76 Mercury thunderbolt

[rickypiet]

My motor WAS running but not well. We took it out yesterday but once you got on the throttle if you ever heard a street bike miss a gear that's what it sounds like...The motor was running at full rpm But NOT PUSHING MY boat...ANY Suggestions WOULD Be Appreciated.

 

[merc850]

Either the motor is tilted too far out, too high on the transom or; the prop hub is slipping (make marks on the inner sleeve and the outer one then run it and see if the marks are out of alignment, a prop starts slipping around 2500 rpm.) Trying another prop to check for slipping.

 

[GA_Boater]

If the prop is slipping, often it moves the boat fairly well until you start to hit the upper RPM range. Does it seem to move the boat OK at lower power settings?

 

[rickypiet]

It works good roughly halfway to full throttle, the minute I try to build speed the boat stops but motor screams...The guy I bought from told me to look into bigger prop..it's on a 16ft bayliner capri. Any suggestions on prop size? Could prop be too small and not be able to propel my boat?

 

[rickypiet]

This motor sounds fantastic, I'm sure this is the only problem as of now. Thanks for the help!

 

[GA_Boater]

Draw a line on the prop, like in the pic below and check to see if the outer part of the line isn't straight with the inner part of the line after your next trip. Props can be rebuilt and the hub rubber replaced.

 

[rickypiet]

Any suggestions on prop size for my 76 thunderbolt 115 inline 6? It's on a 16ft bayliner capri.

 

[rickypiet]

Can I do this in a keg tub? I'm not sure I want to be back on the river until it's fixed. is it necessary to do this in open water? Either way I will work on it this week and reply results...thanks again!

 

[Chris1956]

A prop for your boat would usually (rule of thumb only) have a 19" pitch to it. What do you have now?

I doubt you can run that motor in a keg tub. It will likely just spit out all the water.

 

[rickypiet]

The prop that is on it now is 17" Im pretty sure..could that be part of my problem if hub isn't spun?

 

[merc850]

A tach is a good item to have on higher HP motors, you need to know what RPM you're turning at to choose a prop - the max rpm for your model should be 5300 so you choose a prop that puts you there at max throttle. Each 1' of pitch is usually 2-300 rpm.

 

[Chris1956]

A 17" pitch prop will work just fine on your boat. 17 or 19" pitch is likely optimum.

Sounds like you have the spun prop. If it too much trouble to test, just take it to a prop shop and have it rehubbed. It will be less than $100. Alternatively you can buy a new prop, but whether 17 or 19 is right is still a mystery.

 

[rickypiet]

For some reason my posts are not coming up...I have another issue..I Rebuilt carbs and replaced fuel filter already took it out and was still bogging out..so I rebuilt the fuel pump and the diaphragm had been completely dry rot with holes in both sides. So with all that being done the motor cranks but won't even attempt to start...Please help, I'd love to get a few rides in before winter lol...thanks. . My motor is a 74 thunderbolt 115 inline 6

 

[Chris1956]

OK, the initial idle mixture screw setting is 1.5 turns out from lightly seated. The motor should start at that setting. After you warm up the motor, and with it in the water and in gear, you will need to adjust the idle mixture screws to allow reasonable idle and good acceleration.

Let us know if you can get it started. The wiring on those motors will be rotted out by now, unless it has been replaced. You need +12VDC on both the red and adjacent white wires on the switchbox, to have spark

 

[Chinewalker]

I would go ahead and buy a new prop anyway. If it fixes the problem, great. If not, well, it's a good idea to carry a spare (and tools to change it) with you.