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General Category >> Rubber Side Down! >> Muffler changed, cilinder sound, backfire, re-carb
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Message started by LiPix on 03/11/15 at 01:42:09

Title: Muffler changed, cilinder sound, backfire, re-carb
Post by LiPix on 03/11/15 at 01:42:09

Hello everybody!,

2 weeks ago I changed my stock muffler because it did a metalic sound.

I installed a sportster muffler and everything went fine.

In my opinion I don't need to recarb because the bike now has little more power and it behaves very well in low, mid, and high. Can this be true? Is ALWAYS mandatory to re-carb? <-- First question.

I noticed that now, when I stop the engine, sometimes backfire occurs. It doesn't bother me.

I also noticed that now, the "cilinder" is louder for each explosion. Before it was like a sewing machine, now it's more like a louder tac-tac-tac-tac. Can it be due to the muffler change increasing valve pressure or something like that? Can it be harmful? <--- Second question

Finally, I am always monitoring the consumption. It's exactly the same with the new muffler than when I had the stock one.

I'll upload some pictures soon!

Thank you!

Title: Re: Muffler changed, cilinder sound, backfire, re-
Post by Dave on 03/11/15 at 03:18:48

We have no idea where you live.....the higher the elevation you live and ride at, the better chance the stock jetting is OK.

The bang after you shut the bike off is normal. When you turn off the ignition the engine continues to rotate and pump fuel and air through the cylinder....you have stopped the spark but not the ability of the engine to continue to pull air through the carb where it picks up fuel. This unburned fuel and air is pumped into the hot exhaust system - where it warms up and reaches the temperature at which the mixture is ignited in the muffler.....BANG. You can reduce the chances of his happening by allowing the bike to idle for 15 - 30 seconds before you shut it off so the exhaust can cool down a little bit, or by letting the clutch out with the bike in gear while you turn the ignition off.....your goal is to slow the rotating engine down so it does not pump as much fuel into the exhaust system. If your idle is too high it can make the bang louder as it pumps more fuel into the exhaust - but don't get the idle too low as the engine does not pump enough oil to the cam at low idle speeds.

I have not tried this yet - but it may also work to crack the throttle wide open at the same time you turn off the ignition. This will move the throttle butterfly away from the two small holes where the pilot circuit flows fuel into the carb body during idle....and it may slow down of stop the fuel flow out of these ports.

Title: Re: Muffler changed, cilinder sound, backfire, re-
Post by LiPix on 03/11/15 at 04:53:13

52696473626E757368606D72010 wrote:

Thank you for your answer.

I live in Barcelona, Spain, at 0 feet.. near the beach .. temperature here is about 10-15ºC now, and 23-32ºC on summer.

Regarding to the cilinder sounds when engine is running, do you have any ideas? (question 2)

Title: Re: Muffler changed, cilinder sound, backfire, re-
Post by Gary_in_NJ on 03/11/15 at 05:04:20

Usually a free-flowing exhaust will cause the mixture to go lean, requiring some carb adjustment. However, if your bike already had a rich condition (unusual for a stock bike) it is possible that the new can improved upon this condition.