How long is too long on WOT?

[newcdnboater]

Hi, I have a 1987 3.0l merc with i don't know how many hours on it. Compression is good (138-142) timing set 6 BTDC, dwell at 31, new plugs, wires, cap, oil, filters, new fuel, new coil, carb rebuilt, the list goes on. It starts up well, and takes off well in the water-aside from a hesitation at about 1/3 throttle--have to slam it or it will stall. Anyway, everytime I've been out since all these new parts and rebuilds, I cruise at WOT for about 20 minutes then it starts bucking/hesitating, then stalls. I end up hobbling back to the wharf. Oil pressure is good, temp is reading 150. When I open the engine compartment, a blast of hot air but the engine doesn't feel too hot. I'm getting good output of water but I know I do have to inspect the exhaust further. My main question is-how long is too long to keep it at WOT? If I let it cool off for at least 1/2 an hour it will be fine and then do the same thing. No knocking or pinging. I've read here about the float level too high and the fuel boiling in the bowl but I set the float to spec myself and there isn't any fuel leaking through after it stalls out. I've removed the plugs everytime and they're dry and slightly black. I have the index marks on the choke aligned so I assume my mixture is ok...choke works fine. The engine will start right back up after the stall but will only take about 1/4 throttle or will stall again. Tomorrow I'm installing new points. I finally found a dwell meter so was able to remove the existing points and found they were very misaligned at the bottom. It certainly seems to be starving for spark/fuel/or air when it happens. Could it be that my coil is running too hot? It is VERY hot to the touch but it is new and the old coils was the same.
Sorry to ramble...had to rant.
Thanks.

 

[Coors]

Re: How long is too long on WOT?

http://forums.iboats.com/showthread.php?t=167561

 

[Limited-Time]

Re: How long is too long on WOT?

The quick answer is until it blows up.......................or you pull back on the throttle. You have other issues, Coors post should get you going in the right direction.

 

[capecodder116]

Re: How long is too long on WOT?

Definately sounds like a fuel issue. Assuming you have already changed your fuel separator and fuel filters....One thing you could try immediately after the problem occurs is to pull your flame arrestor off and pump the throttle to see if any fuel is squirting into the carb....if nothing is there, check pick tube up in tank if accesable, and anti-siphon valve. Also disconect fuel line at carb and insert fuel line into a container and crank engine over to see if your pumping fuel...if you are, then chances are you have something in the carb as I did which reared its ugly head when it felt like it.....when all else fails...follow the post on how to troubleshoot your fuel system....as I eventually did....GOOD LUCK..!!!

 

[Bondo]

Re: How long is too long on WOT?

Ayuh,....

It sounds like you're Pretty Handy,.....

You should be doing your Diagnosis When it quits,.... Not Later....
When it Dies,..... What's Missing,..?? Fuel or Spark,..??

Btw,....
Most folks Cruise at 3000/ 3500rpms......
But,..... Like LT says,....
It'll Last at WOT til she Blows.....

 

[Mkos1980]

Re: How long is too long on WOT?

I'm at WOT for 20 seconds or less. Just long enough to give me my shot of energy. Who would want their engine working that hard at so many RPM's for so long.

 

[newcdnboater]

Re: How long is too long on WOT?

It is getting plenty of fuel when it's dying. I think I will change the bracket on the coil to something that will isolate the heat from the block. I believe the coil is getting so hot from heat induction. As far as redlining the engine this long, it's only hitting 3200 at best so it's not working too awfully hard. Which, I have just discovered, was because the throttle linkage cable was mounted too high and not allowing the throttle plates to open completely. There is a threaded grommet attatched to the cable which mounts to the rocker arm cover. The bolt was at the bottom...I turned it to the top and now my throttle arm on the carb perfectly matches up with the cable when the arm is full ahead. Thanks for the help!

 

[Bondo]

Re: How long is too long on WOT?

I think I will change the bracket on the coil to something that will isolate the heat from the block. I believe the coil is getting so hot from heat induction.

Ayuh,.....

It worked Just Fine the way it's mounted for 21 Years....
If it is in fact a Coil Problem,.....
Replace it..... Moving it won't Fix it....

 

[getinmerry]

Re: How long is too long on WOT?

I had a similar problem and it turned out to be the fuel tank breather line. After running hard for a while the boat would start missing and eventually peter right out. The boat would restart, but not sustain any RPM's above idle. Check the tank breather and make sure it's not clogged or kinked.

Otherwise, I would suspect the coil.

Chuck

 

[bruceb58]

Re: How long is too long on WOT?

A good way to test if its a tank venting problem is to remove the gas cap. I would also consider running it on a portable tank to dismiss the the whole fuel system from before the fuel pump.

 

[newcdnboater]

Re: How long is too long on WOT?

I never thought of the tank vent. That's a good thought. Pretty soon I'll have had everything removed from the boat and re-installed.lol. Thanks again for your input.

 

[oops!]

Re: How long is too long on WOT?

hey bud....im not a mechanic at all but sounds to me like the rebuilt carb they gave you is .....well....pooched (is that a nove scotia term?)

thats why it boggs out at a certain rpm and why it chokes out after running a while....(auto choke)

id have it tested.....

 

[chiefalen]

Re: How long is too long on WOT?

u have a fuel water separator?if yes dump in a clear clean container see water in there it's heavier than the fuel so u will see it.dirty? i think 1-anti-syphon valve is clogged or sticking.it's on top of the tank 90 degree turn on it take it off blow thru it it ok? bottom of fuel pickup is a tube with a screen on the bottom it clogged?in the fitting by the carb is a fuel filter it clogged?fuel filter in the fuel pump it clogged?fuel vent as others have posted make sure it not clogged.if u discover water in the fuel than u will have to suck it out.maybe just maybe the accelarator pump is not operating correctly;something to recheck.the riser has to be removed and checked,a boat that old it just has to be checked.good luck and post your progress i'm sure everyone here will help u.

 

[bruceb58]

Re: How long is too long on WOT?

I think the carb being bad is less likely since it runs fine for 20 minutes.

Chief...I think the shift key on your keyboard is broken. It is not going to be the accelerator pump nor the riser either.

 

[newcdnboater]

Re: How long is too long on WOT?

Lots of good ideas. The only thing that seems to make sense is the clogged fuel tank vent. It will work fine for quite a while then start to bog. I did not however yet check the pickup/anti-siphon. I guess the good news is all of the troubles will ensure I know what to look for and check when something arises again. Thanks again.

 

[slia67]

Re: How long is too long on WOT?

Had a similar problem with a coil. After running for 30 min or so, the coil was breaking down under the heat.

I noticed you have already replaced the coil, was it the proper coil and was the boat doing this before you replaced all these parts?