120 mercruiser stalling on wot

[Musik68]

I have an 84 glassmaster with a 120 mercruiser....after 10 mins or so at full throttle it stalls....idles ok then back on throttle...stalls....continues to do this until finally it will barely run at all... temp gauge works runs at about 150 deg....had carb rebuilt. changed fuel filter...new condenser and coil......still it's aggrevating the hell outta me....can any one help?????? what am I missing or not doing right? Any input or suggestions would be greatly appreciated.....leaving for my honey moon next monday and the boat is a big part of our honeymoon....running out of time and my stress level is outta this world...lol

 

[chiefalen]

Re: 120 mercruiser stalling on wot

Ok assuming you rebuilt the carb right, Check the fuel filters.

Carb fitting, fuel pump, Fuel pickup tube. Fuel water seperater. Alot of water in seperater? Dump in clear container Alot of water? Than i would run it on a external tank. See if problem still exist.

If yes, you can try to suck the water out of the tank bye putting a hose down the fuel pickup tube hole and crank the bow all the way up and suck the water off the bottom into a clear container till you see gas. Water is heavier than gas.

Check anti-siphon valve.

The vent clear?

 

[subcrx]

Re: 120 mercruiser stalling on wot

are you using an electronic ignition?

 

[Haut Medoc]

Re: 120 mercruiser stalling on wot

Ok assuming you rebuilt the carb right, Check the fuel filters.

Carb fitting, fuel pump, Fuel pickup tube. Fuel water seperater. Alot of water in seperater? Dump in clear container Alot of water? Than i would run it on a external tank. See if problem still exist.

If yes, you can try to suck the water out of the tank bye putting a hose down the fuel pickup tube hole and crank the bow all the way up and suck the water off the bottom into a clear container till you see gas. Water is heavier than gas.

Check anti-siphon valve.

The vent clear?

I agree with all of the above, but I would remove & inspect the bottom of the fuel pick-up tube & siphon everything I could out of it with the tube removed.......
You can check the tank vent by running with the gas cap off.....
If you can run for 10 minutes under load, I would suspect that you are slowly sucking debris into thescreen at the bottom of the pick-up tube.....
The six gallon gas can is a good trick for isolating problems up to the fuel separator, if you have one.....
If you do not, install an aftermarket...

 

[Musik68]

Re: 120 mercruiser stalling on wot

Carb was rebuilt at a marine shop. As for the gas tank and pick up tube ......already emptied gas and inspected pick up tube.....on wot when it is bogging down or stalling out the carb is gettine plenty of fuel......mechanic friend of mine (however) car not boat,has suggetsted its something in the ignition set up....its a points system..... so where do i go from here?

 

[subcrx]

Re: 120 mercruiser stalling on wot

point setup eliminates my theory.

 

[Musik68]

Re: 120 mercruiser stalling on wot

Anyone can help? Could it be the points? could they be heating up and failing? what about spark plugs??????

 

[Maclin]

Re: 120 mercruiser stalling on wot

Since you are convinced it is not fuel related then it has to be ignition.

If the coil is supposed to have ballast resistance and does not it will basically overheat and saturate the two coils inside and spark goes away after a short time. (Verify the voltage at the coil as correct with the key on, probably 9 volts or so. If the coil has 12 volts and it is a stock coil then that is too high and the ballast resistance has been bypassed somehow, and the spark will not last long.)

If the coil is wired wrong, i.e. pos and neg terminals backwards it will start and run but spark will fade fast as the engine heats up. (Verify coil is wired correctly).

I would put new condensor and points in, gap the points per spec and then check them by a dwell meter and reset the gap if necessary to meet the dwell spec then verify the timing again. Recheck all connections, all must be tight and corrosion free.

Cap and Rotor can also affect how it runs when th engine is warm. Spark plug wires as well must be intact and not cracked and leaking spark out to any grounded part nearby. The spark plugs must be the proper type and heat range for the engine.



Back to fuel:
Are you sure ALL the filters have been changed? On older carbs there is a small stone type filter in the inlet fitting sometimes. There can also be one in the fuel pump inlet sometimes as well in the older applications.

Also be sure to only run fuel with the recommended octane level. Any higher then neeed can cause incompete combustion and leave deposits on the spark plugs.

 

[Musik68]

Re: 120 mercruiser stalling on wot

My buddy who is a car mechanic is coming over tomorrow and going to test some things on the ignition....i will also change points and plugs and follow all suggestions you guys have given me.......hopefully with any luck we will get it straight by sunday. As far as carb goes i only know the marine shop rebuilt it and said he replaced all parts. I see that a few of you talk about a screen on the bottom of the pick up tube.....Mine does not have one.....should it? The tube is cut on an angle on either side of it and goes right to the bottom of the tank....I did drain all gas and hold my hand over one end of pick up tube and blew on it to make sure no pinhole leaks and it checked out ok. I do believe it's in the ignition somewhere but am doing all of this by process of elimination.

 

[chiefalen]

Re: 120 mercruiser stalling on wot

Fuel pump giving you 4-6 psi? Only other thing i can think of.

And yes someone cut the freekin screen off you pickup tube.

Check the screen in the carb inlet fitting should have one there. Anti-siphon valve checked?

Only other thing grounds, did you ever remove the grounds and clean them?

I'm assuming good cables and good connection to battery. Kill switch not messing with you?

Connections to ignition switch good?

 

[Musik68]

Re: 120 mercruiser stalling on wot

I will put new points in this weekend and check anti syphon valve. Will run with an aux. fuel can. I will try everything all you have suggested until I figure the prob. out....Irritates me that it only does it after warmed up. 5-6lbs of pressure from fuel pump to carb is the norm right? I have owned 3 other boats and never had a prob. I couldn't figure out so this one is baffeling me. I do appreciate the feed back tho

 

[chiefalen]

Re: 120 mercruiser stalling on wot

4-6 psi fuel pump.

Sorry for your troubles. Wish i could help.