77 merc 150 bogs down under load

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C_Allen

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Apr 18, 2011
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i got a 1978 hydra sports 18' bass boat its got a 150 hp straight six 1977 merc and at a idle it runs smoth and it will rev up to 6 grand but when i put it in gear it will still run at idle but when i push the throttle up it just dies out. need some help!!
 
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Maxz695

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Re: 77 merc 150 bogs down under load

there is more than likely a fuel starvation rather than electrical have you rebuilt the fuel pump checked hoses for cracks changed any filters and even the check ball in the valve may not be workin and the fuel is going back to the tank. all this must be checked out. have the plugs been changed what kind of maintanance have you done. did you do a link and sinc and check the timing and compression?
 
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CharlieB

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Apr 10, 2007
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5,617
Re: 77 merc 150 bogs down under load

One or two 'dead' cyls will still rev like crazy in Neutral.

Verify compression and spark to be sure the motor is mechanically good and the ignition is working correctly, then start checking the fuel system.
 
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Chris1956

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Mar 25, 2004
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28,866
Re: 77 merc 150 bogs down under load

The motor dieing as the throttle is advanced from idle is a symptom of the carbs idle mixture screw set too lean. A great idle is another symptom of the same thing. Normally the idle is a bit rough when the carbs are set properly.

Check the compression to rule out mechanical failure, and then richen up the carbs. Did you have them worked on recently or is the motor new to you?

Open the top mixture screw 1/8 turn (CCW) and see if acceleration improves. Repeat with mid carb and then bottom carb, testing acceleration after every adjustment. Eventually you will find a setting that gives good, smooth acceleration and reasonable idling.
 
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C_Allen

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Re: 77 merc 150 bogs down under load

the motor is new to me but the first time we took it out it ran fine after i adjusted the throttle we got it up to about forty and the motor was only running about 3 grand but the second time it would only do about 5 or 10 mph and i could put it in neutral and rev up to 6 grand no prob but when i put it in gear and advance the throttle it would just die out
 
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Maxz695

Guest
Re: 77 merc 150 bogs down under load

some carbs have a high idle air mixture screw as well ( like a chain saw Hi and Low adjusments) be sure to check the setting on this if you have it on your craburetor
 
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Chris1956

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Re: 77 merc 150 bogs down under load

He has only a single idle mixture adjustment on each carb. it only affects the motor when the RPMs are below 1200 or so.

BTW - you were likely not hitting 40MPH at 3000RPM. A 21" pitch prop on that motor at 3000RPM on a light speedboat will hit about 30MPH.
I recommend you follow the advice in my previous post. I owned that exact motor for 25 years, so I have some insight.
 
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C_Allen

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Apr 18, 2011
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Re: 77 merc 150 bogs down under load

the compression is fine and its got a 22p laser2 propbut i will try the mixture screw this weekend when i take it back out.thanks for advice
 
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