Mercruiser 120 Carb? Fuel idling questions

[StarLiner]

I am looking for input on 2 questions. I am a first-time and new owner of a 1979 Renken 16' bowrider with a Mercrusier 120 I/O. I am the 3rd owner. She was garage kept all her life. Very clean and all original. There are some minor door dings as they say in the auto world. I am OK in the general maintenance area.<br /><br />The first area. After 2-3 hrs of riding around, she sometimes dies as I am pulling in to dock to tie up. Generally idling in through "No wake" zone she is fine. She runs well the whole time on the water. Any ideas of why she may shut down.<br /><br />Second area. I have been running her aound 2800-3000rpm for general riding. I have had her up to 3500. They say she should run 4400 WOT. I have never run up that high. Kinda of leary. I guess I do not want to her some ear deafening sound coming from the rear of the boat. I did run her up around 3800rpm twice yesterday as coming from the water. She really moved well. I shut down and entered the "No wake" zone. She began running real rough idle and shaking. She wanted to die. I had to pump the throttle off and on to keep her going to the dock. She sat for a couple minutes after I got the trailer in the water. She fired right up an ran/idled fine. She has new plugs,points,cap,wires,fuel filter and carb filter.<br /><br />I am thinking I have some kind of fuel/carb problem. She has a Rochester 2 barrell. The motor all appears original. According to the last owner of the past 10 yrs, she had very low hrs. No meter, too old for that. <br /><br />Any suggestions.

 

[bomar76]

Re: Mercruiser 120 Carb? Fuel idling questions

What's the idle RPM? You should be @ 650-700 RPM in gear. As for WOT RPM, those 4 cyl Mercruisers are tough as a tank, and as long as oil pressure is good you shouldn't be afraid of 4300 RPM. Boat engines are screamers and built for it with heavier valve train components. I am a big believer in dumping in plenty of dry gas into the fuel system since there are 100,000 ways for water to cause problems, I also liberally use carb cleaner (both the dump in tank and use the spray down carb kind in the spring and once or twice a season), and blast out the flame arrestor with spray carb cleaner(take it off the engine first) frequently. I don't know how long the boat may have sat with stale fuel in it before you bought it, nor how much fresh fuel you may have added since purchase, but a lot of problems are stale fuel. The cleaners should help, and you get to ram around the lake while they do their work!

 

[SS MAYFLOAT]

Re: Mercruiser 120 Carb? Fuel idling questions

Sometimes running at WOT can flush gunk out of the carb, but if the gunk is in the idle circuit of the carb, you will have to take it off and clean out the restrictions. Mine would run like a raped ape, but wouldn't idle worth crap. Cleaned the carb, put in a new kit and problem solved. Cleaners and additives cannot remove corrosion particles that have formed inside the bowl of the carb. When they break loose they will clog up the small passages to the idle circuit. Good Luck

 

[StarLiner]

Re: Mercruiser 120 Carb? Fuel idling questions

Thanks for the quick responses...bomar76 and SS Mayfloat.<br /><br />Just a follow up. She idles around 500rpm. That's my best recollection. I will watch closely next run out. I believe the prior owner mentioned around 40-60 on the oil pressure. How does that sound? I did not watch exactly on yesterday's trip. I will check the manual. I was fortunate to get the original engine owners manual. I have owned her since last Oct. She has been a blast for me and my 9 & 5 yr old boys. Knock on wood, she has not let us down/stranded us. I will clean the arrestor. I did does some cleaning prior but she could use a thorough clean. I will try to clean the carb directly. I have been adding fuel stabilizer at the suggestion of the prior owner. I have run 3-4 gallons out of her per trip. However, I top her off before each outing. I could be still diluting through any fuel that had set in the tank. I will try cleaning the carb and run her again in a couple weeks. (need a kitchen pass) I think I will see how that goes. If not, I will give Mayflower's suggestion a shot. Gotta save those unfamilar repairs till last.<br /><br />I will you know how things go.

 

[bomar76]

Re: Mercruiser 120 Carb? Fuel idling questions

Wish you luck!

 

[Trent]

Re: Mercruiser 120 Carb? Fuel idling questions

First of all the "WOT" wide open throttle for this setup is 4200-4600. Run some Berryman's B12 from WalMart thur her(mix per directions) Iv used it for years and it does a great job. <br /><br />Your oil pressure is fine. Use the fuel stablizer at the end of your year....As in your not going to use it for awhile. Otherwise dont use it..Its a waste of money. Todays fuel is good for 60-90 days without stablizer. If you dont want to do that go and get some "Avgas" at you local airport..It dont go bad like car gas.