My classic 3.5hp B&S lawnmower engine will only start cold if I remove the air cleaner and hold open the butterfly. Is this a diaphram problem or is my machine getting worn out. It does go thru a bit of oil and is blowing blue smoke.

Tony, April 2009

Yes, once its warm it runs good and can be started with the air cleaner attached. It does blow blue smoke every now and then and does go through a half quart of oil every 10 hours of operating time.

Tony, April 2009

The butterfly is the auto-choke. This is spring-loaded closed by the diaphragm, and should be opened by manifold vacuum when the engine starts. If it only starts when you hold it OPEN, it implies that the mixture is way too rich, which is a symptom of a ruptured diaphragm.

Does it run OK once started?

phil_saunders(a)bigfoot.com

Phil Saunders, April 2009