1985 Mercury Tower of Power 115 high rpm miss

[Aquasport1985]

Hello. I am currently having a high rpm miss on my 1985 mercury 115. The engine idles great and is fine at low rpm but when I try to throttle up to WOT it does not want to get past 5k rpm. You can feel and hear it missing sporadically and feels like it is dropping a cylinder intermittently. Occasionally you will feel the engine grab for a couple seconds and it runs great then falls off again. In the past year I’ve put a new stator on, replaced power packs and cleaned carbs thoroughly. I have attached a video of it running here. Hoping you can hear the miss. Any help is greatly appreciated.

 

[brodmann]

Sounds like a fuel delivery problem to me. Check filters, kinks in lines, fuel pump diaphram and/or try squeezing the primer bulb when this happens.

 

[Aquasport1985]

Thank you for the quick response. I will try all of the above this weekend when taking it out.

 

[jimmbo]

I couldn't hear any miss.
One thing you gotta do on engines of that Era, is disable the 'Back Drag' feature on the Carbs. It was a way of leaning out the Mix at part Throttle, in order to reduce Fuel Consumption. Problem is that the Gasolines of Today are already Chemically Leaned, and the Back Drag leans it out too much.
In the Image, there is shown a Hose running across the top of the Carb body, that needs to be plugged, but remained attached the fitting on the Carb Throat. It can be left off the fitting on the Fuel Bowl Cover.

 

[TheGM1217]

I have a 1986 90hp tower of power, been dealing with the same problem for the past month. My RPM's were falling off at WOT. Would be running 4500 RPM's then just drop off to 3500/3700 RPM's. Never felt a miss, just lose power. Pull the throttle back and it would run great. I thought that the problem would be in the ignition system. Check the stator, and the coil packs, I replaced the power packs, put new spark plugs in and no fix. A friend suggests a possible fuel pump issue. Bought a Quicksilver fuel pump rebuild kit, took less than a hour to rebuild pump, took it out last night and runs great. As far as I could tell the pump had never been rebuilt. Running better than it has in a long time.
Jimmbo? I know my carbs still have back drag hoses on them. Will it, make a difference on how the engine runs if they are still attached. I've owned the engine for 30+ years and have never had a problem that I know of with them hooked up. THANKS

 

[jimmbo]

The Back Drag feature was introduced in 1975, in order to improve Part Throttle Fuel Economy. It did so by reducing the Atmospheric Pressure in the Fuel Bowl, so less fuel was being pushed into the Main Nozzle. Worked fine with the Gasolines of the 70s. Fast Forward a few Decades, Gasoline is a far different Formula now, and has Oxygenators added which essentially leans the Air-Fuel Ratio Chemically. The Back Drag feature when combined with modern Gasolines, effectively leans the mixture at Part Throttle too much, putting the Engine at risk of excessive Combustion Temps and related Failures.
The Back Drag feature, has no effect at Idle or WOT, However, disabling it has been a recommended procedure for a couple of Decades, in order to prevent engine damage.
It is your engine, so you can do what you want...

 

[TheGM1217]

Thanks, jimmbo. I will certainly look at mine more closely, heck, they might not be connected, and I have never noticed. I certainly don't have 15k for a new engine nor the time to have it rebuilt, if that would even be possible in today's world.

 

[jimmbo]

Power gradually dropping usually points to a Fuel Supply issue. A sudden change can be Ignition or Fuel. My 84 115hp would often find itself running rich on two cylinders being fed by a shared Carb. It seems the Float would get cocked and stuck in the fuel bowl. Sometimes some gentle taps on the side of the carb would free it, other times I would have to take the bowl cover off and persuade the float free. It was easy to tell when it was happening, as the Rpms would drop, at speed, and the motor would run quite rough at Idle, and fuel could be seen dripping out the Bowl Vent.