high rev rattle cd-12

[zanman]

good day all: i have a johnson cd-12 5.5hp and have found that at full throttle the motor rattles.from about 3/4 throttle to full it doesnt seem to rev higher it just rattles louder.could this be a ignition timing problem. the motor runs great from idle to 3/4 throttle (no rattle) but once i hit 3/4 throttle,the boat wont go any faster the motor just starts to rattle above 3/4 throttle.<br />fuel mix is 24:1<br /><br />thanks for your help Z

 

[JB]

Re: high rev rattle cd-12

Hi, Zanman.<br /><br />I agree with your fuel mix.<br /><br />Check the stator of your magneto for mechanical interference or looseness when fully advanced.<br /> <br />Good luck.

 

[Chinewalker]

Re: high rev rattle cd-12

Hi Zanman,<br /> What are you running the motor on? If you're running a 5.5 on a large, heavily loaded boat, it'll hit a point where it simply won't push the boat any faster - called hull speed in displacement hull circles. You're not going fast enough to plane off so you're in essence a displacement hull. Adding more throttle won't do a thing to help and will actually hurt. It might be pre-ignition "pinging" that you're hearing, kinda like climbing a hill in high gear in a car towing a trailer.<br /> One of two things to help - get a lower pitch prop to allow the motor to reach peak revs (no more than 4500 rpm) or get a bigger motor with enough power to plane off.<br />- Scott

 

[zanman]

Re: high rev rattle cd-12

hi:im running the motor on a 14ft starcraft aluminum boat.loaded of course.i think you guys have answered my question and i thank you all.<br />Z