I'm sure it's part of the regional dialect, but what is "over run"? Forgive my ignorance.

When I hear that, I'm usually thinking about glycol refrigeration systems running over through the vent or overflow pipes.

This has also reminded me that I gotta go play with mine a little. Wish I had FI.

Over run is when you shut the throttle so basically slowing down with a closed throttle and the exhaust starts to pop and bang.

_________________
2001 X1 2009 1125CR 2006 Uly X3 2010 Uly a fully Maz'd S1 and a 1991 RS1200 Westwind.

flames from a race bike exhaust on a closed throttle is deemed as overun

_________________
Buell X1
Buell xb12s
Buell 1125r
Ebr 1190rx
Bimota Db7
Norton v4rr??



http://www.aph-performance-engineering.com

Ah ok, "popcorn" as we call it.

Too lean of a fuel mixture when throttle is closed (or too advanced timing) causes improper burn during decel, sending raw fuel into the exhaust system. Then the unburnt fuel ignites in the overheated exhaust pipe (hotter than normal caused by lean condition) causing the popping noise and flames.

Many perfectly tuned engines exhibit this popping as sometimes it cannot be avoided, depending on intake/cams/exhaust combination. Most street applications it can be tuned out without pulling down performance too much. Race bikes, where they want every tiny bit of power, disregard the sound and just go for peak power, regardless of flames out the pipe.

On our carbs, we usually just richen the idle mixture a touch at a time until the popcorn stops.

I like popcorn

_________________
Last year I joined a group for anti-social people
We haven't met yet.

Pash,

Just thought I would update on the new settings. Definite improvement on overall popping but still plenty on that last stage from second into first and then down through first too clutch in then neutral.
Was rereading the post and was wondering if you had left the fuel changes or did you return them to stock when they didn't appear to work.
Cheers

Returned them to stock cos it didn't appear to have any effect.

What sort of RPMs are you getting the popping at?

_________________
08 Specialized Langster

Popping at about 2200 and below. Took the exhaust off and replaced using exhaust paste. Also TPS reset and is now bearable.

You could advance the ignition to 18 at 2000 rpm and 17/21 load on front and rear.

Save your maps beforehand so if there is an issue getting back to idle, you can swap back quickly.

The popping is caused by a combination of incomplete combustion (the burn is slower when the throttle is closed due to lower cylinder pressures and temperatures this is then coupled with an over retarded ignition) and a short exhaust that allows air to be sucked back to the cylinder during valve overlap.

On a carbed Buell (like matey's Blast), advance is about 30 deg down to 1500rpm at closed throttle so the normal way of getting rid of the popping is to speed the combustion up by making the mixture richer.

_________________
08 Specialized Langster