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High Idling Issues
Q:My dad bought a 1995 Polaris XLT Touring (600cc) with a burned up piston and only 1,700 miles on it. This fall we re-built the engine and tested it in the early snow. It ran really good except when we stopped, the motor would never idle down and would stay at about 3800 rpm. The only way we could lower the rpm was by turning on the choke quickly. I know when it is idling it should be about 2000 rpm. We know the carbs are clean, we checked with a heat gun, and all three exhaust ports were within 25 degrees of each other. What else could we do to try and lower the rpm? Thanks! –
93 XLT A:There are several conditions that can cause a high idle on a 2-stroke and I’ll list a few so you can start the diagnosis as soon as possible. First, you need to ask yourself why it had a piston failure with so few miles on it and could it cause my current problem? Did it have a lean condition to begin with (possible air leak in the motor)? Have the carbs been cleaned and synchronized and the idle air mixture been calibrated correctly for the motor? Some things are as simple as the idle speed screws are not adjusted correctly. The idle speed screws are the large knurled brass screws on the side of the carbs and can be adjusted with your fingers or a flat screwdriver. They also need to be even with each other or the carbs will not be synchronized and that can lead to a high idle. Most factory service manuals will give you a base line measurement as to the throttle valve height for initial synchronization. Like we said it is most likely a mis-adjusted air screw, plugged pilot circuit or an air leak. Make sure the air screw adjustment on all three carbs is identical. If you have a UniSyn gauge that would be helpful in setting all three carb slides the same so that they are all drawing the same vacuum at idle. Try adjusting the air screw in ¼ turn at a time to clean up the idle. This will enrich the idle mixture. Also there may have been something in a fuel line that is now lodged in the pilot jet, so if it doesn’t respond to air screw adjustments, check the pilot circuit on each carb again to ensure it is not plugged. Finally, check for an air leak. With the engine running, spray starting fluid around the case half seams, the carb cylinder boot seams, behind the clutch where the case seal is, through the timing access hole onto the other case seal and around the cylinder base. If a leak is found, the engine will immediately increase in idle speed rpm when it sucks in the starting fluid. –
Jerry Mathews, Starting Line Products and Todd Guthrie, Dyna-tek Performance