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Motor Running Rough...Ideas?

5/12/14 @ 3:31 PM
ORIGNAL POST
Shap
User since 4/18/13
Got my boat out on Saturday and she fired right up. But the motor (60 hp Mercury Bigfoot) is running rough and only hitting about 4000 rpm's. Fuses good, plugs are connected. Odd thing is in neutral it runs perfect and rpm's run as usual. Motor was winterized by Mercury authorized dealer as I was moving, waiting to hear back from them. Anyone have any ideas?

Thanks, Shap

DISPLAYING 1 TO 10 OF 16 POSTS
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6/11/14 @ 4:36 PM
Plmlk
Brent Hess
MEMBER since 12/18/07
That is a good point...

If you have a loose wire on the back of the tach, or on the battery, the tach will give you false rpms.

You can usually tell the difference just by listening to your motor, when you floor it (wot) if the hub is spun, its sounds similar to your car being in neutral and hitting the gas.

If your tach is reading incorrectly, the motor sounds normal, but the rpms may be way up.

The Woodlands Resort
(262) 661-3968
6/10/14 @ 11:06 AM
Prop
Prop
User since 4/9/03
Yeah that was my thought as well. I never had a chance to really go over everything. I keep the boat on a lift and I never got to go over everything Saturday before the rain hit. It will be couple weeks before I get back up. I just wanted as many idea's as possible so I can go over everything the next time.

6/10/14 @ 10:42 AM
Stratos219
Stratos219
User since 2/7/09
This might be a stupid question, but are you sure the tachometer is reading correctly?

6/10/14 @ 10:39 AM
Prop
Prop
User since 4/9/03
I have not tried it at full speed due to the high rpm's. I feared damaging the motor. It seems to run fine and at half throttle the speed seems correct. It's at 22 mph and 5800 rpm which is high rpm's for the speed but at half throttle the speed seems correct. Yes it does idle at 1800.

6/10/14 @ 7:22 AM
nihsif
nihsif
MEMBER since 6/15/01
beyond my knowledge if it's not a spun hub...good luck and let us know what you find out

high rpm's at half throttle... does that mean you're not able to achieve your usual speed as well?

6/10/14 @ 2:07 AM
Stratos219
Stratos219
User since 2/7/09
I might be missing something, but you said your RPMs at idle were 1800? That sounds really high for an idle speed.

6/9/14 @ 9:41 PM
Plmlk
Brent Hess
MEMBER since 12/18/07
If you have a new hub kit and your still getting high rpms, sounds like you need to look at the lower unit...

Drain the oil and look for metal shavings.... If something let loose you should see signs of it in your oil.

One more thought, you mentioned that you went up in pitch from a 17 to a 19 which should lower your RPM. Was the diameter of the prop the same?

The Woodlands Resort
(262) 661-3968
6/9/14 @ 9:19 PM
Prop
Prop
User since 4/9/03
Thanks for the reply's but I put a new kit in when I put the new prop on.

6/9/14 @ 8:31 PM
Plmlk
Brent Hess
MEMBER since 12/18/07
I agree with nif, first place to look is a spun hub.

Also, you went up in pitch by putting on your buddies prop, so if anything your rpm's should have went down.

Generally speaking, increasing your pitch should lower your rpm by 200 to 300 rpm per inch of pitch. Since you went from a 17 pitch to a 19, you should have lowered your rpm by 400 to 600 rpm.

Since it went the other way, sounds like a spun hub....

The Woodlands Resort
(262) 661-3968
6/9/14 @ 2:49 PM
nihsif
nihsif
MEMBER since 6/15/01
google spun hub

sounds like that is your problem... can get another insert and you should be good

there is a rubber insert that effectively does what a shear pin used to do

https://www.youtube.com/watch?v=2-lhtReopcI

DISPLAYING 1 TO 10 OF 16 POSTS
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