1979 150HP started to shutter/misfire yesterday

roughshot

roughshot

Petty Officer 2nd Class
Joined
Jul 14, 2010
Messages
164
I was out yesterday and the motor was running fine, then I started her up to get underway and thud, thud, thud, thud. the motor was running and would power up but it was clearly misfireing on a cylinder or two.
I pulled the plugs, didn't seem to look bad but I changed them anyways. Nope, that wasn't the issue.
We were able to get back to the lauch safely under power but I need some help on where to start.
Plugs - done
Plug wires?
Coil pack?
Power Pack?
I've got a manual, just don't have a clue.
I've got a good mechanic, but he's on vacation this week and this is my hunting boat and it's mid season.
I need this thing back up and running right.

Thanks for the help in advance.
 
D

DJ

Guest
Re: 1979 150HP started to shutter/misfire yesterday

1. Fuel tank. Internal? Check "anti siphon valve". That's the valve on the "outlet" of your fuel tank. They tend to stick-starving the engine.

2. Check and CLEAN all battery connections.
 
roughshot

roughshot

Petty Officer 2nd Class
Joined
Jul 14, 2010
Messages
164
Re: 1979 150HP started to shutter/misfire yesterday

The tank is internal, and their seemed to be an air leak when I squeeze the primer bulb.
Battery connections are clean.
 
S

Sixmark

Master Chief Petty Officer
Joined
Jul 11, 2010
Messages
890
Re: 1979 150HP started to shutter/misfire yesterday

Start from the beginning, you changed the plugs but did you check compression, and spark before you changed them out. In a situation like this you always check both.

Check on each cylinder until you find which one is missing, if you can find the miss, try swapping out the coil with one from another cylinder and see if the miss follows to the cylinder that you swapped it to. If it follows the coil pack then you isolated a problem. If you are looking for shortcuts, there really are none.
 
E

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: 1979 150HP started to shutter/misfire yesterday

Once you get the fuel leak rectified, check for spark: put an inductive timing light on each plugwire and check the flashes from the gun to determine the quality of spark to each wire.
 
roughshot

roughshot

Petty Officer 2nd Class
Joined
Jul 14, 2010
Messages
164
Re: 1979 150HP started to shutter/misfire yesterday

Thanks to everyone.
I should get some time tonight to get trouble shooting.
I need this thing back in action by Thursday, cross my fingers.
 
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