YZ 400 popping (lean?)

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Jun 25, 2001
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#1
My YZ400 engine is "popping" whenever I start it and will die unless you bring the RPM's up. This sounds like a lean condition but I haven't touched anything since I last rode it and it ran great as it always has. Any ideas before start searching for answers? I am an old two stroker and don't have any four stroke experience.
 

Boit

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Sep 5, 2000
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#2
If you are saying that the engine ran fine and suddenly started this habit after sitting up for a short period of time, I would suspect that the pilot jet is at least partially clogged. . . if not totally clogged. I ride a Yamaha Riva 125cc 4-stroke scooter to work and the thing sat up for 2 months while I recovered from a motocross injury. When I went to start it, the thing would not idle and generally ran like crap, plus it would backfire through the exhaust if I revved the engine and then closed the throttle completely. . It ran fine before the period of unuse. I found the pilot jet totally plugged with that gummy crud that pump gas tends to leave inside the carb. I cleaned the entire carb and now the thing runs better than ever. It's pretty amazing just how dirty the carb internals get in a relatively short period of time.
By the way, yes, you are correct in attributing the popping to a lean condition. One source of this popping is a leak at the head pipe to cylinder head exhaust port juncture. If you've had to loosen the head pipe repeatedly, then maybe the large copper washer/seal ring may not be sealing anymore. If there is a slight leak here, you can get that popping. Since you describe a generally poor running engine at low RPM, I doubt that an exhaust leak is the culprit.
 
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