With Warren's bike, only ONE cylinder would backfire on decel, and it' didn't do it when it was BRAND NEW with the same exhaust setup. Likewise a leak in the exhaust system will provide the necessary oxygen in the moments before and after deleceration fuel cut engages/disengages. If you make a vehicle's pipes SHORT enough, with low enough back pressure on a bike (or car) it can suck enough air in to backfire even a fuel injected engine. At least for a while, until it runs out of oxygen stored in the media. Not only that, but any vehicle equipped with a catalytic converter is going to quietly COMBUST any excess fuel fed to it. No more blam-blam-blamming down hills on injected cars/bikes. (The throttle today will often have an idle position switch, wide-open throttle switch as well) Now with fuel injection, in order to achieve HIGHER miles-per-gallon (MPG) the ECM normally commands an engine deceleration fuel cut.įuel is completley CUT OFF while you declerate/engine brake UNTIL you get down to idle RPM, or get back on the throttle. So one tiny little piece of red-hot carbon, in the exhaust manifold, on the outside of the exhaust valve and BLAM-BLAM-BLAM.that mixture will ignite. I have even found gravel and other debris in an engine that GOT IN THERE by being sucked in through the tailpipe, as there is NO WAY it got past the air filter. Now when you're engine braking down a hill, air is getting sucked BACKWARDS in through the tailpipe. So you get an enormous amount of fuel being sucked through the engine, consequently a very rich exhaust. Just like CHOKING an engine, this vacuum causes an extraordinary amount of fuel to be sucked through the idle and primary circuits. When you let off the throttle and shut the butterflies, enormous vacuum is generated in the intake manifold. If that doesn't work, pull the headers, run a swab of abrasives through it, and that should clear the pipes.īackfiring on decel, especially going downhill in gear, USED to be common on ALL vehicles. By putting a rock candy on a little stick hanging over the end of the muffler, sometimes you can sucker the little f****** out of there and stop the blams. Gremlins in the headers are little green monster guys that love to make explosions go off when you decellerate. you need to adjust the floats a bit lower, and check the idle screw adjustment on each of the carbs, but because you have a harley, you should also check for gremlins in the headers. Get a timing light, and check it out, make sure it is right before any long trips.įIX HEAD ERUPTIONS IMMEDIATELY FOR FEAR OF ENGINE DEATH FROM EXPLOSIVE DETONATIONīlams in the can are generally caused by a rich running engine. Your timing may be a bit out, because your valve job may have altered the sychronicity of the hit and miss of the plug firing. So lets hope you have the first kind! You say you just had the heads done.good idea! Head jobs are notorious for creating great blams in the can conditions. the kind that ignites leftover fuel in the head and blows your head off. the kind that ignites leftover fuel in the exhaust and blams in the can.Ģ. Hopefully someone else DESPERATE to get rid of an annoying backfire on exhaust will someday find this thread and read the diagnostic procedure, as evidently they don't teach it to factory techs.Įxhaust leak / air getting sucked in on decel/engine braking when the throttle butterflies are closed = backfiring It's that simple.ġ. Have someone rev it up while ANOTHER LOOKED, LISTENED and FELT for an exhaust leak, and eitehr sprayed water, canned smoke, or whatever around all the flanges, manifold gaskets, etc looking for a "jet" of exhaust. Well, you'd warm the engine up and THEN hold a pile of rags over the exahust pipe. So how would you CORRECTLY diagnose an exhaust leak, especially one that only occurred when the engine was hot. Check it cold, rght after starting up (like in the shop) and.no leak! MY GUESS is the crack has been there A LONG TIME, and would only OPEN UP when the engine got hot. So here we are, 5 months down the road, the bike is setting 02 sensor codes and what do the HD techs (finally!) find? The techs at HD and elsewhere told him, "Nope, we can't find a leak!" Exhaust pipe, junction, header, exhaust manifold gasket, SOMEWHERE. You got a backfire on decel, CHANCES ARE you've got a leak in the exhaust. I told Warren OVER AND OVER that backfiring on decel is caused by air getting into the exhaust, combining with unburned fuel, and exploding. Got a valuable "Lesson Learned" to share here, hopefully without sounding TOO arrogant.
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