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KZ1300 Flat spot off idle

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TOPIC: Diagnosing off-idle throttle flat-spot issues.

Diagnosing off-idle throttle flat-spot issues. 1 month 3 weeks ago #5531

  • scotch
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This is a supplement to the 1979 Carb Tear-Down and Rebuild.

Despite being able to strip the carbs down to the core to facilitate a cleaning process, there are three tiny jets per bore which can resist cleaning due to their size and location. With many complaints of poor running after hydrosonic and various forms of chemical cleaning it stands to reason that regardless of the cleaning method it is often not thorough enough; hence unsuccessful.

My take on this is as follows:

The three jets that I believe give us the off-idle transition to mid-range are inaccessable, parse. They are located behind the closed throttle-plate (air filter side) and are staggered very specificaly to give a progressive increase in fuel/air as the throttle is opened. They sequentially supplement the single idle-air delivery until the RPM increases sufficiently to draw the fuel through the main-jet. If these three jets are clear then transition from off-idle should be smooth and linear. If any one of them is plugged then the sequential addition of fuel is interupted giving a flat spot in the throttle repsonse. If all three are blocked then the engine will likely stumble and perhaps quit at this point of throttle input.
To check the 3 jets for fuel delivery turn the idle-air screw all the way in.(off) With a thumb over the end of the corresponding jet tower and the index finger inside the venturi covering the opposite end, fluid (alcohol in this case) can be injected at minimal pressure into the idle circuit using a syringe pushed into the idle-jet tower.
(The main-jet tower is blocked to prevent the fluid from travelling backwards, into it.)
The fluid is forced out of the three jets if they are clear and unobstructed. If any or all do not discharge with a strong and identical stream then the carb requires additional cleaning attention. Similarily the tiny holes in the emmulsion-tube are sized and placed in a specific pattern to give the appropriate atomized fuel/air ratio proportionate to the throttle position and fuel level in the bowl. Any blockage of the ports in the emmulsion-tube will affect performance also but at least they can be cleaned easily.
In both cases the smallest particles of debris can obstruct these jets/holes causing endless hours of grief and frustration. A quality fuel filter is mandatory and because the carb jetting requires air to be mixed with the fuel internally (remember the emmulsion-tube?) prior to reaching the venturi it is equally critical that the air filter system be perfectly sealed and the air filter be in good condition.
To check the three throttle bore holes a .025" spark-plug gap guage should pass through each jet. (1979 Carb) If it doesn't: that jet is blocked. Rust particles, mineral build-up from moisture, dislodged scale etc will block these easily. A previous picture in the "1979 carb tear-down" (Carbs and Fuel Injection thread)shows a "stalactite" formation around one set of transition jets which would have rendered this throttle-bore useless. Hope this informs and helps !

Technical corrections by anyone better versed on this topic are welcomed.





Once the 3 transition ports have been determined to be clear the idle-air screw can now be opened the typical 3+ turns and the fluid discharge from it can be useful in determining whether it is clear.
1980 KZ 1300 sr# KZT30A-009997


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