1985 Mercruiser 120 Sea Sprite I/O

[barmeister]

I would like to get some suggestions on a problem I am having with my Mercruiser 120. This is my first boat and I have been in the engine for tune up (other than wires), electronic ignition now, both fuel filters replaced, new plugs and gaps set, new distributor cap, new roter, new belt, timed at idle (0), new coil.
The boat runs great for about 10 minutes and then starts to sputter and will backfire if I pump the throttle or will stall completely. If I use the trim tab switch while this is occuring, it will sputter worse and sounds like it is loosing power to the ignition system. I have used water remover in the fuel and have added octane boost. The choke seems to be open all the way when running. I also noticed that the tachometer doesn't even try to work. the engine will run at about 3500 rpm when running fine and only at about 2400 when running poorly.
Any thoughts on where to start, or what this might be?
Thanks,
Barmeister

 

[Bondo]

Re: 1985 Mercruiser 120 Sea Sprite I/O

The boat runs great for about 10 minutes and then starts to sputter and will backfire if I pump the throttle or will stall completely.

Ayuh,.... It sounds like a Fuel Delivery Issue...

 

[JustJason]

Re: 1985 Mercruiser 120 Sea Sprite I/O

both fuel filters replaced

Eagh... its possible you have 3. A fuel water seperator if you got one. 1 in the fuel pump, 1 in the carb inlet.

If I use the trim tab switch while this is occuring, it will sputter worse and sounds like it is loosing power to the ignition system.

Possible to.... Put a multimeter on the positive coil post and hit the switch and see what the voltage drops to.

 

[barmeister]

Re: 1985 Mercruiser 120 Sea Sprite I/O

Bond-o
uh... fuel pump...? or what else are you thinking?

thanks,
Barmeister

 

[barmeister]

Re: 1985 Mercruiser 120 Sea Sprite I/O

JustJason
I'll do this today and get back to you... I will look for the 3rd filter (water/fuel separator) ...

Thanks,
Barmeister

 

[Bondo]

Re: 1985 Mercruiser 120 Sea Sprite I/O

Bond-o
uh... fuel pump...? or what else are you thinking?

Ayuh,... Let's see,... Crap in the tank,.. Crap in the tank's fuel pickup tube, Crap in the antisiphon valve, crap in the lines, rotten lines, crap in the fuel filters, crap in the fuel pump, failed fuel pump, crap in the carb, worn out carb,....

Pick 1......

 

[Clipper 800]

Re: 1985 Mercruiser 120 Sea Sprite I/O

I had the same symtoms on my 1998 175 Bayliner 3.0litre Mercruiser.

I finally changed all the rubber fuel lines, double clamped all the fittings. I also removed the fuel pickup from the tank, and the antisiphon valve. Both had some crap in them,looked like rust and some fibrous stuff, like cotton. This fixed the problem,but over time the rubber fuel lines get hard and brittle,and will leak air,so you get a mixture of fuel and air to the carb.

Good luck Clipper 800

 

[barmeister]

Re: 1985 Mercruiser 120 Sea Sprite I/O

I checked the coil positive post voltage before cranking and after starting. It was 10.5V before and 12.2 after starting. when I checked across after starting I was getting 6V. When the engine started to run rough I checked positive post and it was 12.2V. I checked across and it was 6V.

I am going to be looking at the fuel delivery issues more tomorrow.

Thanks for the suggestions and help!

My Best Always,
Barmeister

 

[barmeister]

Re: 1985 Mercruiser 120 Sea Sprite I/O

I had the same symtoms on my 1998 175 Bayliner 3.0litre Mercruiser.

I finally changed all the rubber fuel lines, double clamped all the fittings. I also removed the fuel pickup from the tank, and the antisiphon valve. Both had some crap in them,looked like rust and some fibrous stuff, like cotton. This fixed the problem,but over time the rubber fuel lines get hard and brittle,and will leak air,so you get a mixture of fuel and air to the carb.

Good luck Clipper 800

Thanks,
I'll be checking the fuel pickup line and anti-siphon valve tomorrow. I'll post an update.

thanks,
Barmeister

 

[JustJason]

Re: 1985 Mercruiser 120 Sea Sprite I/O

It was 10.5V before and 12.2 after starting. when I checked across after starting I was getting 6V. When the engine started to run rough I checked positive post and it was 12.2V.

No need to check across it. Put your meter neg on battery ground (use the battery and not someplace on the block), and put the positive lead on the coil pos. Now with it runing, hit your tab switch that you commented about and see if your voltage drops below what it was before you touched the switch.

 

[barmeister]

Re: 1985 Mercruiser 120 Sea Sprite I/O

I will do that in the morning, thanks.

 

[barmeister]

Re: 1985 Mercruiser 120 Sea Sprite I/O

Thanks,
I'll be checking the fuel pickup line and anti-siphon valve tomorrow. I'll post an update.

thanks,
Barmeister

Well, I did remove the fuel pickup line and anti-siphon valve. However, when I turned the connector that holds the ball and spring inside to take it off to look for a possible clog or something, it fell apart and snapped off in my hands. The threads from the valve are still inside the connector to the pickup line on the tank. The tank looks like the original (metal / 1985). Should I consider buying a new tank, valve, hoses etc...? If yes, is fiberglass better?

Thanks,
Barmeister

 

[Bondo]

Re: 1985 Mercruiser 120 Sea Sprite I/O

If yes, is fiberglass better?

Ayuh,.. Plastic...

 

[barmeister]

Re: 1985 Mercruiser 120 Sea Sprite I/O

Thank you Bond-O

 

[barmeister]

Re: 1985 Mercruiser 120 Sea Sprite I/O sputtering

Re: 1985 Mercruiser 120 Sea Sprite I/O sputtering

Hi everyone,

I just wanted everybody to know what has transpired since my last post. I ordered the new fuel tank, disconnected the old aluminum tank, hooked up an external tank. Sputtering after 15 minutes or so, still happened.
I replace the fuel line (intake line) and the hard line from the pump to the carburetor. Still sputtering after about 15 minutes.

I ruled out fuel delivery!

(Just Jason said I should test the coil further and he was right!)

I then took the multimeter out to the boat today and had someone else drive while I checked the positive side of the of the coil (new coil) and used the ground direct from the battery. Before ignition 9 volts, after ignition 14 volts. I then checked it after it started sputtering and I was only getting 11-12 volts. I jumpered from the positive side of the coil to the positive side of the battery to bypass the wiring harness and the sputtering quit!
We ran for another 1/2 hour at WOT and not one hesitation. I have to go out of town for a few days but would like to hear from you guys. What can I look for, or should I just replace the wiring? Thanks in advance!

Barmeister