WOT drop after adding hydrofoil

[bigblue1212]

Hello everyone,

I was having issues getting my 2004 16' Legend (smokercraft) All-sport on plain with my 2003 T50, so I went and bought a hydrofoil. It did the trick! The boat hops out on plain and takes off like a rocket!

There is one thing I noticed though my WOT dropped about 700RPM. I was hitting about 5200 ROM before. Now I am hitting 4500RPM. The funny thing is, after a couple of turns the motor will slowly increase back up to 5000RPM.


Not sure what is going on.

HELP!

[almetelo]

Strange! Shouldn't happen...a least such a big diference... the cases i know dont even increase or decrese WOT

[bigblue1212]

I thought so too. The strangest part is it's eventual rise back up to the appropriate RPM. I was getting 32mph (GPS) at 5200rpm and now I'm getting 24mph at 4500RPM and eventually is rises to 30mph at 5000RPM. I can see a MPH or two and 100-200RPM drop, but no more than that. I can't raise the trim amymore either, it's maxed out. Unless that can be adjusted...

[almetelo]

The height of motor in the stern is OK?

[bigblue1212]

Yep! I didn't touch the height. Everything is even.

[almetelo]

If you're sure a about height... for now just remember one more thing but I dont think it can make such a big diference

The format of hydrofoil follow +/- the format of the hull?

Try to remove the hydrofoil and make sure something wrong didnt happen with motor in the meanwhile (hope not)

[bigblue1212]

I'm going back up to the cottage this weekend. I'll remove the hydrofoil and see if the RPMs come back.

[bigblue1212]

OK so I was at the cottage on the weekend and I put seafoam in the gas tank along with fresh gas. I double checked the motor height and the hydrofoil/cav plate are even with the bottom of the boat. I checked the spark plugs (which look new) and they look fairly white (lean) so I checked the fuel filter,, fuel lines, primer bulb, water separator and fuel pump. Everything looked good. After assembling everything back together I took it out again on Sunday morning and presto 5000-5100rpm every time! Though it did cut out on me twice when doing a fast hole shot ( new problem?) Also, prop size is 13 1/2 X 15, I'm getting 5100rpm at 31mph on a legend v-166. That sound right?

[almetelo]

Hi!

Compared like was before looks like is OK now. (B4 5200RPM/32MPH, now 5100RPM/31MPH) Lost 100 RPM/1MPH which is no big deal.

In my opinion 5100/5200RPM is little RPM, Try 13k (pitch) prop

Good luck

[rodbolt17]

your RPM is still about 800 RPM low.
that motor should be able to spin between 5800-6000 with that days load.

the T50 was designed as a high thrust engine,you most likely should have bought an F50 not a T.

either way it needs to spin at least 5800 RPM with that days load.

[bigblue1212]

The motor and boat were bought used. So I didn't really have a choice in the matter. The hesitation/stall out I was experiencing I'm assuming is when the carbs are switching from the low speed to high speed jets. I do have another prop and threw it on. Its a 13x13 prop. The RPMs were up around 5400 but my top speed dropped to 24-25mph down from 31mph....that doesn't seem right. That's a big difference for a half an inch smaller prop.

[rodbolt17]

find a dealership with test wheel YB1611.
a 10 minute session will tell if you have engine or hull/rigging issues.

way better than monkeying with it.

[almetelo]

Follow the Rod advise! Than you can be completely sure.

But... in my opinion you'll need a prop with less diameter (even a 13K pitch) That motor must be used for big load boats
The props for T50 or FT50 are the same used on 2 stroke 60-140HP and 4 stroke T50, T60 until F115

http://www.yamahaoutboards.com/sites...20T50-F115.pdf

Yamaha just have one 13 pitch prop for that motor. Maybe you must have a look on aftermarket

Good luck

[rodbolt17]

toss the prop overboard and your RPM issues are solved.
however test wheel YB1611 will tell you in less than 10 minutes which issue you have.
engine OR hull/rigging.
remember propping is RIGGING not engine.
if that engine will spin test wheel YB1611 at 5250-5450 RPM then chase hull/rigging.
if it wont, fix the motor till it will and retest.

10 minutes and your not chasing ghosts.

[bigblue1212]

So I went up this weekend and found a few things:

1. The air vent on the built in gas tank was clogged. I cleaned it out.
2. With two people sitting in the aft seats I got 300 RPM more (this also made the addition of 350lbs more in the boat)
3. After a cleaned the air vent i seemd to get 150-200 more RPM. Instead of a consistant 5100-5200 (just me in the boat) I was hitting just shy of 5400rpm.

I also want hitting an added 2MPH.

Thoughts?