View Full Version : Merc problems

roger rednour
04-16-2003, 11:12 AM
i have a older model 40hp merc think it is a 76 model it's a 402 it was running fine one day comming up the lake then it just quite pulling i have cked bottom unit it was fine cant figure out why it quite it still wont go in any kind of gear any suggs

04-16-2003, 11:30 AM

First check the lower shift shaft. It can become frozen in the casing, making it very difficult to shift. You should be able to see this without taking the lower unit off. The coupling between the upper and lower shift shafts should be visible just above the front of the lower unit.

The coupling itself may be the problem. This should also be easy to diagnose.

If both the upper and lower shift shafts seem to be turning freely when you shift gears, then you probably have a problem in the gearcase itself.


robert perneborn
04-17-2003, 02:54 AM
" Roger,
I had a Merc -76 402 40hp, and I seem to recognize your problem.
The splines in the joint between crankshaft and vertical driveshaft might be worn down. If so, the motor is running like in neutral. But since the vertical shaft isn't spinning, the water pump isn't working. You can see there is no water coming out from the tell tale.
If you drop the lower unit, you can see the splines of the shaft are worn, but maybe not totally disappeared. The crankshaft splines seem to give up first.
I also had Merc -74 402 40hp with the same problem. That model didn't have stainless shafts, so corrosion (salt water) did a lot damage to the shaft splines. I bought this -76 model, after I was assured it had stainless shafts. I thought I could use the old motor for spare parts. Since my experience with the earlier motor, I dropped the lower unit periodically and inspected and greased the splines. Although there was a wear, more than half of spline was left when it broke. However, I hadn't been able to inspect the crankshaft, without taking off the power head. It ran till 1997. It seems that the crankshaft wasn't stainless (or bad quality).
You have to repair, or replace both crankshaft and vertical drive shaft.
My decision was to scrap the motor and buy a Johnson instead.
My confidence to Mercury has been spoiled (2 motors with same problem).
I sure hope you have a less severe problem, but this is my experience.
Robert "

Joe Boyer
04-17-2003, 09:15 AM
" Help,
I have a 1994 25 hp merc outboard that bogs down at full throtle, only when there's a load on the prop. It was working good one day then all of a sudden I started having this problem.

Any Suggestions? Thanks in advance. "