Jun 25, 2001
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.


Sponsoring Member
Sep 5, 2000
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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