Ask RT: Why is My YZF R1 Reluctant to Start when Hot?


Here’s a question from RiderTips Reader that his Yamaha YZF R1 reluctant to start when hot - I recently bought a 2000 R1 and now it won’t start on the button when it’s up to operating temperature. The carbs have been through an ultrasonic bath with no joy.




Our Answer




I’d suspect compression problems, but before you dig deeper there are some simple, visual fuelling checks. The tank will need to be up and the airbox cover off, with the bike securely positioned on a rear paddock stand so you can put it in gear with the engine running.

Before start-up, see if there’s any obvious needle wear or the choke is sticking. Pull the choke on and off a few times while you check its operation. Next, fire it up and look down the slides where the needles go into the emulsion tubes. As you open the throttle you should see a fine fuel mist in the inlet, if there are globules of fuel evident the needles could be worn.

Modern fuel ages rubber parts and the carb diaphragms could have sprung a leak causing a lazy throttle slide. To build enough vacuum and replicate the riding experience, the engine needs to be under load as if it was on a rollingroad dyno. Your ‘rolling road’ is the back brake.

Get it into second gear on that secure paddock stand and wind the throttle on and off while scanning the throttle slides. Now drag the rear brake a little so you have to open the throttle to maintain the same revs. All four slides should rise and fall together at a steady rate − if they differ the carbs tops need lifting to inspect.