1989 Mercury 100hp 2+2 - not running above 3600 rpm.

Looking for a little more advance direction.

we recently purchased the motor with a rebuilt powerhead.

This is a 2+2 motor where the top two cylinders run and the bottom two kick in around 1800 rpm. we knew there would be some bugs to work out upon purchasing the motor and are trying to get the motor dialed in.

For the fuel system I have run new lines and installed the larger and smaller filters and inspected the accelerator pump and it has tested fine. I have disassembled the carbs and they were clean from the rebuild and reassembled. the idle screws on the top two carbs were set to factory spec turned out 1 1/2 turns. the fuel system seems to be in good shape.

The ignition system I am a little more unsure of. We recently discovered going through the Mercury service manual that the stator isn't of the wiring configuration in the mercury manual. The mercury stator in the manual is a 6 wire- two yellow, a blue and blue/white, and a red and a red/white. The one on the motor resembles a Mallory or Seirra configuration with what looks like an in line voltage regulator with only two wires to connect to the switch box- a blue and a blue /white. I have new plugs and have tested the spark with an in line spark plug tester and have spark on all 4 cylinders. The boat starts up and idles fine and when you hit the throttle you can tell the bottom cylinders are kicking in and it then seems to fall on its face and doesn't get past 3600 rpm. I am able to squeeze the primer bulb as I am driving but the forced squeezing seems to make the motor fall off slightly rather than pick it up. I did stumble on one thing that allowed the rpms to climb quickly. the one tank was low on fuel and wasn't able to draw all the fuel it would normally through the gas line and the motor climbed quickly to between 4500 and 5000 rpms then stopped running. I then had to switch tanks and was unable to replicate that exact senario. I then was able to somewhat replicate it by running the boat at idle the quickly disconnecting the fuel line from the motor and accelerate to full throttle and it would go to 4100 rpm the then stop running.

Other notes
We have oil mixed in with the gas in the tank for break in and the engine is oil injected. From the best of our knowledge the stator is new and we know the switchbox is new.

I am between the idea that the issue is between the fuel system or the ignition system. It seems like either the motor is getting too much fuel , or the spark is to weak for combustion at the higher speed. could it be from the additional oil in the fuel? Could anyone help shed some light on what can be done to resolve the motor not finding the other 2000 rpms it is capable of with out running out of gas? Any help would be appreciated. Also any thoughts on the aftermarket stator with the two low speed wires? Thanks Glenn

If your saying that unhooking the fuel line it a temporary improvement, than you are flooding the motor with gas. If you have not rebuilt the fuel pump that do that next. They leak gas into the vacuum signal ports all the time. Those only last ~5 years on average.

The stator is a O.E. "red stator conversion" from Merc. That will work.

I forgot to mention in my initial post that I had rebuilt the fuel pump when I installed the new fuel lines. I am not familiar with the vacuum signal ports. Just out of curiousity where would they be located and is there a way to find out if fuel is getting into the ports or should I be looking in the direction of the carbs and the floats. I mentioned I had them apart but just enough to see if they where clean and didn't remove the floats ect. Thanks for responding I appreciate the help.

I know this is an old post, but I have the same motor with the same issues. Did you ever get anything to fix it? If anyone with any experiance with these could jump in that would be great. I have rebuilt the motor and all parts, but still am not making the power it should, or at least I don't think it is. I can't find the problem with the primer bulb, it will never get hard, I am not sure its supposed to or not on this one. Seems to free flow. I have rebuilt the fuel pump and replaced the bulb and fuel pickup as well. Pumping will not help power up. I have the motor on a 17' lund cherokee with 15 pitch prop. I was thinking about changing prop pitch next.

The bulb must firm up when you operate it !!-------So look for a leaking float valve in one carburetor.----A problem with the enricher circuit.--Or a ruptured diaphragm in the fuel pump.---Or air leaks in the fuel line including the pick-up tube in your fuel tank.

Some later Mercs use electronic spark advance. If yours is one of these, the high speed circuit might be shot.

You can tell which type you have by seeing if the throttle arm has a link that rotates the trigger coil below the flywheel. If not, it's electronic spark advance.

Jeff

Some later Mercs use electronic spark advance. If yours is one of these, the high speed circuit might be shot.

You can tell which type you have by seeing if the throttle arm has a link that rotates the trigger coil below the flywheel. If not, it's electronic spark advance.

Jeff

Well i rebuilt the pick-up when I replaced the bulb and that end is all good. when the hose is unhooked from the motor the bulb will stay firm for days. I did rebuild the fuel pump as well. I do have a manual and followed it. I also rebuild the carbs. I may have to check and see if the floats are not adjusted properly or see if there is a problem with the fuel pump. Any way to check the fuel pump to make sure it is working correctly? I still think it is an electrical problem. I think the sator has been replaced at sometime, it has the red stator now. I am not sure about the trigger or the switch box. I replaced the rectifier last year, but that didn't help much. I did a complete rebuild on the motor, had 2 cylinders burned up due to cracked oil lines. I have put 20 to 30 hrs on it since the rebuild but I don't think it is making the power it should. I don't mean to hijack this thread, but its the same issues I am having. I just seems to bog down at wide open at bit and I have never got over 4000 rpm even with one in the boat and light load. The non-firm primer bulb does bother me a bit, but I have pumped the bulb when at wide open throttle and I do not gain any power. The boat is a fiberglass 17' lund cherokee. Any ideas would be great.

fastjeff said:

Some later Mercs use electronic spark advance. If yours is one of these, the high speed circuit might be shot.

You can tell which type you have by seeing if the throttle arm has a link that rotates the trigger coil below the flywheel. If not, it's electronic spark advance.

Jeff

Click to expand...

It has the arm that rotates the trigger

Simply put the bulb does not need to be hard when the motor is running.------I sure do not know why folks think that is must stay hard !!

hi,
not trying to hijack thread either but am having a similar problem, cleaned carbs accelarator pump diaphragm changed filters and injectors that run from accelerator pump seems to be in the bottom two cylinders because when I pull plug wires off of them it sound similar to when they are on.Must be electrical??
going to get very expensive trouble shooting electrical anyone know of a good place to get used mercury parts?or have any advice?
thank you

o and one more thing I have a new fuel pump diaphragm but the old one looks fine is it worth changing?
any help would be greatly appreciated

If you can feel the bottom 2 cylinders kick in at 1800 RPM then it may not be a spark issue. Check all 4 cylinders' spark w/a spark board to compare them...should jump 7/16". If spark is good you need to clean all 4 carbs. Inspect the reed valves w/carbs off. I found a chunck of F'glass holding one open.