under prop'd or tach issues?

[tigerfishes]

thanks in advance for your help....
I have a 1977 19' Century at about 2500#s with a 190hp OMC(302 ford w/351 heads i'm told) it has the OMC sterndrive with a Alum 14/18 prop newly rebuilt. I don?t know the model or manufacture.
i have only had the boat a few weeks. it used to be that i would run it out at about 4600 rpms and we would do about 24-25 mph(non-gps) with 2 or 3 of us on board. i was almost at full throttle it seemed.
then we replaced all the wires, plugs points cap,etc...
now it seems like when i'm going the same rpm my speed is unchanged but now i seem to have a bunch of throttle left. i can burry the throttle past 6000 rpms(i'm talking getting another 4 or 5 mph after it hits 6k) and get easily up-to 37-38 mph. (I don?t know for sure but would think this boat capable of that speed at 4600 prms with the right prop?) i can run at 5200 and 26-28mph and the engine stays cool to about 130 degrees whereas it used to run about 150-160. when I was pushing it up to about 34mph/6000+ rpms for about 10 minutes the temp climbed to 175 and seemed to hold there. the "new" prop got some dings/bites in it after a boat ramp issue but doesn't seem to have affected things too much that i can tell.

here is my dilemma... is it possible that the tach is now not reading properly or wasn?t before and now is? or is it just with the "tune up" it has more power and now i need a 20p or 22p prop instead of the 18p? or needed it from the start? i only got that one because it was the same as what was on it when i bought it.

or is there something that i'm missing? i have the prop that i took off that i was planning on getting rebuilt since i'm told that i can only get as 14x14 and 14x16 now new. this is the old style prop with a sheer pin. I was going to get it repitched up to 20 or 22 but thought i would ask you all for advice.

My speedo was right on when checked against a phone with GPS speed on it...
my inclination is that i need more pitch in the prop... but how much more?

 

[steelespike]

Re: under prop'd or tach issues?

I think you have 2 things going on the tach is reading high and the prop is too small ,I would guess a prop for all around performance and top speed would be about 21-23"
Varify your tach,check that connections are clean and tight.Note the position of the switch on the back operate it a few times to clean the contacts. See if you can borrow a gps.I think the 18 would be good for skiing but likely will over rev.

 

[tigerfishes]

Re: under prop'd or tach issues?

thanks... i think i have a line on a 22"... rebuilt and ready to go...
and working on the tach issue...

 

[kikroks]

Re: under prop'd or tach issues?

you can even step it up on the prop size as long as it clears the upper housing I just changed my prop from a 14.5 x20 to a 15x16 because I needed to run at a higher rpm at a lower speed for wake boarding I was at 1800 rpm at 20mph now im at 2300 at 20mph it made it easier to hold 20mph without constantly playing with the throttle , lost about 8mph top end with a 4deg change. dont know if this helps . its a 1986 20'6 formula 350 260hp alpha 1 .