mercruiser 120 i/o loss of power at WOT!!

[snowthrower]

I have 1967 seaswirl with a 120hp merc I/O. I just bought this bought from my father and he had the same problem.<br /><br />I took it out the other day and it ran well but sluggish, on the way back to the launch with the throttle barried the boat had a suge of power and ran well for a while. I stoped the boat (kept at idle for about a minute) then took off again it was sluggish again, after 1/4 or so I heard some pinging and the boat began to surge and slowly lose speed until it died I waited 30-60 sec and the boat refired right away.<br /><br />Now this is what has been done to the boat since I had it (one month) New: cap, rotor, plugs, plug wires, fuel filter, fuel line, points and condenser. The timing and gap are correct (to the specs dad had).<br /><br />What else can I do? I think rebuild the carb but where do I get a kit? Could it be a stuck choke? This has a rochester w/hot air choke, can I clean the choke assembly (insides)? <br /><br />If any body has the tune-up specs (point gap, plug gap, idle mixture, timing) for this motor that would be awsome.<br /><br />Thanks in advance for the help!!!!

 

[DJ]

Re: mercruiser 120 i/o loss of power at WOT!!

Check to make sure your fuel tank is venting. Run it with the fuel filler cap loose.

 

[recess]

Re: mercruiser 120 i/o loss of power at WOT!!

Sounds like a fuel starving problem. If the filters are good, then the fuel pump is next. As a pump dies, it gradually pumps less and less. The pinging sound you heard could be a "lean" mixing of fuel and air due to fuel starvation at higher speeds. Be careful as this can burn out valves. Check on fuel pump pressure. Your manual will tell you the proper specs. Check your oil level NOW! If it is higher than usual, then you are leaking fuel into the crankcase. Good indication you blew a diaphram. Just replaced mine today. Crankcase was overfilled about 1/2 qt. and very thin. My pump did not have the sight glass to identify a diaphram rupture. Mechanic told me the new pumps are designed to leak into the crankcase rather than into the bilge. Don't know that I like that answer.

 

[recess]

Re: mercruiser 120 i/o loss of power at WOT!!

Sounds like a fuel starving problem. If the filters are good, then the fuel pump is next. As a pump dies, it gradually pumps less and less. The pinging sound you heard could be a "lean" mixing of fuel and air due to fuel starvation at higher speeds. Be careful as this can burn out valves. Check on fuel pump pressure. Your manual will tell you the proper specs. Check your oil level NOW! If it is higher than usual, then you are leaking fuel into the crankcase. Good indication you blew a diaphram. Just replaced mine today. Crankcase was overfilled about 1/2 qt. and very thin. My pump did not have the sight glass to identify a diaphram rupture. Mechanic told me the new pumps are designed to leak into the crankcase rather than into the bilge. Don't know that I like that answer.

 

[ronjon1944]

Re: mercruiser 120 i/o loss of power at WOT!!

also recheck your point gap, I did not get the screw tight one time and the gap moved on me.

 

[snowthrower]

Re: mercruiser 120 i/o loss of power at WOT!!

As for the tank venting, that is good I just had the tank out, cleaned and inspected it, then replaced the filler hose and the vent tube, made sure every thing was working good before reassembly.<br /><br />Oil level is perfect. Fuel starve sounds like a good thing to look into. <br /><br />Last night I took the choke apart and cleaned every thing in there (it was kinda bad) I also hosed the carb good with B-12 chemtool. I still have a hesitation as if I bring the RPM's up fast, you know just pin it and it does not take off very well but if I ease into into it then it's fine.

 

[TwoBallScrewBall]

Re: mercruiser 120 i/o loss of power at WOT!!

Th hesitation when you gun it wquick is likely the accellerator pump. With the engine off, pump the gas while looking down the carb throat. You should see two streams of gas pump into the throat as you pump the gas. Either have someone move the throttle lever for you ro disconnect the linkage so you can move the throttle manually at the carb. No streams, bad accellerator pump. Easy to fix, comes in a carb rebuild kit.

 

[cc lancer]

Re: mercruiser 120 i/o loss of power at WOT!!

Your engine is one of the toughest little engines GM ever built. It started out in the Chevy II, and I abused the day lights out one and it took it, even on the drag strip.<br />After reading your comments twice I say you have a fuel problem......water, or as posted by DJ, the vent is stopped up.<br />The age of the boat makes it wide open for condensation in the tank. If the diaphram was gone, the engine would not idle, pinging is a sign of lean mixture, if the choke was half closed you would have some black smoke and the engine would have a very rough idle.<br />First carefull remove the fuel filter, pour the contents in a clear glass container, let it sit for a while and look for dropletts of water in the bottom.<br />Check the tank vent, be sure it is clear, if the screen is missing, there may be something that is blocking the tube.<br />After this run the boat again, check your new fuel line, see if there is a "wet" spot at the fittings, or any where along the lenght of the line, you could be sucking some air.<br />The fastest way to check for water is to put the boat on a incline, and drop the front as low as possible, warm the engine with the muff, then QUICKLY ram it wide open for a SECOND, if it did OK you found your problem, because the water will be at the front of the tank. Good Luck!!!

 

[snowthrower]

Re: mercruiser 120 i/o loss of power at WOT!!

Ok, the tank should have been perfect, the day before I went out I had removed the tank, and "washed" it with acetone to remove any moisture, The vent hose is brand new and has no blockage. The choke was my first thought but I think it's fine, the butterfly does not even close when I start it. <br /><br />Where do I get a rebuild kit? I have found them on the net but does napa or anybody sell them? Does anyone know the part#. Or a make, model, year of a car I can cross referance?

 

[cc lancer]

Re: mercruiser 120 i/o loss of power at WOT!!

if I bring the RPM's up fast, you know just pin it and it does not take off very well but if I ease into into it then it's fine. <br /><br />You are not getting a spark advance on the distributor. What are you setting the timing at?<br /><br />Do what Steve said about checking for the two squirts into the Ventura's.<br />Take the carb. apart, being carful not to tear the gasket and see if one of the "holes" is stopped up. If there is any white gunk under the floats, the carb is simply stopped up.<br />Yes NAPA has a kit for the carb, what is the number on the carb?<br />Also get the points on one of the lobes and see if you have any side to side play in the distributor shaft.

 

[cc lancer]

Re: mercruiser 120 i/o loss of power at WOT!!

Snowthrower:<br />Here's the specs. on the Model 120 engine.<br />Plugs.......035 Thousands<br />Points......022 "<br />Dwell.......28-34 degrees<br />Timing......8 degrees before top dead ctr.<br />Fuel pressure...3 to 6 PSI

 

[snowthrower]

Re: mercruiser 120 i/o loss of power at WOT!!

I have the timing set at 8* btc the points are gaped at .022 I do have some advance, I can watch it advance with the light. It advances about 6-8* (estimate). <br /><br />Oh and yes I get two good streams from the carb. <br /><br />Do you think it would be worth the time to take the distributer apart and clean, and lightly lube the advance assembly?