This sounds like a classic case of being too lean. As an engine warms up it runs richer, and that is consistent with your symptoms.
First I would try backing the fuel screw out about 1 turn and test for positive response. If you have to back it out more than about 3.5 full turns, go up on the pilot jet one size, and come back to about 2.5 turns and adjust the fuel screw for maximum idle speed.
Choke on the carby not operational
If the choke(enrichener) were not operational, it would be hard, if not about impossible, to start without extraordinary measures.
then what do you mean by taking a long time to warm up Are you going off the temp gauge and how long it takes to move up the gauge
has it got coolant or is it air cooled
if it has coolant then check that the thermostat is not stuck open and inoperational
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SOURCE: 2005 yamaha yzf 250-unstable idle
I had the same problem, Turns out that my shims were finished, replace the chims and check clearences.
SOURCE: yamaha big bear no spark
I'm not familiar with all quads and all the kill switch locations. Does it have a tip-over kill switch? Is it working? Is the quad in neutral? Is the neutral light on? Many engines will not spark if the neutral light is not on. Are the headlights off when starting?
How good is your battery? A low battery can kill spark. Test for spark by grounding the threads of the plug against the cylinder. Have a friend kick the bike while you watch for spark. Assuming no spark, then check all the kill switches with an Ohmmeter to be sure each switch is working properly. Sometimes a switch goes bad. Try disconnecting all kill switches then check spark.
Check the wiring harness, even under the gas tank and seat. Are all the connectors clean and with a tight connection? Any abraded wires? Check the ignition switch also. The switch can go bad as well. Are the flywheel coils good > is the battery getting a charge?
Still no spark? If CDI then get it checked.
If points then file or replace them along with a new condensor and reset the gap then time the engine. >> I hope this helps.
Please rate this answer. Thanks trsrhntr!
SOURCE: won't idle steadily
Check the rubber boot that joins the carby to the engine to often have seen the hose clamps over tightened therefore creating a split in the rubber letting to much air in giving the rough idle eventually you wont be able to start it at all (have a good thorough look inside an outside of boot doesnt take much)
SOURCE: my yamaha pw 50 just bogs down and wont take off
clean the carbs, if it bogs you hve to rich main jet if it shuddders you have too lean/dirty jets
SOURCE: 400 Eiger ATV, starts and runs fine, stalls the
If 2 stroke check crank seals, check plug cap for proper resistance,check for good fuel flow to carb.Spark should jump at least a 1/4" gap,NOT JUST the .030 or plug gap.
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