need help

[jsohl]

I have a 1985 VIP with a 120 I/with a wot of 3800-4300 running a 23 pitch prop at 4100 rpm and only going about 20 mph and burning through 17 gal of fuel in about 5-6 hrs is this normal any help appreciated

 

[steelespike]

Re: need help

Is that a Mercruiser 120 I/O? What size is the VIP,dry weight? What kind of load.Is the motor a 3.0?
A 120 hp will burn about 12 gph(gallons per hour) at wot (wide open throttle)
Very likely the 23" is too much prop But your numbers don't add up.
If you read and answer the questions in the stickys at the top of the prop forum we can help better.

 

[jsohl]

Re: need help

Yes it a I/o and its a 17 ft with the dry weight 1150 usually carrying two adults and 3 small children and the motor is 1984 2.5 and my speed was checked with GPS

 

[steelespike]

Re: need help

My info indicate rpm range is 4200-4600. gear ratio 1.98. The 23" prop is very likely too much prop, but that said at 4100 rpm you should be doing about 40 mph. Your gps should be accurate at 20 mph the boat would be just on plane.
AT 20 mph rpm would be about 2000. It would appear the tach is not accurate also would indicate the motor is tired perhaps on 3 cylinders. even a healthy motor would struggle trying to get to the wot rpm range with a 23" pitch.