78 1400 wide open falls on it's face

[tramp666]

I did sea trails today after redoing my carbs, water pump, stern, trim motors, electrical system and fuel system. I have 2 complaints.. 1) it seems to cavitate or ventalate when I get on it. and 2) at wide open it screams for a minute then falls on it's face. Like maybe the fuel pump can't keep up or something. Any ideas?

 

[Clams Canino]

Re: 78 1400 wide open falls on it's face

Assume the float valves won't open enough or the fuel pump / fuel system won't keep up.

Make sure it's trimmed all the way in for the hole shot.

You a racer?? You run your car in a lower gear at 5500 WOT for minites at a time??
So why put your outboard through it? WOT is for hole shots, cruise at 4200, unless you wanna tear it down every few weeks like the real racers do.

-W

 

[Fishstick1962]

Re: 78 1400 wide open falls on it's face

Are you running on a portable tank or a built in tank? The vent side of the tank may be clogged. Mud Dobbers are usually the culprit around here. See if your pump ball is colapsing when this happens. If you are using a built in tank try running on a portable tank to eliminate the fuel vent.
If the ball is not colapsing... try pumping it when this happens and see if the rpms come back up. As previously stated, your fuel delivery system may not be able to keep up with the demand.
IMHO....
Good luck!

 

[tramp666]

Re: 78 1400 wide open falls on it's face

Thanks for the reality slap. I just wanted to test the performance after doing all this work to it. I don't want to tear it down every week thats for sure. I used a lot of gas yesterday but learned a lot as well. I guess my WOT was acceptable for what it is. I need to do something about the cavitaion/ventalation deal. Will a stringray xp hyrdo tail thingy make this problem go away? Any ideas in this area? It is trimmed down all the way but still wants to stur up the bubbles and over rev. It's mounted on a 17' tank of a tri haul (rebel) bass boat.

 

[CharlieB]

Re: 78 1400 wide open falls on it's face

Just for giggles, look at the mounting height of the engine, the cav plate relationship to the bottom of the hull/transom. If the cav plate gets even or above the hull line cavitaion can become an issue.

 

[CharlieB]

Re: 78 1400 wide open falls on it's face

One other thing, the wrong prop thrust washer can space the prop too far back from the gearcase allowing exhaust gases to cause cavitation at lower speeds.

 

[tramp666]

Re: 78 1400 wide open falls on it's face

one more thing to throw out there. The prop has some small dings a dents on it that may come into play, how nice does the prop need to be to perform correctly? The cavitation plate is just above the bottom or equal to the bottom of the haul.

 

[CharlieB]

Re: 78 1400 wide open falls on it's face

Is it possible for you to post a picture showing the cav plate and bottom of the hull?

The cav plate MUST see CLEAN water under the hull to prevent the prop from sucking air and cavitating.

Has this craft had this problem always with this engine or is this a 'new' problem?

This engine could have been mounted an inch too high and a previous owner may have been willing to just NOT run it hard enough to exhibit the problem.

 

[tramp666]

Re: 78 1400 wide open falls on it's face

Previous owner needs a punch in the nose. But that's another story.

I took some pictures this weekend, so I'll try to post them. It's not the original engine. The tri haul is a 76, and the 1400 is a 78. I bought a searay xp hydro stab at walmart this weekend (the waletail) and put it on to see if that helped but I had my wife with me and she don't like to go fast so we putted around. Then the trim motor went bad on me (in the up position) and I didn't know how to manully drop it so we limped back to the dock.

I think the motor is a tad high. The cavitaion plate is just level with the bottom of the haul. I was hoping that the xp would kind of hold or redirect the water round the prop to stop/decrease the bubbles. But didn't have a chance to try it. I'll be rebuilding the trim motor this week and hopefully try again next weekend.

 

[CharlieB]

Re: 78 1400 wide open falls on it's face

Spend your time on the trim motor and skip the tail.

If the engine is mounted too high the prop will push all the water out from under the cav plate and 'suck' air down from the front, your may be able to watch this if you are very careful.

You'd see the vortex swirl between the hull and the front of the gearcase about a heartbeat before the RPM's climb.

Remount the engine.

Whaletails will help get on a plane a bit quicker and allow you to stay on plane at a lower speed. The drawbacks are less top speed and sometimes jerky handling in tighter turns.