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

• Introduction
  • What causes backfiring?
• Fixing Backfiring
  • Idle Screw (Classic Only)
  • Idle Mixture (Classic Only)
  • Exhaust Gaskets (Classic+GS)
  • Poor Jetting (Classic Only)
  • Carb Intake Rubbers (Classic Only)
  • Airbox to Exhaust Header Line (Classic Only)
• More details on Carb-Related Backfiring

For other related FAQs:

• Hard Starting / Poor Running FAQ
• Carb Cleaning FAQ
• Carb Misc FAQ
• Exhaust Gasket Replacement FAQ
• F650 Exhaust Rejet Xl2K
• Jet Kit FAQ
• Fuel Injection FAQ
• Idle Mix Screw FAQ

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.

• "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.

• 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.

• 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 This FAQ.
• 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.

• 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:
  1. 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 models).
  2. One where one of the Exhaust Manifold pipes meets the Collector pipe to Cylinder Head, which look like this.
  3. 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.

• 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.

• 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.

• 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 Canisterectomy.
• 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.

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."