7.4 MPI not getting to WOT RPM

[BigDog98]

Hello all, I recently purchased a 1999 270 SeaRay SunDancer with a 7.4 mpi and a 22p Bravo 3 drive with dual prop. When I purchased the boat in January of 2016 I had a survey done and a seatrial. Everything went well boat would top out at 4600 rpm as should. The surveyor did a compression and a scan and everything checkout out clear. Long story short, I have noticed that the rpm would drop when I go WOT , now I don't do this regularly but I noticed that my wake is not the same. The weird part is that some days it will do 4600 and others like today it would top out at 4200 rpm and at the speed the gps shows about 29-30 mph. However I will say that it was a bit of a choppy day not calm but I had my out drive all the way down and trim tabs all the way up. Then I slowly started to trim the drive up and the rpm would go from 4000 to 4200 and that was the max. The only spring maintenance thing that I did was change the oil and impeller. The surveyor at the time of the inspection told me that the plugs looked really good and near new so I did not bother with replacing them this year. Does anyone have any input to as what it may be? What to check and look for? Thank You

 

[achris]

How long has the boat been sitting in the water? You may have enough growth on the bottom to account for the 'lost' revs.

 

[BigDog98]

How long has the boat been sitting in the water? You may have enough growth on the bottom to account for the 'lost' revs.

When the boat was winterized I stripped all the paint to bare gel coat , applied barrier coat and all knew anti fouling paint . Boat has been in the water since the end of April.

 

[achris]

When the boat was winterized I stripped all the paint to bare gel coat , applied barrier coat and all knew anti fouling paint . Boat has been in the water since the end of April.

3 months is plenty enough time to knock 400rpm off the top end. You'll probably find the revs come back when you pull the boat for winter and clean it again.

Chris......

 

[tpenfield]

There is an old boat selling 'trick' that you keep the fuel tank fairly low and the boat fairly light so that it reaches a good WOT RPM and speed. Once you load it up with fuel and 'stuff' the boat will be quite a bit heavier and will probably not achieve the RPM and speed that it did on the sea trail. This may be the case, not saying it is, but maybe :noidea:

As mentioned, bottom condition, prop condition, sea condition also contribute to the top end performance.

A friend of mine has that same boat, except the 7.4L carbed. Takes a lot to get it going and keep it going, so it will not take much to effect it. That boat is so much better with twin engines as it is on the border line of needing twins, (which is 27-28 feet).

 

[BigDog98]

There is an old boat selling 'trick' that you keep the fuel tank fairly low and the boat fairly light so that it reaches a good WOT RPM and speed. Once you load it up with fuel and 'stuff' the boat will be quite a bit heavier and will probably not achieve the RPM and speed that it did on the sea trail. This may be the case, not saying it is, but maybe :noidea:

As mentioned, bottom condition, prop condition, sea condition also contribute to the top end performance.

A friend of mine has that same boat, except the 7.4L carbed. Takes a lot to get it going and keep it going, so it will not take much to effect it. That boat is so much better with twin engines as it is on the border line of needing twins, (which is 27-28 feet).

Well during the seatrial there was a half tank of gas and 5 passengers. Topped out at 4600 rpm. But today I went out again and it did 4600 almost went up to 4800 when i hammered the throttle with the same amount of fuel as the day before but one less passenger and conditions were better then yesterday , almost flat calm conditions and on gps it showed 31-32 mph with the tabs up and the drive a little up. then i had to lower the tabs because conditions got bad again. I was thinking maybe its the spark plugs and the cap that i did not replace. Tomorrow i will check to make sure all plugs are getting spark.