Gear Ratio Check

R

Reming870

Petty Officer 2nd Class
Joined
Jul 15, 2010
Messages
191
I have a thought that maybe my Gear Ratio has been altered.

How can I check the ratio to confirm?

Reasons I think this:
1. The motor is now running mint after my electronic ignition conversion and timing correction.
2. The 16 x 17P prop is a slug on the boat and takes forever to plane. 3,000 RPM WOT and struggle to plane.
3. I put a 16 x 11P on the boat and can now go to 4300 RPM WOT at 23 MPH.

1984 Mercruiser 140 HP / Alpha 1 outdrive / Supposed 1.98:1
 
R

Reming870

Petty Officer 2nd Class
Joined
Jul 15, 2010
Messages
191
Re: Gear Ratio Check

Just updated the OP. It is a 140 HP Mercruiser 1984 with Alpha 1. 1.98:1 is the tag ratio.
 
Kainon

Kainon

Chief Petty Officer
Joined
Sep 13, 2009
Messages
608
Re: Gear Ratio Check

hows the fuel ? hows the engine compression ?

Hows the prop ?

Got water IN the boat ?
 
ziggy

ziggy

Admiral
Joined
Jun 30, 2004
Messages
7,473
Re: Gear Ratio Check

i didn't write this and can't remember who did, but i thought it was good.

"How do I check gear drive ratio?

Do all these steps in order.
1. Shut off the battery switch
2. Pull the spark plugs out of the engine.
3. Put the shifter in forward.
4. Turn the engine till the prop starts turning, continue turning till the timing mark is at TDC.
5. Put a mark on the prop hub and the drive housing that align with each other.
6. Turn the engine by hand while someone watches the marks on the prop, and count the revolutions the crankshaft makes to turn the prop 1 complete revolution.
7. If it took 1.5 revolutions of the crankshaft, then you have a drive ratio of 1.5:1 which is what is needed for a small block V8. (a 260)."
 
R

Reming870

Petty Officer 2nd Class
Joined
Jul 15, 2010
Messages
191
Re: Gear Ratio Check

Kainon said:
hows the fuel ? hows the engine compression ?

Hows the prop ?

Got water IN the boat ?
Click to expand...

120 PSI in each cylinder. I am not sure what it should be, but all 4 are consistant with #3 cylinder slightly higher at 130 PSI.

Fuel is very clean. I replaced the seperator and emptied what was in it in a glass and let it sit overnight. No debris, no water. Also changed the remaining in-line filters. Again, the boat runs mint at 4200 RPMS with the 11-pitch prop. Carb is also new. Same problem with the old carb as well. I had the new one in the box, so I thought I would put it on anyways.

Both props are new and are in new shape, no defects.

No water IN the boat. Just me and 1/2 fuel tank. The occasional wife or kid on a test run. If the boat was haevier than should be shouldn't the RPM's still be high and the speed low, but this is not what I am experiencing.

I have been through just about everything and this is leading me to the gears. Please let me know if I am missing anything else.

The 17P struggles to plane with just me in the boat. I estimate about 15-seconds to plane and I cannot get over 3,000 RPM's.

The RPM's I quote and the speeds are accurate. I have checked it with a shop tachometer and GPS.

I have also concluded that the valve cover has been changed at some point because the serial numbers on the cover do not match the parts specs in the books. For example, the plug spec will not fit in the current head, but the older serial number plugs will. I have to use the Autolite 404 when the serial number calls for the 144. All numbers are on the valve cover.
 
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