engine stumbles/dies at throttle up

Hi folks. 351 1986 Silver Nautique. Took the kids out skiing on the river yesterday evening and had no problems, boat ran great. Finished up and had some smooth water and no takers for another run, so I ran the boat up to WOT just for fun, hit about 43 Mph. After about 45 seconds or so I felt the engine get rough and the speed dropped. Dropped down to an idle and headed back to the pier. I tried throttling up some and the engine stumbled and almost died. Put on idle and in gear and it was fine, soon as i tried to throttle up at all it stumbled, eased back to an idle and it was fine.

Figured maybe i sucked some junk up and clogged the fuel filter, so back on the lift I pulled the filter and dumped out the gas into a white bucket. Color looked off and some gunk was in the bottom of the bucket. I also noticed some gas leaking from the fuel line near the filter.

This morning I cleaned the filter housing, replaced the filter element, replaced the fuel line all the way the tank and emptied the tank and put in new gas. Put the boat in the water and, after some cranking, it fired up. Once it warmed up, it idled fine, but as soon as i throttled up, under load or in neutral, the engine stumbled and died.

With my daughter at the wheel, i pulled the flame arrestor and had her throttle up and down. When she throttled up, the accelerator pump squirted in some gas but only when she first moved the throttle. If she continued to push the throttle forward, even as the engine stumbled, no more squirt from the accelerator pump.

With the boat in idle and in gear, i pushed the accelerator pump lever, watched it squirt in some gas and the motor immediately stalled. Started it back up, tried it again and it immediately stalled.

I don't have a vacuum gauge to check the fuel pump. i am pretty sure the fuel pump has never been touched, so replacing it's guts is OK with me if that could be the problem. After 32 years its probably due for some work anyway.

There is a ant-siphon valve on the tank, which I did not replace this morning.

Thoughts?

What you did at idle, stroking the accelerator pump and having it stall is "normal" big shot of gas at idle like that with no additional air from the throttle will stall it

The accelerator pump doesn't squirt all the way to full throttle either.

I just went and verified this by doing both of those things, if you're wondering why I say that they're' normal Easy to do.

I'd do the tank setup like SNobsessed said and see how it runs.

Right after it quits, does the accelerator pump squirt gas when you pump it?

Yes would mean you have fuel in the bowl and it's not starving for fuel.

No squirt would mean it's empty and not being kept full by the pump

And remember, if you think it's a fuel problem, there's a good chance it's an ignition system problem.

Thanks for the input. i've been running an electronic ignition for years now without any issues, but I will check it. First I will try the extra tank setup tomorrow and see if that solves it.

I've been looking into fuel pumps, just in case. Anyone know if the pump in an 1986 351 is mechanical or electric? Boat is a half hour away, so I can't check right now.
Thanks,
Andrew

Glad it was an easy fix.

Was the valve plugged up with junk ?

andrewmarani wrote: No, it wasn't clogged. Which bugs me. It looked perfectly clear and clean. .

It'd bug me too

The issue came back yesterday evening out on the river with my family. Puttered back to the dock and after staring at the engine for a couple of minutes we decided it was the carb.   Took off the spark arrester and stared at the carb for a couple of minutes while we moved the throttle back and forth and realized that the secondary throttle plates were not moving. Discussed this for a couple of minutes and decided that those throttle plates were moved by vacuum from that flat bell shaped thing (now known to be the vacuum secondary housing) on the right side of the carb.

Pulled the carb, took it home and removed the secondary housing and found a destroyed gasket between the carb and the housing, see picture below. Pried off the gasket to see nice large hole, see other picture.

Edit: I tested the rubber gasket inside the housing by pushing in the plunger, then holding my finger over the hole. The plunger didn't return to it's original position until I removed my finger from the hole, showing the rubber gasket inside the housing was working correctly.

I have a new gasket coming Friday and will install everything over the weekend. They only sell the gasket in bags of 20, so I will have 19 extras. Anyone need some send me a PM with your address and I will mail you some for free.

Thanks,
Andrew

It ran fine for about 10 minutes yesterday, but after that we couldn't get past maybe half throttle without it stumbling. When we watched the secondary throttle plates sitting at the pier and pushing the throttle to WOT, the plates never moved. I didn't check the tach but the engine was definitely not racing and was stumbling a lot. Seems like the secondaries not moving would indicate some issue with the vacuum, either a clogged hole or a damaged rubber gasket inside the housing. I did try moving the secondary throttle plates manually with the lever on the left side and the engine raced when I did that.

When I pulled the housing off last night the gasket had completely closed the hole and when I pulled the gasket off, there was a plug of something slightly amber colored in the hole that I had to pick out with a toothpick.   

I replaced the anti-siphon a month ago.   Screen filter at the carb inlet looked clean, but I didn't check closely, I will do that tonight.

andrewmarani wrote: So one false fix (the antiphon valve), one problem fixed that likely wasn't the issue but needed fixing (the melted gasket) and now, maybe, the real issue solved.

Sounds like a reasonable statement since like TRB mentioned earlier that gasket wouldn't cause your issue.

But that white stuff partially blocking a main jet would.

White soft and a little gummy sounds kinda like a stray little piece of teflon tape.

Also makes me wonder if you have a screen in the fuel inlet fitting to the carburetor that would have kept that piece out of the bowl. TRB asked about that too.

The fitting in the middle of this picture has a screen in it unless somebody poked it out over the years It's where the fuel line attaches to the front float bowl

Clear silicone will turn that color when exposed to fuel. It gets gummy and clogs everything in its path.