Changing Lower Unit Gear Ratio (PROBLEMS??)

[bassassassin104]

I have a 1993 Mercury XR6 150 on a 1994 18 foot skeeter and have recently replaced my factory 4.25 inch mercury gearcase that had a 1.78 gear ratio with a Mercury 4.75 inch gearcase with a 1.87 gear ratio and I am running a 4 blade 24 pitch tempest prop. I ran it today for the first time and it is twisting way too hard. It wants very badly to run 6800 RPM. I am currently considering several possible probs. One is the gear ratio change, however this Lower unit is very common on 1995 and newer Merc. 150's. It is possible that it is under propped but the hole shot isn't great so I don't think that is it. The hub of the prop isn't spun it was pullin hard at 64 mph and I got out of it when I realized how high it was wrapped out. Another possibility is the Tach. could be malfunctioning but I don't have a way to test this without buying a new tach. Thanks for reading my novel here thanks for any input you may have.

 

[rob_bluhm]

Re: Changing Lower Unit Gear Ratio (PROBLEMS??)

Do you have a little blade that hangs down from your cavication plate just above your prop? if you do lossen the nut that holds it down and move the blade to the right or left then tighten the nut up and try it out if it still acts up keep up this procedure till it acts right.

 

[rickdb1boat]

Re: Changing Lower Unit Gear Ratio (PROBLEMS??)

bassassassin104 <br />As Dhadley asked in your other post, what were the WOT RPM's before you changed the lower unit? The 24 pitch Prop sounds right with the old lower unit. I believe if you were within the recommended RPM range before you changed the LU, then you are underpropped now, due to the gear change. I doubt that your Tach would have gone bad while you changed the LU. If you are truly in doubt of your Tach readings, just pick up a cheap one and install it. Maybe even a used one! Also, Please try to keep the same thread going instead of opening 2 posts. That way everyone can follow what going on much easier and offer better advice. Good Luck,

 

[Waterbugtoo]

Re: Changing Lower Unit Gear Ratio (PROBLEMS??)

"IF" your prop has an "original" 24 inch pitch...<br /><br />Have a prop shop that knows what they are doing repitch your prop with a 25-26 progressive pitch. This will mostly retain your hole shot, and bring the WOT RPM to an acceptable level. Crankin 6800 is very close to stripping the plastic oil pump gear.

 

[bassassassin104]

Re: Changing Lower Unit Gear Ratio (PROBLEMS??)

well turns out my tach was the prob all along. Thanks for the help.