1992 Mercury 115HP Acceleration Problem. UPDATE

[Charlie5001]

Hi everyone. I am new to this forum and this is my first post. I was hoping someone out there might be able to steer me in the right direction with the problem I am having with my motor.

I have a 1992 Mercury 2-stroke 115HP 4 cylinder. The problem is mid range acceleration. Between idle and about half speed the motor bogs down some and cuts out if you gradually increase the throttle. Once the throttle reaches a certain point then something kicks in and the motor runs good. It accelerates better from a stop if you put the hammer down on the throttle.

The motor always starts right up, idles well, and runs strong between half speed and full speed. It is on a 18? Deep V Wellcraft CC boat with T-Top. Top speed is about 35mph. Here is what I know:

1. Has 120-125 psi compression on all cylinders
2. Shows to be charging 13-14 Volts on volt gauge and on the depth finder
3. New spark plugs
4. New water separator
5. Just performed the carburetor synchronization procedure as listed in the service manual
6. Stabil kept in the gas

Thanks in advance for any information or suggestions of what I should check into.


Chuck

 

[CharlieB]

Re: 1992 Mercury 115HP Acceleration Problem. Please help.

Re: 1992 Mercury 115HP Acceleration Problem. Please help.

The link and synch is critical on your 2 + 2, the accelerator pump adjustment is also critical to give that 'shot' of fuel needed to start the lower cyl's.

Test the accel pump squirt and nozzles to be sure that they are clean and flowing properly when the throttle opens and actuates the pump.

 

[Charlie5001]

Re: 1992 Mercury 115HP Acceleration Problem. Please help.

Re: 1992 Mercury 115HP Acceleration Problem. Please help.

Thanks for the reply CharlieB.

I have been working to learn how the 2+2 system works and your information about the accelerator pump makes sense to me. Now I?ll study up on that part of the system and check it out. I?ll post a reply later of what I find.

 

[CharlieB]

Re: 1992 Mercury 115HP Acceleration Problem. Please help.

Re: 1992 Mercury 115HP Acceleration Problem. Please help.

The bottom two carbs do not have idle circuits, per se, thus do not have enough fuel to fire at idle. As the throttle is opened the accel pump provides the initial squirt of fuel AND the airflow thru the carb begins pulling fuel.

As the throttle is continued to open the accel pump continues to provide a small squirt of fuel to enrich the mixture enough for acceleration. Once the throttle is opened the main jets on ALL carbs control all fuel flow.

So you can understand that link and synch AND accel pump adjustment is mandatory for smooth operation.

I set the air gap on my accel pump a skoosh tighter than the book, only enough that the pump begins to squirt just AS the throttle lever picks up and moves the throttle plates, NOT before.

 

[horsefli]

Re: 1992 Mercury 115HP Acceleration Problem. Please help.

Re: 1992 Mercury 115HP Acceleration Problem. Please help.

I had the same problem with my 1998 merc. 2+2 it was in the accelerator pump, i think it had trash in it, i removed the carb. side and left the incoming side hooked up and pumed it real easy with a screw driver about ten times and so far so good.
I think it had some trash or something in it or was just sticking not sure but that seemed to fixed it.

 

[Charlie5001]

Re: 1992 Mercury 115HP Acceleration Problem. UPDATE

I wanted to reply about what I found when I rebuilt the accelerator pump. I took it apart and the diaphragm looked to be OK. I noticed the plastic piston was worn down about 3/16? compared to the new one, and also was worn at an angle.

I cleaned it up and put it back together with the new diaphragm. I pumped a couple times to see if it would pump air and it was sucking and blowing from both ports.

Took it apart again and found trash under a brass disc at the bottom that functions as a check valve. Tip: I held a magnet to catch the tiny spring when I popped it out. Re-assembled and it would suck air in one port and blow out the other.

The surface of the plastic throttle cam was rough so I replaced it as well for $10. I?ll add cleaning and lubricating the throttle cam to preventative maintenance in the future.

The yard is too wet to get the boat out for a lake test to confirm that the problem is fixed. I?ll post a reply when I am able to do so. Thanks again for the helpful replies!!