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

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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

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and NOT modified anything else like your Intake Snorkel or your Exhaust Pipe, you'll have to ask your Dealer for a Newer Version of the FI Software to fix your bike.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.

Fixing Backfiring

1. Idle Screw - Can be Adjusted - Classic F only

2. Idle Mixture - Can be Adjusted - Classic F only

3. Exhaust Gaskets - Can be Replaced Classic F and GS/Dakar

4. Has your bike been rejetted/New Pipe/Open Airfilter? Poor jetting can ruin a well-running bike.

5. Carb Intake Rubbers - Can be Replaced - Classic F only

6. The Airbox to Exhaust Header Line - Can be "Fixed" Permanently - Classic F only

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 beast.

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 gases inside.

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."