Deceleration Backfiring

Below are actual problems submitted by Virago Owners and Club members. Read through them and see which one matches your particular problem.  Feel free to send-in your problem solving experience.

 


Situation #1

I am a new member of the VOC and need some advice on a problem my 1992 750 cc is having. In the two and a half years I have owned it (bought new) I have enjoyed almost every one of the 39,000 miles I’ve put on it. But in the last 4,000 miles it has a tendency to do a compression backfire when I have been holding the throttle at a steady pace and then let off fairly fast. It doesn’t do it all the time, so when it does it startles me. It sounds just like a rifle shot. I have taken it to the shop. My mechanic readjusted the carbs saying that they can’t be adjusted out anymore Next time they will need to be replaced. It is still backfiring. He now says that it probably needs a new “emission valve. Has anyone else had this problem, and what all does it take to change one or do you think that it might be something else? My husband does minor work on our bikes and would do it for me if it would not involve pulling the whole engine.

(The Answer as scanned from the VOC Newsletter Mar/Apr ’95)


      IN REGARD TO THE COMPRESSION BACKFIRING PROBLEM WHEN DE-ACCELERATING, I THINK I MAY HAVE A CURE. A COMBINATION OF INSTALLING HIGH QUALITY SPARK PLUGS (SUCH AS SPLIT FIRE) GAPPED METICULOUSLY, CLEANING THE PLUGS AND CHECKING THE GAP EVERY 1,000 MILES, AND USING AN OCTANE IMPROVER IN THE GAS HAS VIRTUALLY ELIMINATED MY PROBLEM. I SAY VIRTUALLY ELIMINATED AS I WILL STILL BACKFIRE WHEN QUICKLY BACKING OFF THE GAS IF I’M AT HIGH ALTITUDE. MOUNTAIN ROADS ABOVE 7,000 FEET ARE STILL A PROBLEM BUT THIS IS UNDER-STANDABLE WITH THE THINNER AIR AT ALTITUDE.


Since the publishing of this recommended fix, several other members have asked for solutions to backfiring problems.  Following are some of the added recommended fixes and actual solutions:


      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 consistenty. This, in turn allows some unburnt fuel to get into the exhaust pipes. Then when the engine does fire, these unburnt 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 don’ 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. Some backfiring on older modles is par for the course, so these owners should learn to enjoy it.
Ride safe,
Mac McCurdy.


Situation #2

As of yesterday, the backfiring problem has been resolved. Guess what it was……the spark plugs! When I had the 600 mile service, the dealer installed short-reach plugs. The XV1100 takes long-reach plugs. Can you believe it!! Happy now, although a bit dishearted about the dealer….Joe Addison.


Situation #3

Found out that the problems that cause the backfire are definitely located in the area of the emission system (air induction system). The air induction valve is the culprit. This is an expensive item. The mixture control valve, which is made of plastic runs $83.00 here in Wichita Falls. In Ft. Worth, $127.00. However, it was working ok. The problem is the air induction valve which is made of some kind of metal. The cheapest solution is to by pass the emission system. This is done by blocking off the the vacuum line from the reed valve to the air induction valve. A local motorcycle mechanic stated that he does this quite often by placing a thimbel in the line between those two valves. He stated that the thimbel is the right size and is cheap. I tried it and it worked. The backfire is gone, the engine runs fine and the spark plugs are the color they are supposed to be. The temperature here today was 105 degrees with a heat index of 110 degrees. I was afraid at first that the mixture was going to be too lean. The bike ran fine. This worked for the 1984 and 1985 XV1000, it should work for the other bikes as well. The Clymer Yamaha manual for XV535-1100 Virago 1981-1995 displays the air induction system on pages 138-139. Good luck with it, Rick


Situation #3

I own a 1983 virago xv920k with which I have had the decel backfire problem. The mixture control valve works exactly like a flush valve for a commercial toilet! Intake manifold vacuum acts on a diaphragm which in turn actuates a plunger allowing air to be inducted into the front #2 cylinder. This causes a drop in vacuum and would allow the plunger to immediately close the system to air induction were it not for the fact that there is a small orifice separating the high vacuum chamber (which actuated the plunger) from the now lowered intake manifold vacuum. This orifice slowly allows air to equalize the pressure/vacuum on both sides of the plunger diaphragm and closes the intake manifold to air induction.
I suspect that this allows a much leaner mixture to be introduced into the exhaust manifold. Too lean, that is, to be ignited by the hot surfaces therein. This is the only reasonable guess I can make as to why the mixture control valve only acts upon one cylinder. Does any one else have any theories? I finally cured the decel backfire by installing new exhaust manifold gaskets which apparently were letting oxygen rich air to be drawn into the pipes and igniting unburnt gases introduced by valve overlap. I talked with many people who had this problem with after market pipes (which I have) and I wonder how many of them remembered to replace the gaskets?
I think I have found a reliable way to test the mixture control valve. It can be tested with a vacuum pump. QUICKLY draw a vacuum on the small pipe while blowing through the end of the valve (where air is drawn in) seal the valve tightly with your lips. Air should be expelled through the large pipe for a short time and then stop quickly.
Ted Harris

 


Situation #3

      I don’t care how good a rider you are, how cautious you are, or how long you have been riding. Still, simple, stupid things do occur.
I provide Virago fixes for other riders and sometimes I can’t seem to fix my own bike. On a recent ride, my bike was really starting to backfire . . . not just once or twice, but on every little hill where I would decelerate. This went on for several miles, then I noticed that upon accelerating, my bike would gasp and choke on the low end of acceleration. Also when I stopped, my rpm’s on idle would vary and not stay constant at 900 rpm as it normally does. I had just set the valves, balanced the carbs and changed the oil and the bike ran great on three previous rides.
After 62 miles, I finally found the problem:
My bike does not take much choke in the morning to get it started and as soon as it is started, I cut the choke down to about half choke. Then, once on the bike, I run the choke down the rest of the way. This particular morning, after starting the bike and cutting the choke back half way, the phone rang . . . a rider cancelled out of the ride. No problem!!! Jumped on the bike, went to our meeting place. Fifteen riders showed and off we went.
That slight diversion in my normal starting routine caused me to not close my choke all the way and it stayed open half way for that first 62 miles. What a difference in the performance once I closed the choke . . . DUMMY !!
Oh! By the way, better gas mileage, too.
Joe Beautz


Situation #3

Editor’s note: Ted has requested that this information remain proprietary for use by Virago owners only and not to be used for commercial purposes.

      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 motorcyles 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 wern’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 reassmbly 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. Its 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.
Ted Harris




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