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Starting Member
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Last Login: 5/17/2006 4:49:36 PM
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I have an '03 Road King that, until a few days ago, was completely stock. I bought and installed a Vance & Hines fish-tail slip-on muffler, screaming eagle air filer, and Power Commander. Install went well. I downloaded the best mapping from Power Commander I could find (with their assistance). The download "took." I also turned on the RevExtend, which allows me to get an extra 1000rpm before I have to shift to the next gear. The folks at Power Commander suggested that, even without taking my bike to the dealer and having them put it on a Dyno, I should get 95-95% of the total expected performance. They felt taking it to the dealer would be unnecessary, since I only slightly modified my bike.
Problem - my bike backfires (pops) on decel much more than it did when I had a stock bike. It used to do this a little, and of course it wasn't as noticeable when I had stock pipes, but the backfiring I am experiencing seems unreasonable. A tech at Power Commander modified my mapping (twice) to try to eliminate the problem. I'm wondering to what degree I can expect to eliminate backfiring.
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Senior Member
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Last Login: 7/6/2008 10:52:54 AM
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I won't even pretend to be an expert on this but here are a few things to chew upon.
You aren't really talking about a back fire. It's afterburn. a backfire is when flame shoots out of the carb. Afterburn is combustion in the exhaust pipe and can have many causes. A leak between the head and exhaust can contribute. You can make it happen by shutting off the ignition and coasting with the throttle open, then closing the throttle and switching the ignition back on. Not really recommended as this can cause engine and muffler damage. I think that is because you are purposefully moving a load of unburned fuel/air mix into the exhaust and igniting it as opposed to the small bit that ignites in the usual pop you're getting on engine braking.
I think a dyno run is going to focus on things where you are at full throttle and at acceleration. On decel different things are happening with respect to the carb and the engine.
On acceleration you are opening the throttle which either directly opens the carb by lifting a slide or opening a butterfly vavle or you are indirectly lifting a slide by opening a butterfly valve in the carb throat with the intake vacuum lifting the slide via a diaphragm under vacuum.
On decel, especially snapping the throttle shut you are missmatching the throttle opening to engine speed. The engine is nothing more than a pump and at this point is moving fast enough to pump a given volume. However the throttle is closed and this influences the way fuel and air are mixed and delivered to the combustion chamber.
I know a number of engines are fitted with air cutoff valves to eliminate afterburn on decel. I dunno if your modified exhaust and ignition mapping can eliminate the popping without one of these and they are fitted to the carb on a portion that is part of the casting that is the carb. I don't think you can just slap on an air cutoff valve.
Hopefully someone else will come along with some much more sound information, but I think you may be stuck with the popping to some degree as it was present before. It may also be louder due to a more open exhaust system not muffling it like the stock system did.
Dirt bike forum in need of members (free)
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Senior Member
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Last Login: Yesterday @ 4:51:02 PM
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Not to be sarcastic....but, boy, I'm sure glad I got a carburator....just for us electronic dummies.
If you had a carb, instead of EFI, I'd say, with some authority, that you're now running too lean.
You've increased the amount of air going INTO the motor and probably decreased the back pressure of the OUTGOING exhaust.
So,if you had a carb, I'd say go up one or two sizes on the main jet. Run it for about a mile at 60....then kill the motor and coast to a stop. Pull the plugs. Hopefully, they are now brown...not white. Keep increasing the jet size until the plugs are brown.
That's the old-fashioned seat-of-the-pants method...always worked, always accurate.
However, with EFI....I sure wish I could be of more help.
Any electronic EFI tune-up weenies out there?
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Junior Member
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Last Login: 11/4/2008 8:16:40 AM
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This very same question was raised by a guy in the "Downtime Files" of MCN a few years ago. I wish I could remember exactly how long ago, but I would say maybe 3 - 4 years ago.
One thing I would recommend physically checking on the bike itself is for a leak between the head and exhaust pipe as Bwana suggested. I had a '92 Shadow 1100 that began "afterburning" when I had removed the exhaust pipes for engine work and then reinstalled them. New copper exhaust gaskets and properly torquing the acorn nuts eliminated the problem completely. I do remember the tech guy recommending checking that in the "Downtime Files" response.
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Average Member
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Last Login: 10/16/2008 9:44:53 PM
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| Barring the obvious potential exhaust leak, you're running too rich. You've opened up the exhaust and intake, and are accentuating a process called reversion where extra fuel is being pulled through the chambers on the exhaust/intake stroke on decel. You can try a leaner map but you WILL need to read the plugs to verify proper mix at ALL rpm/load ranges. Or, spring for a dyno tune from somebody that understands the difference between a daily driver and a drag bike. Elvis is in the building. For now anyway....
If I could walk like that, I wouldn't need a doctor.
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Junior Member
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Last Login: 11/11/2008 9:02:29 PM
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cwolf,
We'd like to use your question in the Downtime Files section in the upcoming issue of MCN. Please e-mail me at fsantos@bowtieinc.com.
Cheers,
Franke Santos
Associate editor
Ride safe,
Franke
Saving for a Ninja 650
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Starting Member
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Last Login: 7/27/2007 2:04:27 PM
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| Check the intake gaskets (sometimes they only leak when really hot), Head pipe gaskets and muffler clamps most dealers change them every time they are removed. I sounds like it needs a little more fuel. Does the popping start at just under 2000 rpm if you have a tach? Go to techlusion.com and see why and how its explained. hope this helps I just got done with the same problems on my FLHXI and once dialed in no more decel popping.
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