Mercruiser 120 intermittently dies

Hey Guys,

The last time I took my boat out on the Saginaw river (about 3 weeks ago), it ran great except after I was cruising for about 30 mins, the boat would intermittently die on me about every 20 minutes or so after that point. The first time it happened, I was at WOT and passing a 700' Lake Freighter, and it didn't completely die, it acted like it was starving for fuel or something. (similar to if it was either running out of gas or i was jerking the throttle back). As I got further up river into a no wake zone, the boat again all the sudden acted like it was starving for fuel and then died. It would take a little bit of cranking to get it re-started, but it would eventually start back up. The same situation repeated about 6 more times about every 20 minutes under low throttle. I found that if I waited about 2 minutes to restart, it would start up easier.

My boat, which is a 1987 2.5L mercruiser 120 Alpha I, ran great the whole rest of the summer. It had been sitting for about a month or so when it happened. I am currently on the other side of the state away at college, but i am going to go back home this weekend to take her out as the weather looks nice. Anybody have any ideas on what could be causing this?

I initially thought maybe it was over heating but, temp is exactly where it should be. I didn't want to go adjusting the carburetor unnecessarily as it runs fine. And yes the gas tank was full!

Re: Mercruiser 120 intermittently dies

Try running it without the gas cap on. Perhaps the vent is clogged allowing a vacuum to develope. At least something to try.

Otherwise, may be time for a complete tune up.

Re: Mercruiser 120 intermittently dies

^^^

+ have ya replaced the fuel filters (which would be a part of the tune up) lately. that's always a good place to start too.

Re: Mercruiser 120 intermittently dies

I did do an almost complete tune up in the spring. New plugs, wires, distributer, timing, cleaned the carb and adjusted, etc.....

I think the one thing I didn't do was the Fuel filters. I remember that I never got around to it, because the one that the guy at the dealer sold was not correct and I just forgot about. Do any of you guys know what would be the correct filter? If I remember correctly, I believe my manual kind of confusingly stated there are multiple filters. If someone could link me to the correct filters I need (through iboats store would be great!) that would be a real help. Sorry, I dont have access to my manual, its on the other side of the state back home, and unfortunatly you cannot access the mercruiser manuals online any longer.

Re: Mercruiser 120 intermittently dies

Fishermark said:

Try running it without the gas cap on. Perhaps the vent is clogged allowing a vacuum to develope. At least something to try.

Otherwise, may be time for a complete tune up.

Click to expand...

That is a good idea, It really did seem like a vacuum. it is possible the vent had gotten clogged up with wax or something. Its not to big of a deal, i just don't want to be stuck out on Saginaw Bay (the part of Lake huron that gives Michigan its mitten shape, seperating the hand from the "thumb"). I will replace the fuel filters and if it starts to bog out just unscrew the gas gap and see if that relieves it.

Re: Mercruiser 120 intermittently dies

If you're running E10 you may want to check your fuel lines.

Re: Mercruiser 120 intermittently dies

Hey Guys,

Well I went back home this weekend, and was able to try a few things. I replaced both fuel filters, as well as the fuel filter on the carb. I also cleaned the carb out again, although it was still pretty clean. I also cleaned the fuel tank vent and line. I buttoned her back up, and took her out with a couple friends. She started right up and ran great. All of the sudden at wot throttle, sure enough, it started to hesitate again. When we came to a no wake zone, we had to keep the rpms right around 2500 or so, and sure enough it died. This time it was very difficult to restart. After about 10 mins of cranking it started back up, and we headed strait back for the launch, where it died out several times on the way back (although it was a little easier to restart). I tried taking the gas cap off, and that did nothing. I did notice, it seemed to almost back-fire which caused it to immediately die out. I am completely stumped.

It runs perfect at all RPM levels the rest of the time. Could it possibly be an ignition issue? Like I said, it only hesitates and dies about every 20 to 30 mins so could it possibly be the ignition coils are heating up or something? The plugs and wires are new, and the distributer is in good shape. The carb is cleaned and set, as well as the timing. all filters on the engine are now brand new. The boat will have to be winterized soon, so hopefully I can get this figured out.

Re: Mercruiser 120 intermittently dies

Bluestream said:

If you're running E10 you may want to check your fuel lines.

Click to expand...

It probably is E10 since i never purchase gas from a Marina. What exactly should I check in the fuel lines? All of them were replaced last year, and they arent clogged. The only lines I have not replaced were the metal lines on the engine. When i replaced the rubber lines, I disconnected them and blew them out with air to make sure they were clear. I have never really purchased gas from a marina, and have never had issues before. Plus, it runs perfect 95% of the time, and starts easily. It jsut randomly dies after about 20 mins of operation. then it runs fine again, and then dies.