motor running too many RPM 6800 (prop?)

[bassassassin104]

I have a 1993 mercury XR6 150 on an 18 foot 1994 skeeter and I just put a new large gearcase lower unit and prop on it. I took it out today and it ran great, but it wants to run 6800 RPM and that is way too much. I have a small hub merecury tempest 24 pitch on it right now. IS the fact that it is a small hub a problem, I was told it wouldn't be any suggestions would be much appreciated.

 

[12Footer]

Re: motor running too many RPM 6800 (prop?)

Does it act like a slipping clutch on a car?<br />If so, it is the hub. Have the prop rehubbed, and all should be fine.<br />If it is constant, than, I would bet you are under propped,and in need of more pitch. But I am far from an expert when it comes to the particulars,just that it needs more ..How much more, I can't help with..There was a site that had a prop selection guide,and Ill see if i can hunt it up, and will post it here later if I find it.

 

[12Footer]

Re: motor running too many RPM 6800 (prop?)

Here is the site..I hope it helps.

 

[bassassassin104]

Re: motor running too many RPM 6800 (prop?)

I don't think it's the hub the prop accelerates well. I gave more info inthe other post I placed. The post is called Changing Gear Ratios on Lower Unit. Thanks for the feedback.

 

[bassassassin104]

Re: motor running too many RPM 6800 (prop?)

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.

 

[ob]

Re: motor running too many RPM 6800 (prop?)

bassassassin,That's the first thing that I would do if you are uncertain as to the accuracy of the tach is to obtain another one or have yours checked out.It is feasible that you would be somewhat underpropped since your engine is not exerting as much to turn the 1.87 to 1 gear ratio.

 

[steelespike]

Re: motor running too many RPM 6800 (prop?)

Im no expert but it seems to me you need to go with a 3 blade prop.The michigan wheel site may be some help.www.michiganwheel.com

 

[Dhadley]

Re: motor running too many RPM 6800 (prop?)

The most important thing is - what was it turning before (with the same 24 Tempest)?<br /><br />The gear ratio change could account for about 200 rpm difference depending on your rpm. The other thing about ratios is - are you sure you have a 1.87 now or could it possibly be a 2.0:1? Going from a 1.78 to a 2:1 could possibly add 400 to 500 rpm. The odd thing is that your holeshot should be fantastic. <br /><br />After you verify what you have, check the tach. Is it a Merc tach or aftermarket tach? Check your manual to see where the rev limiter is. Its most likely at either 6200 or 6700. If its indeed at 6200 the your tach may be fooling you.<br /><br />Also, what speeds were you at before?<br /><br />Good luck!

 

[bassassassin104]

Re: motor running too many RPM 6800 (prop?)

before I changed lower units I was running an old high five with an unmarked pitch. It ran about 6200 Rpm's and 60 miles per hour with that prop and old lower unit. My tach is made by TELEFLEX if that helps at all. Thanks again for the input.

 

[ob]

Re: motor running too many RPM 6800 (prop?)

bassassassin,I think you just found your answer.The prop you have now is apparrently causing your excessive rpm at WOT as compared to the other prop and lower unit gearing ratios.You'll need to prop up.

 

[Dhadley]

Re: motor running too many RPM 6800 (prop?)

Ob hit the nail right on the head. You made more than one change at a time. Now you are not comparing apples to apples. <br /><br />First verify your tach. Make sure its set correctly according to T-flex. Run the boat with a shop tack to compare. <br /><br />I still think you may possibly have a 2:1 gearcase. IF your tach is correct, with 1.87 gears at 65 mph and 6800 your slip is at about 22-23%. With 2:1 gears its at about 12%.<br /><br />Let us know about the tach.<br /><br />Good luck!

 

[bassassassin104]

Re: motor running too many RPM 6800 (prop?)

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