Exhaust burble
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From: R.C.
Even though Mikuni is explaining carbed systems the principles and causes are still the same just electronically metered instead of mechanically.
4: Backfires in Exhaust
Note:
It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.
The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.
Why This (normally) Happens:
1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.
Note:
It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.
The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.
Why This (normally) Happens:
1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.
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Joined: Jun 2011
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From: Puyallup,WA
My invidia N1 burbles and pops at low RPM's not often but it does and I love the sound. It is really loud though I like to think that I got use to it over time because I DD my car but really it's probably bc i'm going deaf.
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From: Clatskanie, OR
My test pipe / Q300 does it sometimes at low rpm but you have to REALLY be listening for it to hear it on the stock ECU. On my AEM EMS v1 base map that comes with the software it does it A LOT and quite noticeable. Its in the fuel map like others have said. The AEM EMS basemap runs stupid rich, I wouldnt doubt that I'd throw some flames if I were to step on it with that map
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Joined: Oct 2011
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Alright so it's kind of a hit or miss with the stock ecu i guess. For anybody that has the invidia q300 exhaust, how do you like it? Could i DD with it or is it too loud? And yes, i have read all of the reviews on it but i would like to hear from more people.
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From: Clatskanie, OR
If you have back-firing at 0% throttle, then something is wrong. Heck, if you have back-firing at any point something is wrong.
Joined: Apr 2006
Posts: 322
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From: Tampa, FL
I have a Q300 v4 single and a test pipe. As was said before, you will get burble on the stock tune if you listen close, but after getting retuned, It's loud and clear. If you want that rough, racy sound, a set of cams will get you a more aggressive tone coming out of that exhaust. Start with bolt ons and keep adding till you're happy.
Joined: Jul 2009
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Originally Posted by ScandinavianFlick' timestamp='1332379088' post='21531859
It's a result of excess fuel being ejected unburnt.
The older Chrysler K-cars do this; think Turbo Le Baron lol. They injected the fuel to help cool off the catalytic converter on de-acceleration. Nowadays, to my knowledge, no cars do this. It's just a waste of fuel...something that doesn't sell cars these days.
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