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intermittent power reduction at high speeds 1995 5.0 L

C

ccacio

Member
I have a 1995 19' four winns with a 5.0 omc. New fuel filter, cleaned carb, new plugs wires and new alternator. Seems like I get intermittent power reduction when I plane out and seems to happen more when I have a lot of weight on the boat, but not an unusual amount ( 3 *****s, 3 children). Any ideas?
 
Re: intermittent power reduction at high speeds

Be sure that your fuel supply system is free all the way to the tank pickup tube. Loss of power may be attributed to "running out of gas" at higher fuel flow (like when the boat is fully loaded) due to restriction. Could be clogged screen, collapsed hose, bad fuel pump, sticking needle valve. Lots of little gotchas to sort out first. Check all that and report back.
 
Re: intermittent power reduction at high speeds

Anything else the motor doing when this reduction happens ? I don't want to suggest things.

By chance you boating in the ocean alot of big waves maybe, just asking ?

Got a water sep on the boat if yes dump it in a clear container and let sit. You will see if water is in the fuel cause it will sit on the bottom of the fuel.

No water in the fuel all filter clear, fuel pickup tube clear no vent restriction anti-syphin valve good, then i would do a compression test, vacuum test.

Post the result.
 
Re: intermittent power reduction at high speeds

Thanks to both of you for the suggestions. I am boating on a small lake. I am going to check for interference in fuel supply this weekend. Something else to note, engine will kill if I lay the throttle down fast. If I gradually bring it up to about 2k RPM and then lay it down it will not kill. I will post results after I narrow down the possibilities.
 
Re: intermittent power reduction at high speeds

You should not throttle down rapidly if you can avoid it. Smooth. That's the ticket. Let the jet ski drivers look like lubbers. Boats don't have wheels. I know that's obvious but that means there is nothing except the engine flywheel for inertia. A car engine gets "pushed" by the rolling gear which keeps it from stalling (along with an idle air bypass device).
Get up on plane by opening the gas gradually but not so fast that you're tail dragging and then once on plane back off until you are at cruising speed. When slowing do the same in reverse so that the boat comes off plane nice and easy without dropping the bow or making a big stern wave. In all cases when there is traffic slower is better.
 
Re: intermittent power reduction at high speeds

Thanks. I checked the water fuel seperator, no water. A few small particles of gunk settled to the bottom of gas but nothing serious. Checked the anti-siphon valve and found a few little pine needles. I cleaned them out and the springloaded "ball bearing thing" seemed to operate freely. Perhaps this was the issue. I ran the boat and it was fine but the true test is when I have a heavy load of people.

A couple other things I noticed... 1. I really had to crank the engine quite a bit to get it started. 2. Dont know if its my gauge (my trim guage and tach seem funky once in a while) but seems like I burned gas up pretty quick. Once I refill I will know how much I really had left.
 
Re: intermittent power reduction at high speeds

How you running it ?

This is how i run a carbed motor. Cold i pump the throttle 2-3 times leave the throttle down 1/3 of the way and crank it. It starts i leave the throttle alone should run at about 1000-1500 rpm. Choke opens you hear the motor change, put the throttle in neutral for another minute or 2.

Then i slowly give throttle till i'm at say 300-3500 rpm i run 2/3 wot.

I want to stop i slowly back off the throttle till i come to a stop put throttle in neutral and aloow the motor to run at least 4-5 minutes at idle.

Shut off motor. Say i want to start after 30 minutes i try it in neutral no pumping nothing, crank it should start.

No start then a pump or 2 and back into neutral and crank no start then it's 1/3 down the thottle as a cold start.

Why i just write all this cause the idle at warm up and idle before shut down
 
Re: intermittent power reduction at high speeds

As o2batsea mentioned earlier about the restrictions, have you checked the air vent and line for the fuel tank, it needs to be able to breathe in as much air as you are burning gas or else it will create a vacuum and it could cause your problem. Next time out and it happens, crack the gas cap open and see if that helps or if you hear a whoosh.
 
Re: intermittent power reduction at high speeds

Thanks Cheifalen. I will try your starting technique.

Boatin bob is there any way of checking the air vent by disconnecting the tube to and blowing to see if there is any restriction?
 
Re: intermittent power reduction at high speeds

follow the fuel hose from the fuel pump to the tank. That 90 degree fitting is a anti=syphin valve, unscrew that fitting, the fuel pickup tube will come up with it.

There maybe a screen on the bottom of the tube, clean?

Blow thru the valve it clear and working?

To check the vent have someone blow air thru it from the rear and you will know if it's clear cause you will hear and feel the air with the fitting removed.
 
Re: intermittent power reduction at high speeds

Has to run from the tank to a thru fitting in the hull usually in the rear near the fill hole.

Follow the hose from the tank to where it exits the boat.

Think about this a second, when you draw gas from the tank air has to take it's place, or a vacuum happens.

So the air will be drawn in from outside the boat to the tank via a hose connected to a fitting with a screen on it usually
 
Re: intermittent power reduction at high speeds

Definitely get the concept. Do they call that vapor lock? I will check that hose as well.
 
Re: intermittent power reduction at high speeds

Is it possible that the few small pine needles I found in the anti-siphon valve were contributing? Also should I be able to blow thru that check valve with out any restriction? The spring load on the little bearing seemed to be fine and moved freely but blowing thru it was a little difficult.
 
Re: intermittent power reduction at high speeds

1992 / 454 cu.in. king cobra / runs great and has lots of maintenance / most every time out cruising at 3000 rpms , after about 15--20 min. boat motor starts shaking violently and have to slow down , if stop and sits a while ,it may run fine again ...??????????
have tried octane booster etc. / non ethanol fuel / :( any sugg.
 
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Re: intermittent power reduction at high speeds

ccacio said:
Is it possible that the few small pine needles I found in the anti-siphon valve were contributing? Also should I be able to blow thru that check valve with out any restriction? The spring load on the little bearing seemed to be fine and moved freely but blowing thru it was a little difficult.
Click to expand...

It should be hard, that spring and ball need to be able to stop the fuel from flowing to prevent it from siphoning out should your fuel line rupture. A dirty pickup screen or clogged vent line can cause fuel starvation.
 
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