Engine breaking up at high RPM. Only after running for a long time

[christoner2002]

I am back with a new problem. 1997 Larson 206 SEI MerCruiser 5.7 Alpha One
My boat is acting up, only after a long time of running. If you go wide open, it will sound like it is hitting a rev limiter, and will start to pop, if you stay there to long. Thought I had it fixed tonight when I changed the dist rotor pickup in the dist. I ran the boat tonight for over 30 miles, and ran it wide open for alot of it.(edited. I didn't run it wide open the whole time by any means, I think anyone with a brain knows not to do that.I would move around the rpm range edited) Ran perfect. Just after saying I think it is fixed, it started to break up again. You can run at 3800, and it runs fine, but if you go wide open(4600) it acts up. Could it be the coil? I will list everything I have replaced(long list). Heads rebuilt do to burnt valves. It was a 2bbl carb, and has a mercruiser gm 4bbl manifold, and a brand new Edelbrock 1409 carb. Any other questions, please ask. Any help would be great!

1997 Larson 206 SEI
Mercruiser 5.7 Alpha One(was a 2bbl, now a 4bbl)
Compression good on all cylinders
New Plugs,wires,cap,rotor,pickup in Dist.
Heads rebuilt, new head gaskets, intake gaskets
New Edelbrock 1409 Marine carb
New water/fuel seperator, new fuel filter
No water in fuel
Timing is set to Merc specs
Temp is around 170 when up to operating temp

 

[AZSeville]

Re: Engine breaking up at high RPM. Only after running for a long time

WOT for more than 30 miles might explain why your having so many engine issues.

 

[christoner2002]

Re: Engine breaking up at high RPM. Only after running for a long time

I did not say I ran it wot for 30 miles straight. I ran it all over the rpm range, and did run it up in many short spurts to wot. Thanks for the input though. It was very helpful.

 

[Shamus O'toole]

Re: Engine breaking up at high RPM. Only after running for a long time

Could be leaning out at the higher RPM. Would realy need to look at the plugs.

 

[christoner2002]

Re: Engine breaking up at high RPM. Only after running for a long time

Thank you Shamus. I did pull the plugs before. They looked good. clean, not rich, not really white. I will check them again tomorrow. Thank you for reply. There is no pinging in the engine, just the breaking up at high rpm,

 

[QC]

Re: Engine breaking up at high RPM. Only after running for a long time

FWIW, Merc says you can run WOT forever. May limit engine life, but has nothing to with engine "problems" . . . that's all the help I got, but might save you any more beatings

 

[christoner2002]

Re: Engine breaking up at high RPM. Only after running for a long time

Ok. Thanks QC. lol I bought the boat with the problems. And that was my mistake. It was a good price, and my labor was free. It was back popping through the carb, so I thought carb was junk. Replaced intake and carb. Bought a reman junk Qjet. Compression was down on a couple cylinders. Hoped I could make it through the season. Got worse. Pulled heads, rebuilt. Installed new carb and new plugs again. Now, this is the problem.

 

[Shamus O'toole]

Re: Engine breaking up at high RPM. Only after running for a long time

still popping through carb?

 

[amdburner]

Re: Engine breaking up at high RPM. Only after running for a long time

What are you using for fuel? I had this problem with my 3.0L after filling up with E10 at a store that had been selling non-ethanol gas. I had to run at less than full throttle or the engine would backfire through the carb. The next fill-up of non-ethanol cured the problem. Just a thought.

 

[dalenoman2005]

Re: Engine breaking up at high RPM. Only after running for a long time

Wow, i'm having exactly the same problem. I have a 20 year old 3L merc that ran like a dream... couple months ago i filled up while trailering at a no-name gas station. since then, i can't maintain more than 3500rpm without the exact symptoms you describe. it's as if i'm running out of gas - motor skips, coughs, and starts loosing rpm. if i ignore it for a little while, it'll drop down to less than 2500 and my speed will drop in half. if i reduce the throttle to about 3000 rpm or maybe a little less, the problem completely goes away.

I find it more likely to happen quickly if i've been trolling for a while. When it starts happening, i play with the throttle to try to stay just below the point where it happens, and gently increase it. over the next 15 minutes, i can work it up to close to 3500rpm, but the higher i go, the more likely i am to create the issue again.

i've since run a 2nd tank of gas through it from another station and still have the problem.

taking it to my mechanic today. getting a little teary eyed after reading the list of things christoner2002 has done so far without yet solving it. yikes!

I'll be watching this thread closely. christoner2002, please continue to post your experience with this. Really like to hear what finally solves it for you.

 

[kwoolard]

Re: Engine breaking up at high RPM. Only after running for a long time

if you haven't already, check inline fuel filters or screens you have between the tank and the carb. My buddy's boat did the exact same thing, at 3500 or above (about where the secondary's open up) the engine would spit, sputter backfire but as soon as you dropped it below if was fine. Turned out that his inlet screen was clogged with some trash and the carb couldn't supply enough fuel at higher rpms, causing a lean condition.

 

[christoner2002]

Re: Engine breaking up at high RPM. Only after running for a long time

To me, it seems more like a ignition system breaking up. If you let it do
that for any long time(about a minute) it will pop through the
carb. Plugs will be coming out to check them. I never thought of a fuel grade
problem. I have gassed it the same place every time since I have
owned it. I am pretty sure they have ethonol(10%) in their fuel. Of course,
I will try that next refill. It just seems strange that it runs perfect for over a hour straight(not wot the whole time) and then will start acting up. I have tried running the blower the whole time to cool the engine comp. down more.

 

[6meter]

Re: Engine breaking up at high RPM. Only after running for a long time

Yes, it could be your coil. Had the same symtoms with my first bad coil. When it starts to break up, grab the coil and see how hot it is. To hot to handle isn't good.

 

[Matt89GT]

Re: Engine breaking up at high RPM. Only after running for a long time

Have you checked the timing?

 

[wolfantry88]

Re: Engine breaking up at high RPM. Only after running for a long time

I have a 4.3lx and had same problem last weekend with sputtering/ cutting out at wot. almost seemed like it was trying to shut off for a sec then it would go again. I have put on new coil, ignition pickup in dist, cap, rotor, wires, plugs, carb rebuild (weber 4 bbl) new fuel water filter. also checked pickup tube and cleaned or replaced all fuel lines. engine doesnt run above 145 no matter how long its run.
might try and adjust the fuel/ air screws and see if i can get a better mix at wot.

 

[christoner2002]

Re: Engine breaking up at high RPM. Only after running for a long time

Talked to the Mercury/ MerCruiser dealer today. Asked him about the coil. He said the same thing, it will be to hot when it is acting up. He also said it will boil the oil inside of it, and that will sometimes show up in the area where the coil wire plugs in. Mine does look really bad in there, and has lots of oil. He gave me one to test the problem tonight, so we will see....... Checked the plugs, and I think they are ok, to maybe a little lean. Please let me know what you think on the plug.

 

[Beefer]

Re: Engine breaking up at high RPM. Only after running for a long time

Maybe you'll have better luck. I just got back from my coil test. Ran strong and fine for about 1/2 hour, then, sure enough, bogged down, and died. I had difficulty getting it to start and stay running. The coil (original) was really hot to the touch, so while I was sitting in the Gulf with 3-5 footers, I changed out the coil. Nope. Still nothing. I did get her to restart, and got to the edge of the inlet, and she died again. I dropped anchor, and called Bout US to come get me. When I got back to my dock, and she was cold, she fired right up like nothing ever happened. I think the carb rebuild is next.

No wonder boats are referred to as women....

EDIT: As we were all sharing the same problem, I thought we could combine our efforts. Mods (understandably) moved my posts to a new thread.

 

[Shamus O'toole]

Re: Engine breaking up at high RPM. Only after running for a long time

plug looks ok.

 

[wolfantry88]

Re: Engine breaking up at high RPM. Only after running for a long time

just went fishing out in the sound, boat ran good all the way up to coming back in, sputtered, died, started, died....... got it in though. have no idea what is the problem.

 

[ChrisCraftJohnny]

Re: Engine breaking up at high RPM. Only after running for a long time

From this picture.....

Plug looks lean. Too white. Is the porcelain around the center electrode a light tan color?

Light tan = perfect.

White to Very White = lean condition.