KTM\HUSQVARNA\BETA Overly Sensitive Carb Settings and Pipe "SOUNDS"
KTM/BETA/HUSQVARNA Jetting Issues and Carb “Finicky-Ness” is not really a carb issue.
New Buzz-Term “Pipe-Bang” also not a Carb issue.
Note: I am going to try and keep this VERY simple in order to get the points across. Intent is not to get into a fully involved technical discussion on what happens, but, rather, an over-view with some “liberties” taken as to exact internal engine function.
Much internet “chat” regarding the sensitivity of the carburetor with respect to temp and elevation changes.
New buzz term “Pipe-Bang” has surfaced as well. Pipe Bang is said to happen when you are riding and suddenly chop the throttle (close) and coast while under gear loading.
We, personally, have never experienced the phenomena, but it appears to be a real entity.
BOTH of these “issues” are related and can be cured or, at least, lessened to a more normal/acceptable state of operation
Let’s break it down to the “basics”, and get to the heart of what is happening, at least what we have determined.
1)The Mikuni Carb is not inferior to the Keihin NOR does have a greater sensitivity to elevation or temperature changes when compared to the Keihin. I know this may seem hard to believe, but, I can assure you, this is the case. BOTH carbs function EXACTLY the same in terms of fuel metering. They both work great! Your Carb is NOT alive--> so it does not have "feelings". It is ,therefore; not sensitive.
2) The real issue is related to engine mis-fires. The VERY poor OEM head (combustion chamber) design is prone to many engine mis-fires. Engine mis-fires are the result of trapped Fuel/Air (F/A) Mix NOT being 100% combusted during the combustion phase of the engine.
These mis-fires cause un-burnt Fuel/Air (F/A) Mix to “linger” and travel to different parts of the engine (ie pipe, crankcase etc).
When these F/A mix make its way back down into the crankcase, it causes some charge impurity and dilution. This will weaken the next charge and can cause an overly rich condition.
This same F/A Mix can also contribute to more mis-fires due to the Air/Fuel Ration being incorrect for proper combustion. This will usually manifest itself (from a rider’s view) as a rich carb condition (blubber etc).
This cycle repeats itself over and over and causes much “un-happiness” to the rider. This translates to the rider blaming the carb manufacturer and lobbing profanities at them as to their “poor” product and its inability to obtain and maintain proper jetting.
3) Pipe-Bang… This is a term I feel (and rightly so) will never make it into any SAE papers or textbooks. It only exists as a result of one or more poorly designed components attached to any given engine. While in this modern time, we would hope that engine components would not mimic those from the 2-Strokes infancy (60’s and 70’s era), unfortunately, this is not the case with these new Enduro Engines.
What is causing the Pipe-Bang? SAME as what is causing the carb sensitivityà Engine Mis-Fires!
These mis-fires will allow un-burnt F/A mix to enter the exhaust system. The exhaust will then “house” them, bouncing them back and forth within itself. Eventually, they will get ignited and explode within the pipe vs. on top of the piston where it could do some “good”.
These “in-pipe” ignitions/explosions will do what all explosions doà “BANG”.
We have determined that engine mis-fires are the cause of carb issues and pipe-bang (I hate using that word!). What is causing these mis-fires??
Mis-Fires are the result of incomplete combustion. The incomplete combustion is a result of a VERY poor factory combustion chamber design.
Fix the combustion chamber design and you fix the mis-fires (given proper jetting and ignition timing). You can still have mis-fires with a better chamber design. It is inherent with the 2-Stroke Engine but they will be MUCH less.
Same with what would "appear" to be a carbthat is changing calibration... less mis-fires = less carb issues.
In fact, we ride from 5,000ft to over 10,000ft nearly every ride we do. We also ride from 20F to 100F temperature throughout our riding season.
I can tell you with 100% honesty, that we never make ANY carb adjustments!
WHY? Because we have engines that are properly jetted and combustion chambers that are very efficient.
IF any of you are experiencing adverse runability with temp changes or elevation changes, you have “other” factors that are causing these issues.
You should NOT have to alter jetting as a result of a small elevation change or temperature change!! Even a large one (like we do).. IF you have an efficient combustion chamber that is not as prone to mis-fires!
It, really, is that simple!!
I’ll stop here, for now. Hopefully, this gave some clarity to these issues?