1978 Mercury 150hp PROBLEM TOP END!!

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boostinside

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Hi, i have a 150 blackmax that i have no idea why but it idles great and runs solid up to 3/4 throttle, and just before it can plain out( so going full throttle) or any beyond 3/4 it sputters like its not getting fuel. We replaced all electrical components and timed engine, also replaced fuel lines and still no fix! About to give up. Any ideas?? assuming compression is good... all cylinders were at 120 two months ago, also replaced fuel filter.. im stuck please help.
 
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boostinside

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Re: 1978 Mercury 150hp PROBLEM TOP END!!

boostinside said:
Hi, i have a 150 blackmax that i have no idea why but it idles great and runs solid up to 3/4 throttle, and just before it can plain out( so going full throttle) or any beyond 3/4 it sputters like its not getting fuel. We replaced all electrical components and timed engine, also replaced fuel lines and still no fix! About to give up. Any ideas?? assuming compression is good... all cylinders were at 120 two months ago, also replaced fuel filter.. im stuck please help. also inspected carbs, looked clean and cleaned passages just in case. new fuel pump as well( accelerator).
Click to expand...
 
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aussieflash

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Feb 5, 2011
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Re: 1978 Mercury 150hp PROBLEM TOP END!!

Check compression today,takes 10 minutes.Im assuming you already replaced Stator,Trigger and Switch box's from your thread ^^^.
You say carbs look good....did you strip them down and soak in carb cleaner,replace ALL GASKETS ($8 set)?Check floats not sticking or saturated/cracked.Check and replace needle/seat assembly ($10).Reset float height to exact spec.
 
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CharlieB

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Apr 10, 2007
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Re: 1978 Mercury 150hp PROBLEM TOP END!!

Disconnect the fuel hose just before the carb(s), crank the motor and see if you have a STRONG pulsed fuel flow, Then temporarily install a clear hose, start and run the motor, watching for any air bubbles passing thru showing an air leak sucking into the system somewhere before the fuel pump.

Remember to remove the clear hose when finished testing/repairing as clear hose is NOT rated for fuel use.
 
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Chris1956

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Re: 1978 Mercury 150hp PROBLEM TOP END!!

Pump primer bulb as you try to accelerate. If that fixes it, the fuel pump is likely bad. If the primer bulb collapses, you have a restriction upstream of it. Likely bad antisiphon valve, clogged fuel tank vent or something else.
 
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westexasrepublic

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Jun 17, 2009
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Re: 1978 Mercury 150hp PROBLEM TOP END!!

84' 150hp Mariner here with the exact same problem. turned out the rubber diaphragm on the fuel pump had a nice slit down the middle. cleaned out and used rebuild kit to replace all the gaskets and runs like a champ now.

Oh also retarded the timing 2?, so instead of advancing to 18? on a cold crank im at 16?
 
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boostinside

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Re: 1978 Mercury 150hp PROBLEM TOP END!!

Chris1956 said:
Pump primer bulb as you try to accelerate. If that fixes it, the fuel pump is likely bad. If the primer bulb collapses, you have a restriction upstream of it. Likely bad antisiphon valve, clogged fuel tank vent or something else.
Click to expand...
ball gets hard, no collapsing..plugs are black, and 2 are wet and black. Im thinking the floats need adjusting. May be too much fuel, also when carbs apart didnt inspect float too closely
 
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