1988 115 hp merc stalling on de celeration

[leog]

my engine idles fine shifts fine gets to top end fine when i cut back on the throttle the engine dies. It starts back up fine but keeps doing the same thing on DEceleration. any help is appreciated. I can do basic things and checks but i am definitely not a mechanic. thanks all

 

[Texasmark]

Re: 1988 115 hp merc stalling on de celeration

I bought the same engine new. Never had that problem. What is the maintenance/use history on the engine? Where I am headed is that with "back drag" carbs the timing is held at a fixed value and the carb butterflies are moved in the closed position......you punch out of the hole, get on plane and up to speed, say 38. Then you ease the throttle back and your speed drops off to say 35. If you had a fuel flow meter, you would notice that your speed dropped 3 mph but your fuel consumption may have dropped 20%. This enhanced the mpg. There was obviously a limit to how far you could cut the throttle before the timing retarded along with closing the carb's butterflies.

Another thought: Out of the water, or lower unit out of the water, engine off, shift into F gear and ensure that CCW turning of the prop offers resistance....to rotate the prop you will be turning the crankshaft. Turn it CW and it rotates easily and makes a clicking noise....that noise is your overrun clutch in your lower unit...your clutch dog that your shift linkage moves forward and backwards along the prop shaft to hook into the F gear which is up front in the LU or R gear which is at the rear part of your LU.

If the overrun clutch isn't functioning, your boat should come off plane very abruptly, and it would be loading the engine really big time because instead of the prop free wheeling, being driven by the rushing water, not the engine, it is locked to the engine and is keeping the engine up and running in rpms, but you just strangled it by cutting the throttle. Doing that while you are manually cutting the throttle would kill the engine.

Keep me posted.
Mark

 

[leog]

Re: 1988 115 hp merc stalling on de celeration

thanks texas but i dont understand all the abbreviations can you give me a more basic explanation? i do appreciate the response though

 

[awehnert4]

Re: 1988 115 hp merc stalling on de celeration

F=forward, R=reverse, LU=Lower Unit

 

[Texasmark]

Re: 1988 115 hp merc stalling on de celeration

And: Back Drag carbs was a name Merc gave to it's engines when they designed the linkage such that it would do as I said which makes for fuel efficient cruising. Nothing special about the carbs themselves.

Punching out of the hole is a Bass Fishermans term and means to get the boat out of the water and on top, hence to get above about 18-20 mph and on top of it. When the boat is at rest water is against the transom (rear of the boat that your engine mounts on/to) from the rear. When "on plane" the force of the boat moving through the water moves the boat away from that water and there is none against the rear of the transom.

Clutch dog is just a sliding gear that is connected to the prop shaft by a pin/key. The engine turns the drive shaft which goes down and into the LU (lower unit....the part of the engine containing the drive gears and shaft, shifting mechanism, water pickup, water pump, drive shaft, and prop) and makes a right turn becoming the prop shaft upon which the prop is mounted. The dog is moved back and forth by the shifting mechanism to engage either the forward gear, no gear, or the reverse gear.

Overrun clutch is nothing more than the fact that both the F gear and the front of the dog that connects to it are shaped similar to a saw blade. In one direction they have vertical mating surfaces and lock....this is the drive mode. Trying to turn the prop CCW looking from the rear is where you feel resistance and if you push hard enough on the prop the engine will turn. When you are up and running and decide to slow down you cut the throttle. This causes the engine to slow down but the prop is locked into the water flowing by it....it is a wheel. So it turns faster than the engine rpms and "overruns" the engine. If it weren't for this clutch, if you were running 30 mph and cut the throttle you would be working your teeth out of your windshield. Grin. You can feel this work if you turn the prop CW and feel and hear the clicking as you turn it.

Mark

 

[leog]

Re: 1988 115 hp merc stalling on de celeration

Thanks mark. Comprende now.gonna look at things again this week and willupdate