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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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148 is the standard main jet and from my experience you are simply too rich all around.
First do not make any changes using a fuel more than 21 days old (alcohol blended fuel) or more than 90 days old for non-ethanol fuel. Use a minimum 91 octane (non-ethanol blended fuel) or a 93 octane ethanol blended fuel, but no higher than a 95 octane either alcohol blended or alcohol free fuel.
The stock pilot jet is a 38, but engine modifications do not necessarily necessitate bigger jets (more fuel) - I highly modified a Kawasaki F7 175cc in 1972, my pilot jet was fine, but I had to reset the air screw from 1 1/2 turns to 3/4 turn to make it idle acceptably (I modified it for racing and idle was not a priority - the original Suzuki RM400 had no provision for idle - close the throttle and the engine would die and was then flooded on the restart). You may have to turn you fuel screw in more for a good idle. If it will not idle cleanly at 1/2 turn or more, change the pilot jet to a 35. Changes to the intake that may require more fuel include a free-er flowing air filter and lid removal or modifications to the air box or lid to allow a less restrictive intake. Changes to the intake that require less fuel (smaller jets) include larger venturi carbs, higher valve lift, larger valves and larger intake openings (ported heads usually require jetting down).
Once idle is fine (set idle mixture on a hot engine - if set immediately upon start up, it will be rich when the engine warms to normal operating temperature), work on the mid range (slide needle) - in third or higher gear with the throttle set at about 1/2 opening and cruising at a steady speed, the engine should not surge (lean) or blubber (too rich) - generally, the middle notch on the needle is adequate (unless you run a high or low octane fuel or an alcohol blended fuel) - you should not be using a fuel higher than 94 octane unless the engine builder has made changes requiring the slower burning and harder to ignite high octane fuel.
Now that the idle and mid range are set, take it out and stab the throttle wide open, a stumble from idle to WOT is not abnormal, but indicates a momentary lean condition upon the instantaneous vacuum drop caused by the sudden opening of the throttle. The accelerator pump on the carb may need adjusted or modified to deliver a stronger shot of fuel during this moment. If the stumble is not severe, it can be worked around by a change in fuel, setting the idle fuel screw on the rich side and (if adjustable) raising the float level can help.
Beyond the stumble, you want to get the WOT to run clean with the engine at maximum RPM - I think a 170 main jet is a bit on the big side - try a 160 or a 165 - standard was a 148
Jetting is as individual as the machine - when they come off the assembly line, they are all the same and the jetting installed at the time of manufacture is generally adequate for most locations, but each machine can benefit from tweaking for the specific location, fuel quality and changes like exhaust and air filters.
If you are happy with it and is is not fouling spark plugs or pinging, it's fine.