~~Cutting out / Backfiring at high RPM!!~~~

[FecalFajita]

So I finally got the boat (1987 Sunsport, 351 w/ clip down distributor cap) running reliably after almost two years of steady work on it.

One problem remains though, When I get the motor up to 3200rpm and above, it randomly backfires and knocks, losing power. Sometimes not at all, sometimes up to once every 10 seconds. It seems to do it more with weight in the boat, and I could be imagining, but it also seems to happen when I hit big waves.

The boat rips and runs great besides this...

It did it before replacing or changing anything, but things that are new;

plugs (properly gapped, don't remember the spec.)
wires
distributor cap
rotor
electronic ignition
coil
timing set to 6 degrees

fuel lines
fuel filter
carb was recently rebuilt

Anyone have any ideas on what's going on here? 90% of the time it runs great and it's hard to keep the boat in the driveway with the way the weather has been...but I'm sure this isn't too healthy. Thanks for any input

[Longhorn21]

knocks, losing power, and backfire are all signs of the timing being off or worn chain. I'm not saying thats what it is, but could be. What kind of boat do you have cause i thought most pcm engine are set to 10 degrees

[87SunSportMikeyD]

Hey Sean - To be honest bud this is kind of what mine is doing too. Sometimes it starts to sputter once or twice a second which causes power loss and surging unless I throttle back.

They thought it was a transmission damper plate, but not the problem. They checked the chain and say it seems looks but fine. He is going to start tearing into the motor and is looking around to price out new motors.... Gulp....sniffle.....

Longhorn is there any way this can be electrical related like distributer that is affecting my ability to set timing? We have a timing light and have adjusted timing many times.
Both boats are 87 SunSports w/ pcm 351

[FecalFajita]

Wow, how did I forget that?

Boat is a 87 sunsport.
351 with a clip down distributor cap.

[fastab]

I ran a 1990 Comp up till this year and know a number of others with similar boats. We would run it with lots of weight and started having this problem. It would run irratic etc. It was not an electrical problem but rather water in the fuel. On the 90 the fuel vent is located at the back of the boat just below the gunnel molding and back splash or all of the people on one side can cause it to push water into the tank.

Water does not flow the same as gas, it has a hard time flowing thru the jets.
It is heavier than the fuel and will sit in the bottom, at times of high load the fuel is going to flow fast(more demand) the water in the carb float bowl can cause the engine to sputter, die, run poorly depending on how much is in the carb and fuel filter. When you shut the boat of the water will settle to the bottom and the boat may run fine.

If you have a fuel vent on the back or side below the gunnel I would relocate it to where there is no chance of water getting into it.

[FecalFajita]

Water is one thing that I haven't thought of. The vent for the tank is on the port side in the same area as the handle/rope tie off in the rear. It doesn't seem like water could get in there easily, but who knows.

What's the best way to go about checking for water in the gas? Run it low and dump the remainder of the tank, fuel lines, and whatever is in the carb? Or is there an easier way?

Also, google has brought up a few cases where people have had the plug wires in the wrong places, so I've been looking into what the correct firing order is, and I can only find info on the newer style (bolt on cap) distributors. Is the order the same for the older style (clip down cap) ?

I only had a minute to look at it today and I could be wrong, but it doesn't seem like it matches the patterns I found.

[Longhorn21]

Go to plant nautique website and they have the manual their for free. It's got the firing order in it

[fastab]

If the fuel vent is at the back of the boat when you have people moving around all to one side the vent can go under and take in some water if this happens several times you continue to buildup water in the fuel. The water sits on the bottom of the tank. The fuel is picked up from the bottom of the tank, so if there is enough water it will pick it up first.


Water will sit at the bottom, while there is no movement. If you have a water/fuel filter that has a cartridge you can take it off remove the cartridge and see how much water is in the cartridge and other dirt, if it had a spin on you can dump it into a clean container to inspect. Let it settle the water will go to the bottom. You can also get rust from the filter housings. Rust/dirt can do similar things. You would have to pull the carb apart if there is anything in it other than water or fuel. On mine it had a Holley and I would pull one of the bottom bolts for the float bowls draining the fuel into a clean container, I would usually cut the top of a emty pop can. You may need a new gasket with for the bolt so you may want to find this before you do it.

If it is a water problem the solution is to clean the fuel tank, carb and reroute the vent.

[rollenrock33]

My 87 Sunsport is doing the same thing. Only at high rpm's though. The water in the fuel makes sense but I really don't see how fuel can go through the one way vent on the port side gunwale. I thought being that its one way (only air goes out, nothing can go in) this would prevent water getting into the fuel tank? Anyone try putting dry gas in the tank? I know we use it in the winter to get rid of condensation. Would be an easy fix. Any other ideas?

[87SunSportMikeyD]

We double checked our plug wire order a bunch of times and replaced them too. We spent some time checking the spark from each plug and noticed there seemed to be two plugs that were not sparking strongly or barely at all. We were considering replaceing the distributer cap or possible the whole distributer assembly.

I will call and have him double check for water in the gas right now as well as debris in the carb and integrity of inside of fuel line.

Just called mechanic and he seemed to think that the water would only affect the motor for a few minutes and then would be gone.