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RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT FULL?
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Author:  Mark T [ Sun Aug 04, 2013 7:03 am ]
Post subject:  RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT FULL?
Its been a while since I've been on, but here is latest boat problem. I was having problems at high throttle on our 1994 sundowner with a 5.8 OMC ( ford 351 ). changed the cap and rotor, since they were a little charred / etched.
Engine sounded and ran better, but misfired at hire RPM's (4,000 + ). Tried to change plugs, but stupid engineers designed it so manifolds needed to come off so into marina it went. $840 later I get boat back, they also changed the oil. (ouch) They said they tested it and it ran fine.
We hauled boat up north 270 miles to Torch Lake Michigan and now it doesn't misfire at high RPM's but it just gradually slows down and sounds like it might stall.

Up here there are tons of Four Winns so I happened to find a guy with a 1995 sundowner at the Sandbar on Torch and he had same engine, said it needed 2 new fuel pumps ?? Before I go buy those, any thoughts or other things I need to look at ?

Am I going to kill the boat by running it at lower RPM's until we bring it back ?
Author:  Walt [ Sun Aug 04, 2013 8:02 am ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
Possible fuel pump. First, I would check/replace the fuel filter. if that doesn't do it, crack your gas cap to see if the vent may be blocked causing a vacuum on your fuel tank. Look over your fuel lines for any kinks, and even get a pressure gage on it. all of the above are much cheaper than one, no less 2 fuel pumps.
Author:  rpengr [ Sun Aug 04, 2013 8:58 am ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
Mark T wrote:
...Am I going to kill the boat by running it at lower RPM's until we bring it back ?

Assuming that the problem is fuel starvation, you could cause damage at high power settings due to detonation and/or overheated valves. However, below about 65% power, this will not happen. So if you limit yourself to 3300rpm or so, then I don't think you could hurt anything getting back, even if engine is too lean. (I have this knowledge from aircraft engines, but the same should be true for any piston engine)
Author:  wkearney99 [ Sun Aug 04, 2013 1:28 pm ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
I'd also wonder about fuel flow. Could be any number of causes. Likely candidates to check, fuel pump, fuel filter and the hoses. A hose could be collapsing under the vacuum pulled by the pump during higher delivery rates, starving the system. Hoses don't last forever, when's the last time yours were replaced?

Outside chance there's something in the tank that's only being pulled up against the intake at higher rates of flow, but that's unlikely. Seen it happen in old cars, but I'd think a boat would be moving around randomly enough to not have this happen.

Is this fuel injected or carbureted? There's also a chance the carb is having issues.

There's potentially issues that could come up with timing given that you'd had cap/rotor work done. Or possibly other ignition issues. I'm assuming you're not hearing misfiring or other spark-related noises? Any time you've got someone working in the engine compartment you have to assume there's the chance of something having gotten knocked loose.

I ran into that with a plug wire that wasn't fully seated. Sounded all the world like something related to the transmission, but was just a single cap not pushed fully onto the plug. I must not have fully attached it when I'd done the plugs some time before, but it took several weeks to work loose enough to be a problem.
Author:  230 Mike [ Sun Aug 04, 2013 3:24 pm ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
I agree, on a non-EFI engine, probably pump or hoses and I would almost suspect hoses first.
Author:  deafwish [ Sun Aug 04, 2013 4:35 pm ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
When was your carb' last serviced, if ever?
Another thing that is often overlooked in fuel maintenance, is the in-carb fuel filter.
Mine was horrible when I first got my boat!
Author:  LouC [ Sun Aug 04, 2013 9:19 pm ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
The likely culprits are:
fuel: fuel filter, fuel pump(s) anti siphon valve, fuel vent
backfire flame preventer (dirty?) they can get pretty oily
Igniton: distributor: is the distributor giving full advance at 3,000 rpm and above?
What about spark plug wires, do any have higher than normal resistance?
Author:  Mark T [ Mon Aug 05, 2013 6:20 am ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
Thanks for the suggestions, it is fuel injected, so ruled out the carb solutions, hoses have probably never been replaced so will do that.

Fuel filter was replaced, no change ( see my " Bad day at the lake thread in boating banter section )

The anti siphon valve was one I never thought of and I have had problems when I go to fill up at marinas where we cannot fill up without the gas constantly foaming up. This does not happen at an auto gas station though and from what I understand marina's have a higher delivery rate, so will try replacing that.

I would have hoped Pier33 Marina would have checked timing when they replaced the plugs ?? but will ask just in case.

plug wires are only 140 hours old, do I replace or how to test them ?

Thanks as always
Author:  LouC [ Mon Aug 05, 2013 8:35 am ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
The anti siphon valve does not have anything to do with filling the gas tank ( fuel vent does). The anti siphon is screwed into the top of the gas tank and is a spring loaded valve that prevents fuel from leaking into the bilge if the line from the tank to the pump ruptures. If the valve corrodes internally it restricts fuel flow. The engine will start and run at low speeds but fall on its face trying to plane the boat.

The wires, remove one at both the plug and dist cap and check resistance with an ohm meter, should not be more than 1k ohms per foot of length.
Author:  230 Mike [ Mon Aug 05, 2013 9:59 am ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
Sorry, didn't realize it was injected. Far less likely to be hoses in that case (though it's never a bad thing to replace them).
Author:  jayjay4735 [ Mon Aug 05, 2013 11:32 am ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
In regards to checking the anti siphon valve, this may be a silly question, but will gas start pouring out when you remove it?
Author:  LouC [ Mon Aug 05, 2013 1:41 pm ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
No its on top of the gas tank so gas will not pour out. If the boat was running, and there was suction on that line from the fuel pick up to the pump and the gas line ruptured, the anti siphon would stop the flow of gas as soon as there was no suction against that line because of it rupturing. The pump has to be able to overcome spring pressure in the valve to get gas to come out of the tank. The issue is that if the fuel line/fuel pump is lower than the top of the tank, gas could siphon out if that valve was not there.
If you disconnect the fuel line at the tank there is no way gas is siphoning out. Be careful if you have to change one of these valves because they are screwed into an aluminum fitting on the top of the tank and you can strip it. I'd spray PB Blaster (don't get in the tank, just on the threads of the fitting and make sure the wrench you use fits perfectly.

This valve is something often overlooked because they are not used in automotive applications (at least not in old cars with carbs, I know that they do use valves in the gas tank filler tube to keep gas from flowing out if the filler tube gets torn off in an accident).
Author:  jayjay4735 [ Mon Aug 05, 2013 4:19 pm ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
Lou,

Thanks for the advice, I guess I just figured the fuel lines would be constantly filled with gas, but now that I think about it, I guess that doesn't make sense considering what the purpose of the anti siphon valve is. So if I disconnect the fuel line will it be very noticeable if the valve is stuck and not working properly?
Author:  LouC [ Mon Aug 05, 2013 4:27 pm ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
I don't for sure know how to test these valves because there isn't really any information in the factory shop manual from OMC that I have (I guess because the valve is part of the tank assembly installed by the boat builder). I would guess that you use a vaccum pump and pull a vaccum on it and see what the results are. If it requires a lot of vaccum to open the valve its probably bad. I'd google "how to check an anti siphon valve" and see what comes up. I know about the valve because when I first got this boat, it ran OK the first season and the second it would start, idle, rev up in neutral but would not plane the boat. I did everything I could myself and then gave it to the best OMC shop around. They fixed the shift system and also went through the whole boat fuel and ignition. They found that the carb was dirty but also it had a stuck anti siphon valve. The mechanic explained to me that the boat builders use a cheap aluminum one but he used a better quality Evinrude/Johnson one. No problems since then (2003-4).
Author:  Mark T [ Tue Aug 06, 2013 8:47 am ]
Post subject:  Re: RUNS GREAT AT 3/4 THROTTLE, STUTTERS AND SLOWS DOWN AT F
Thank you so much for the lesson, I have the boat at a different Marina now and they will check on all of the suggestions given..
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