Mercury 1150 rpm problems

laganiere

Seaman Apprentice
Joined
Jun 18, 2009
Messages
49
First of all thanks to everyone for helping me with past boat problems. Now I have a new problem(s). My boats rpms are jumping around at wot, almost like it hesitate then takes off then hesitates, does this the whole time at wot, from idle to full throttle it takes off awesome. But half to wot it skips around, it has a 17 pitch prop on it but runs to many rpms about 6200 so I'm only running about 3/4 throttle thinking a19?also the cavitation plate on plane is still under water when at wot not much but you can't see it. Any thoughts? Motor is a1150 mercury a 1976 its on a 1978 ranger aztec 17ft I can not raise the motor on the transom anymore so I am thinking of needing a jack plate.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,971
Re: Mercury 1150 rpm problems

Let's start with the RPM changes. Have someone pump the fuel line primer while underway at WOT(keep the RPMs under 6300). If the motor runs smooth, it is likely the fuel pump needs a rebuild, the fuel line has air leaks or the fuel filters are cloggged, or some combination of all three.

I am surprized you can get that much RPM from that motor on a 17 footer. Are you sure the Tach is accurate? The motor should be raised a bit. Can you use a 2x2 under the transom clamp? If not, leave that adjustment for last, after you get the correct prop and the motor running properly.

A 2" increase in pitch will lower RPMs about 400. So you would need a 19" pitch prop to drop WOT RPMs to a safe range (5800), provided the motor does not develop more RPM when you actually hit WOT.
 

laganiere

Seaman Apprentice
Joined
Jun 18, 2009
Messages
49
Re: Mercury 1150 rpm problems

Squeezing on the primer bulb does seem to make it run better,i have already rebuilt the fuel pump with new diaphragm, gasket, and check valves I will replace the fuel line from the tank to the motor, all fuel lines on motor are new a of this spring. I am not able to stick a 2x2 under the transom clamps, other wise it would clamp to that instead of the transom. I do believe the tack is correct. I am gonna see if I can borrow a 19 pitch prop to try. Any recommendations on which one will work better a 4 or 3 blade prop?
 

merc850

Commander
Joined
Jul 7, 2010
Messages
2,046
Re: Mercury 1150 rpm problems

Check the carbs
carb-filter.jpg
if you have this type check the filter screens
 
M

Maxz695

Guest
Re: Mercury 1150 rpm problems

Make sure your coil wires are inserted all the way and that they are in good condition.(Not Arching)
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,971
Re: Mercury 1150 rpm problems

As Merc850 says, there are screens under the brass tee on the top and mid carb and under the brass elbow on the bottom carb. They occasionally get gunked up.
 

merc850

Commander
Joined
Jul 7, 2010
Messages
2,046
Re: Mercury 1150 rpm problems

Remove the whole fuel line and the tops; just hold the top gently with a wrench and undo the line with another one - 9/16 I think.
 
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