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RSRBOB, Technician
Category: Motorcycle
Satisfied Customers: 1078
Experience:  Former Factory Service Rep, Dlr Line Tech, Service Manager, General Manager, Store Owner
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Eric I'm working on a 2002 Honda crf450r I replaced the

Customer Question

Hi Eric I'm working on a 2002 Honda crf450r I replaced the shims and when I turned the bike over I had my hand on the exhaust and it did a blow air out then suck air in through the exhaust pipe is that normal ? Or is my timing off ?
Submitted: 6 months ago.
Category: Motorcycle
Expert:  RSRBOB replied 6 months ago.
Hi and thank you for your question.It is not unheard of, especially at low RPM's that there is some negative (vacuum) pressure at the exhaust pipe. So that in itself is not indicative of any particular problem. That raises the question in my mind, what made you put your hand over the exhaust in the first place? Will the bike not start after you did a valve adjustment? Lets start at the beginning please.Thanks,RSRBOB
Customer: replied 6 months ago.
Well to start off me and a friend just put the cam and shims back in and the spark plug then we just tried to turn it over he had his hand over the exhaust for no reason and he told me that he felt a blow out then suck in air so I held my hand over it and felt it blow out then suck in and I was wondering if that's a problem and I haven't tried to start the bike just making sure that the (vacuum pressure) isn't a problem
Expert:  RSRBOB replied 6 months ago.
Thank you for the additional information.If the engine is turning over by hand, the cam timing cannot be off far enough to cause a problem. If a valve were going to hit the piston, you would feel the kick starter stop when it hit the valve.If you are worried about it, it is always worth a quick double check. Set the arrow on the gear in the window and look at the mark on the cam. If everything lines up, you are good to go.Just feeling negative pressure through the exhaust is not enough to indicate a problem.RSRBOB