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Old 08-20-2012, 08:30 PM   #1
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Question Causes of backfire

Just curious, what are some of the common causes of backfiring. Does backfire happen at low idle speeds or after starting up the bike.?? Just mechanical issues I like to keep a watch out for.
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Old 08-20-2012, 11:34 PM   #2
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Backfire through the carb or out the exhaust? Differing causes / differing effects. Which malady do you have or are you just anticipating one?
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Old 08-21-2012, 12:49 AM   #3
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Backfire out of the exhaust is caused by running too rich. I've also found it can be caused by the idle screw being set too low, causing the bike to run rich at idle. Lots of guys like that low, loping sound but the engine is tuned for 1,000 RPM's at idle, not 500 or 600.
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Old 08-21-2012, 01:46 PM   #4
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Here's some info from the Cobra website.

Backfiring in the exhaust system of your motorcycle is created by the same three necessary elements for combustion inside the engine; oxygen, fuel and ignition. Because the heat of the exhaust will supply the ignition, let's discuss the other two. The backfiring during acceleration is a serious engine related (cam timing, bad valve seat seal, etc.) problem and can not be fixed with air/fuel mixture changes.

Backfiring on deceleration is much more common and often occurs when exhaust and fuel mixture changes are made. This is what we are going to discuss.

All fuel injection systems turn off the fuel during deceleration when the throttle is fully closed, then turns the fuel back on down to idle once the engine reaches a predetermined r.p.m. This is a most important indicator of what is happening. If a motorcycle backfires down through the entire r.p.m. range during deceleration, then most likely the injectors are not turning off as intended, and this indicates a possible issue with the fuel injection system. There are numerous possibilities, including but not limited to; improper Throttle Position Sensor (TPS) setting, bad cold start and/or temp sensors, throttle cable adjustment, etc. Visit your dealership or shop manual and fix this problem or problems before proceeding. Remember that your bike may well have had this pre-existing problem; adding the less restrictive exhaust or tuning fuel may just make it more noticeable.

Now, if backfiring is only present in the lower r.p.m. range usually the lower third of the full r.p.m. range, that is confirming that the injectors are coming back on and now that fuel is igniting in the exhaust. But, there has to be oxygen (fresh air) present to create that ignition.

There are numerous ways oxygen can end up in the exhaust system, especially if you are riding a large V-twin motorcycle. On deceleration, with the throttle plate closed, the pistons are still pulling a large vacuum that is not being fed by the throttle opening, so any tiny opening at gaskets, seals, joints, slip fits, must be suspect. Even though exhaust isn't coming out of a lose connection, it doesn't mean air can't be drawn back through under high vacuum.

So, first, what have you changed on your motorcycle? This must be the first place you look. Also, the exhaust system itself may be drawing fresh air back into the exhaust. Very short and/or no baffles can allow exhaust to rush out but fresh air to rush back in, along the sides of the inside of the pipes. This is known as fresh air inversion, and is common with very short exhaust pipes, large diameter straight pipes and/or no baffles. Also a one-into-two rear head pipe design on a stock Harley dresser allows fresh air to be drawn up through the left hand muffler.

The intake side also needs scrutiny; even if there have been no modifications. Fresh air can be introduced by intake manifold leaks, or in many Japanese bikes, exhaust gas emission systems.

Harleys have a rubber intake gasket that can become overheated and damaged and the slightest movement while changing an air cleaner system can open up a leak. The Japanese twins use reed valves or mechanical one-way valves that can get stuck, allowing fresh air to get past and into the exhaust. Some aftermarket air cleaner assemblies donā't deal effectively with converting the original EGA hoses and valving.

Based on the above data, there are some simple tests you can perform to help you solve your motorcycle's backfire issues:

1. Once fully warmed, decelerate the engine from a high r.p.m. and listen for where the backfiring occurs. Fix any EFI issues first.
2. Add fuel to the cruise range, then ride the bike and see if there is a change. Worse means a big air leak, usually from the intake side. Better means a small air leak, usually from the exhaust side.
3. On Harley Baggers with dual slip-on exhaust, try plugging off the left muffler.
4. On very short or open pipes, try baffles or increasing the baffle effect.
5. On bikes with exhaust gas recirculation systems, try plugging off the feed tube to the system.

A step-by-step logical approach should help you solve and eliminate backfiring.
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Old 08-21-2012, 05:29 PM   #5
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Does anyone here think it could also be air filter oil in the cylinder? I'm working on some problems of my own. OEM exhaust, didn't screw with the fuel/air ratio, OEM air filter and air box. The only thing I've really done differently is switch air filter oils (in addition to the bike sitting for a few weeks). Not meaning to hijack the thread, and I'm not looking to troubleshoot on this thread, just wondering if oil in the chamber could cause backfiring through the air box.
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Old 08-21-2012, 09:24 PM   #6
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Greetings everyone,

Thanks all of you for the info. Bear2123 in the last couple of weeks the bike did backfire a few times. It only seemed to do it as I was decelerating from 3rd gear down through 1st gear. Just small backfires. I checked my battery connection and all was fine. Did have the valve clearance check done the other day. Miles at the time was 5,930. Sure enough the valve clearance was getting tight. On the ride home the bike purred like a kitten.

chi-rider, thats a good article you posted very interesting material!
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Old 09-02-2012, 04:36 PM   #7
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Very informative thread,great work.Thought I'd continue this thread rather than opening a new one.Hope no one minds.
Needless to say, I find myself here because of a related problem.Just finished adjusting the valves at 14,000km(approx. 8500 miles)on my VN800A.Front jug needed,rear jug no. Only put maybe 8 miles on it so far,and it runs like a champ mostly.But I get a sporadic backfiring on acceleration.Only happened for about 7-10 seconds on 2 occasions,once revving it when it was first put back together,and once in first gear on the road.
Never had backfiring out the pipes before,just sometimes puffing through the carb just starting out in first.Only change other than the valve shims was I left off the upper breather box and readjusted the air screw to 3 turns out from 2 3/4 turns.
Anybody have ideas?
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