1976 mercury 1150 stalling when lake but not with clam shells

Goofy420

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Hi all, I just bought a 77 Galaxie with a 1976 mercury 1150 tower of powe . When the clam shells are on the motor out of water the boat seems to run fine. Then I get to the lake and it stalls when throttling up. I have rebuilt the fuel pump, reset the carburetors, reset the timing, spark advance, and throttle stop. I have ran sea foam through it and wide open throttle 5mph. I am stumped at this point, any help or suggestions greatly appreciated
thank you.
 

Chris1956

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Assuming the motor runs on all 6 cylinders, and the prop is the right pitch, the carbs are the next most likely issue.

The carbs on those motors must be set on the water, motor warm, under load. If all the rest of the link and synch is good, you likely need to richen the carbs a bit to allow smooth, fast acceleration. I recommend a fresh 50::1 gas/oil fuel supply, for setting the carbs. Start with all carbs 1-1/2 turns open.

So warm the motor on the water, in gear at idle. Open the mixture screw (richen the mixture) on the top carb 1/8 turn and try to accelerate. She will still likely still stumble. Open the carb on the mid carb 1/8 turn and try to accelerate. Repeat with bottom carb, and then repeat sequence starting with top carb and testing acceleration after each adjustment. You may need to run thru sequence 3 or 4 times to get it right. Listening carefully to the engine accelerate can help. You can find the bottom carb will be 1/8 turn open, the mid carb 1/4 turn open and the top carb 1 turn open from initial setting, or something like this.

Generally top carb will need to be the richest, followed by the mid carb and the bottom carb will be set the leanest. Remember the richer the idle mixture the worse the idle, so you want a compromise that gives good acceleration and reasonable idle.
 

Goofy420

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Thanks for all the info the top carb idle screw was just about falling out when I looked at them I have Them all set to one and a 1/2 turns
 

Chris1956

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If the compression is even across the cylinders, the motor is good.
 

Texasmark

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In the manual on a later model Merc looper but the same analogy applies, it says to open the low speed adjustments about a 1/16th of a turn (forget the actual number but just a tad....I think Chris uses the width of a screwdriver blade for his metric) and test the hole shot. Continue to open till the engine will take the throttle and deliver the shot. My Tower idled like a sewing machine.....beauty of having 6 bangs per rev. (opinion). I was running the Laser SS prop which was ported and the ports unloaded the engine during the shot and helped me get the best of both worlds....great shot and silky smooth idle.
 

Goofy420

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Which one is the low speed? I have 3 carbs, the top carb the screw was falling out
 

racerone

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All 3 carburetors have a low speed mixture screw.-----It is very common for these engines to idle very nicely and fall flat on acceleration, very common.-----So test by opening EACH mixture screw 1/8 tur at a time.------The stalling may go away.-----Costs nothing to try that.-----And unless you know the condition of the impeller it needs a new one in the water pump !!
 

Goofy420

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Ok so I took it on the lake and tried to adjust the mixture screws. I got it to where it didn't stall but top speed was only 5mph.
 

Chris1956

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If you followed the carb adjustment procedure I outlined in post #3, and she still can only get 5MPH, either the prop is too great a pitch for the load of the boat, or some other engine issues is at work. What prop and what boat do you have?

Did you clean the carbs, rebuild them with new gaskets, inlet needles and seats and new floats and set the carb float heights to spec?

Idle pickup timing is when the carbs are still closed, but ready to open. That spec should be 4-6* BTDC. The adjustment is a brass collar on the distributor body, about 1/2 way up.

Max spark advance adjustment is one of the set screws above the distributor. It should be at 21* BTDC.
 

racerone

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The motor is NOT running on all cylinders I think.-----Can not tell from where I sit.---How did you " reset the timing " as per post # 1 above ?---Check belt timing..----Remove distributor cap for cleaning.----Does spark jump a gap of 3/8" on all leads , yes or no ?----All carburetors opening up ?-----Boost venturi in place on all 3 carburetors.-----Water in fuel ?
 

Goofy420

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To Chris, when I bought the boat it ran pretty good hitting 45mph to 50 mph then the second trip is when it began stalling bot still would hit 35 to 40mph. Third trip is when it just didn't run at all, I noticed the fuel pump was cracked and leaking from the top so I replaced and rebuilt it. The 4th trip after doing the "link and synk" I noticed the fuel mix screw on top carb was falling out. 5th trip tried adjusting mix screw . I have not rebuilt the carbs and am thinking (hoping) that is the issue.

To Racerone no I have not messed with distributor yet, fuel is brand new,
 

racerone

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Inspection and trouble shooting is what I recommend !----I am not sure what you mean or infer by the term " messed with distributor "----But good luck with the project.
 

jimmbo

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If the Engine still has good compression and good spark, I suspect your Timing and Spark Advance Adjustments. However, I will not rule out Carb/Fuel Delivery Problems, possibly a combination of everything
 

Chris1956

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Check to see if the lower spark plug wires are arcing to ground. They do, when they get old. Also, makes sure the engine wiring harness is still good. Usually the insulation falls off by now.
 

bugelingkayaker

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Have you made any progress on this yet? I have a merc 850 from 1973 Ive been working on that I’m having an identical issue with. Wondering at this point if it might be an issue with reed valves inside the power head. Is there an easy way to test that these are working properly and not getting stuck? Might be a dumb idea but Since they are basically just a one way valve can you just set the piston to bottom dead center and blow in the spark plug hole and if you can’t blow in that means it’s closing properly...?
 

bugelingkayaker

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Check to see if the lower spark plug wires are arcing to ground. They do, when they get old. Also, makes sure the engine wiring harness is still good. Usually the insulation falls off by now.

Can the arcing to ground be fixed with just some electrical tape if you are still getting bright blue spark when removing the plugs? Or is there really no alternative to replacing them?
 
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