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Discussion Starter · #1 ·
I have been having issues with intermittent backfires and stalls, particularly when blipping the throttle on the downshifts under braking. Typically I get a pop and then the bike stalls. I release the clutch and we crash start again. All good.

I fitted the new Zard low slip on last night and when I removed the stock silencer I noticed a reasonably strong smell of unburnt fuel coming from that enormous, heavy pre-silencer box of the stock exhaust. I have only had a couple of short rides since last night but so far no pops and no stalls. It may be early days so let's see how we get on.

Considering the mapping is supposed to be so lean I was surprised to smell fuel. Thoughts?
 

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The "pre-silencer", as you called it, is a catalytic converter (not a silencer).

This device uses a catalyst (platinum or palladium) to convert three harmful compounds in the exhaust into harmless compounds.
Those three are:
  • Hydrocarbons (in the form of unburned gasoline)
  • Carbon monoxide (formed by the combustion of gasoline)
  • Nitrogen oxides (created when the heat in the engine forces nitrogen in the air to combine with oxygen)
Carbon monoxide is a poison for any air-breathing animal. Nitrogen oxides lead to smog and acid rain, and hydrocarbons produce smog.

The catalyst helps to convert carbon monoxide into carbon dioxide. It converts the hydrocarbons into carbon dioxide and water. It also converts the nitrogen oxides back into nitrogen and oxygen.

"Blipping" your throttle extensively, will add significantly to the amount of unburnt fuel that enters this system, hence the smell of gasoline.

Fitting an aftermarket exhaust (without catalyzer) leans the system out even more than stock, so it reduces the amount of unburned fuel (even with blipping).
 
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Here's a little test to check whether you are running lean - without the use of an exhaust analyzer. It's easy to do...
  1. Bring the bike up to operating temperature (on an air-cooled bike like the Scrambler this takes just minutes).
  2. Find your self a nice straight stretch of empty road, and SLOWLY accelerate to around 5000 rpm's in first gear.
  3. Snap the throttle closed - WITHOUT PULLING IN THE CLUTCH - and let your engine braking slow the bike all the way back down to idle.
If you hear any popping during step #3, then you are definitely running lean. The more it "pops", and at which rpm range it happens, shows you HOW lean you are.

On a twin, If the poping starts at about 4000 rpm (or above), then you are EXTREMELY lean, and should take steps to rectify this. Running very lean significantly raises head temperatures, and on air-cooled motors this will lead to serious damage over time (water cooled motors can deal with these increases in heat much more effectively).

If the popping doesn't start until around 2000 rpm's then you are MODERATELY lean, and this should not cause long term problems - but it certainly isn't optimal, and the bike will run hotter than it could.
 

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Here's a little test to check whether you are running lean - without the use of an exhaust analyzer. It's easy to do...
  1. Bring the bike up to operating temperature (on an air-cooled bike like the Scrambler this takes just minutes).
  2. Find your self a nice straight stretch of empty road, and SLOWLY accelerate to around 5000 rpm's in first gear.
  3. Snap the throttle closed - WITHOUT PULLING IN THE CLUTCH - and let your engine braking slow the bike all the way back down to idle.
If you hear any popping during step #3, then you are definitely running lean. The more it "pops", and at which rpm range it happens, shows you HOW lean you are.

On a twin, If the pooping starts at about 4000 rpm (or above), then you are EXTREMELY lean, and should take steps to rectify this. Running very lean significantly raises head temperatures, and on air-cooled motors this will lead to serious damage over time (water cooled motors can deal with these increases in heat much more effectively).

If the popping doesn't start until around 2000 rpm's then you are MODERATELY lean, and this should not cause long term problems - but it certainly isn't optimal, and the bike will run hotter than it could.
Isn't fuel cut off when throttle position is closed? That's how EFI works in cars. So obviously it'll be lean if there's no fuel being put in the engine during decel.
 

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Isn't fuel cut off when throttle position is closed? That's how EFI works in cars. So obviously it'll be lean if there's no fuel being put in the engine during decel.
Yep, the fuel is cut.

But under normal circumstances the bike won't pop. It's only when you are leaner than normal to begin with that it does. This test exacerbates this condition, and the popping shows you how lean you truly are.

The stock bike won't backfire under the same test - because it's not lean enough to.

It's an "old-timers" tuning trick from back in the days where we didnt have access to exhaust analyzers of any kind. A really "expert" tuner could dial in a set of carbs this way really well - but of course it was a ton of trial and error.

One could also learn to "read" spark plugs, but that is beyond the skill level of most.
 

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The stock bike won't backfire under the same test - because it's not lean enough to.
My stock UE backfires all the time on more aggressive downshifts (~5k rpm). The stock Icon I demo'd did as well.

EDIT: I should say "pops" not backfires, different phenomena of course.
 

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My stock UE backfires all the time on more aggressive downshifts (~5k rpm). The stock Icon I demo'd did as well.

EDIT: I should say "pops" not backfires, different phenomena of course.
The test explicitly states "in first gear" - downshifting invalidates the result by rapidly altering the rpm beyond the desired range. It also specifies to accelerate "SLOWLY" up to 5000 rpm.

The test needs to be conducted EXACTLY as described for any validity.
 

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The test explicitly states "in first gear" - downshifting invalidates the result by rapidly altering the rpm beyond the desired range.
Ah, yes. My mistake - I was talking about downshifting into 1st gear. You're right, this is outside of the scope of your testing. Sorry about that!
 

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Assuming one comes to the conclusion that the engine is running lean, how can one get the mixture enriched? surely the EFI has to be reprogrammed? And whats does the Lam
de Sonde do if not make the mixture correct?
 

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So I just did the test on my stock bike and it pops pretty much immediately all the way down to 2 or 3k rpm.

Is it possible that this test works for carbs but not EFI?
 

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Assuming one comes to the conclusion that the engine is running lean, how can one get the mixture enriched? surely the EFI has to be reprogrammed? And whats does the Lam
de Sonde do if not make the mixture correct?
The lambda sensors only affect fueling in a narrow range of conditions, when the throttle position is relatively stable and rpm's are pretty much constant. During other times fueling is controlled by fuel map only.

The fuel maps need to be altered to rectify a lean condition, this can be accomplished via a "piggy-back" type solution like the power commander by DynoJet (there are others that are similar).

The closed-loop section of the system (lambda feedback) can only be altered via reprogramming the stock ECU with a Rexxer unit - which can also alter the fuel maps
 

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Don't forget that to make this test at all worthwhile, the exhaust must be airtight at the headers.
Yes, very important point.
 

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So I just did the test on my stock bike and it pops pretty much immediately all the way down to 2 or 3k rpm.

Is it possible that this test works for carbs but not EFI?
Nope, works for any fuel delivery system, Make sure you followed the testing parameters explicitly, if so you may have a leak in your exhaust system.

The stock bike is lean from the manufacturer, but it should not be enough to cause what you describe.
 

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Discussion Starter · #16 ·
So I just did the test on my stock bike and it pops pretty much immediately all the way down to 2 or 3k rpm.

Is it possible that this test works for carbs but not EFI?
How long did you warm it up for? It takes a good 5-10 mins of riding to get warmed up and stable.
 

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Tried that with low mount Zard yesterday. After accelerating sssslllooooowly popping started at around 2.5-3krpm. After accelarating less-slowly-but-not-that-fast popping started at ~4krpm.
In Finland we got air-cooling covered anyways by our climate.
 
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