backfiring on
Up-shifting. An exhaust gasket (item #3 below) is the most likely culprit,
rather than items 1-2 or 4-5.
|Hang on a minute, all but one those fixes are for the Classic F?|
|Sorry, if you have checked your Exhaust Gaskets and NOT
modified anything else like your Intake Snorkel or your Exhaust Pipe, you'll
have to ask your Dealer for a Newer Version of the FI Software to fix your
bike. There are no idle mix screws or idle adjustment knobs on the GS/Dakar.
1. Idle Screw - Can be Adjusted - Classic F only
- Some folks cure backfiring by turning up the idle
slightly. This is the brass knob on the end of the black flexible thing hanging
out the bottom of the carb set. If you just have mild backfiring, you can try
this. Many bikes never run at the low end of the recommended 1200-1400 Idle rpm
range in BMWs Manual. Refer the Idle Mix Screw FAQ.
2. Idle Mixture - Can be Adjusted - Classic F only
- No this is NOT the same thing as the IDLE. The Idle Screw just limits the
minimum throttle when you take your hand off the throttle. The Idle Mix Screws,
and there are TWO of them, one for each Carb, actually controls the Gas/Air
Mixture at Idle. Obviously this is not applicable to the GS. For the
location of the Idle Mix Screws and useful tools to turn them, see
- Most riders find that enriching the idle mixture
by backing off the screws by 1/8 to 1/2 a turn will eliminate the popping.
Enrichen as little as necessary, work in small increments, and test the results
before backing out the screws another 1/8 turn.
- For more information on the Idle Mix Screws the Carb Cleaning FAQ.
3. Exhaust Gaskets - Can be Replaced Classic F and GS/Dakar
- These seem to be one of the most frequent cause of
Backfiring Problems and an item that has been seen to fail quite often.
- "Recently went to replace gasket where header pipe meets
cylinder head. After pulling left header pipe I found culprit of noise, blown
out header pipe to collector gasket. This was a corrugated sleeve that failed
under the clamp. Replacement gasket is no longer corrugated. Added bonus was the
commonly reported backfiring problem, I thought I had to live with, is now
completely gone. Try this $11.00 fix if your bike is backfiring."
- There are actually FOUR Exhaust Gaskets (THREE on a GS) on the Stock Exhaust Pipe:
- One on each of the Exhaust manifold connections to the
Cylinder Head, which
look like this. This is a
close-up of one
of them. It is the Silver Ring, which is very soft (crushable) metal
sandwiching a Gasket Material. (Note: There is only one on Fuel Injected
- One where one of the Exhaust Manifold pipes meets the
Collector pipe to Cylinder Head,
which look like this.
- One where the Exhaust Collector Pipe meets the Exhaust
Proper, i.e. at the crossover pipe.
- VERY often you can fix the problem by simply tightening the Exhaust Header/Cylinder Clamp Nuts. Here is the RHS Upper One, RHS Lower one, LHS Lower One. You can also try tightening the Exhaust/Collector Clamp.
- Testing: You can check these with a bit of smoke (like from a
cigarette) in a still area, wafting around the joints. Headers also crack and
split sometimes. Watch for the smoke being blown away by leaking exhaust gases.
- For Exhaust Gasket Replacement see that FAQ.
4. Has your bike been rejetted/New Pipe/Open Airfilter?
Poor jetting can ruin a well-running bike.
- I just went through this. I agree loose the DynoJet. I went back to
stock and have a Staintune exhaust. Check the orifices on the lower edge of
the Airfilter side Venturi on both carbs make sure they're clean. My bike
runs great. I also got rid of the pipe from the header to the air cleaner.
No more backfire on Decel. James.
- If you want to rejet, see the
Performance Modifications FAQ for
details of how others got on.
5. Carb Intake Rubbers - Can be Replaced - Classic F only
- Testing: For the
carb intake rubbers, you can an unlit propane torch, just barely cracked
open, and see if the idle changes when the gas gets sucked in. Don't get
carried away or you will flame the bike. Less messy than using WD-40. See the
Carb Cleaning FAQ for details on removing your
carbs. The extra air from the split boot will cause the popping.
6. The Airbox to Exhaust Header Line - Can be "Fixed" Permanently - Classic F only
- Remove the line from the bottom of your airbox to the right
exhaust header. This is the steel/rubber/steel piece that follows the right
header pipe rearward. Plug the airbox (better yet, pull the air filter and pull
out the exhaust re-circulation valve), cut the pipe behind the exhaust end
connection, and replace it on the threaded piece it came off of with a 3/8" ball
bearing inside it. That ended my backfiring.
- Check your Air Injection Hose Routing if you've done a
- The mis-connected carb vacuum hoses can also encourage backfiring - the
right side carb is sucking too much air thru the canister and up the drain
tube, right? That means the carb fuel mixture is also slightly leaner, more
prone to backfiring. Putting the hoses back correctly may help with the
backfiring without having to adjust the idle mixture screws richer. The
mis-connected hoses can also degrade low end response slightly.
- You might want to try closing your mixture screws a half turn (or so) each before you go remove the line. That's faster, cheaper and LOTS easier.
More Details on Carb-Related Backfiring
By Ted Harris. From the Virago Site.
"When it comes to machinery I am
a perfectionist. A machine, must be carefully maintained and assembled in order
to perform to its full design potential. I own a 1983 Virago XV920K with which I
had an annoying and persistent backfire every time I decelerated. Other
motorcyclists and mechanics with whom I spoke were most sympathetic when
discussing this problem but invariably dismissed the problem as something to be
expected when using low restriction, after-market pipes.
Now, an engine is not supposed
to backfire and, when it is persistent, I am not convinced it is harmless to the
engine. If there is an explosion in the exhaust pipe when the cylinder is trying
to exhaust its spent combustion products a back pressure is created which
interferes with the next cycle. This could result in incomplete extraction,
irregular charging with the fresh mixture, and overheating. In addition valves
are not designed to seal against pressure from their backsides and a broken
valve head rattling around in the combustion chamber at 7000rpm engine speed is
not something I am desirous of experiencing. As a side note, I got a real
appreciation for the forces involved in the internal combustion process one day
when I had removed one of the spark plugs in order to clear raw gas from the
cylinder. I had forgotten to disconnect the igniter and, to my very great
surprise the engine started and began to run! WOW!!! the forces involved can
only be described as awesome. And remember the open cylinder was not even
firing. The force of the exhaust pressure alone, at idle speed, was responsible
for this incredible display. I am still disquieted by the act of straddling this
A man named Daniel Bernoulli
(1700-1782) was the first one to describe the effects of a Venturi. Basically
stated it is this. As the speed of a fluid increases its pressure decreases.
And, a fluid (exhaust gas is a fluid) when forced through a restricting
orifice/Venturi (an exhaust pipe is a restricting orifice) increases its speed
and creates a vacuum.
This principle has a dramatic
demonstration any one can perform. Grasp an ordinary sheet of paper by two
contiguous corners and let it droop down in front of your mouth. Now blow across
the top of the paper and the paper will rise toward the vacuum created. This
effect is what makes airplanes fly and motorcycles backfire. It application is
truly profound and if you have the time I highly recommend a reading of the
Bernoulli family biography. No slackers in the brains department these people.
Its not the after market pipes
causing the backfire you see. Its the air leaks caused when the gaskets weren't
replaced and other leaks sealed when they were installed. Fresh air is drawn in
through those tiny holes and provides sufficient oxygen to ignite the unburnt
Recently I removed the rear
wheel from my motorcycle and had to exert pressure on the front pipe in order to
extract the axle bolt. Upon reassembly the backfire had returned, though not to
the outrageous extent as before. Attempting to deny that my old problem was
incurable after all.
I tried to convince myself it was due to a heightened
sensitivity on my part.
Yesterday I received the new
shock assembly I had ordered and the rear wheel was removed again. Upon close
inspection the evidence was unmistakable. Soot marks were present not At the
front exhaust flange but on the #1 (rear) cylinder downpipe where the slot is
cut for the pinch bolt. Taking great care in the reassembly I removed the old
gaskets and sealed the slot with a paste made for the purpose and obtainable at
any well stocked automobile parts store. The backfire problem is cured and there
is not the slightest bit of doubt in my mind that the cause was as I have
explained. One caveat is in order here, Do not waste time trying to listen for a
leak in the exhaust pipe with the engine running, You won't hear it. It's too
small. If the exhaust pipes have ever been removed or replaced of if the exhaust
gaskets have been stressed in any way, replace the gaskets."