1971 Merc 200 20hp gear issues

[Duck_Hunter12]

took boat out for a spin and would go about 5-6 mph and the the engine would rev up. So i searched on here and thought it was a spun prop hub, I bought a brand new prop and the same issue remains. I grabbed the prop, while the engine wasn't running. and spun it while in foward, was able to spin easly in the reverse direction. I tried the same when the motor was in reverse and it would not spin easily in either direction. Do you think my forward gear is out, also will all 20hp lower units from 69-81 work with mine?

 

[Duck_Hunter12]

Re: 1971 Merc 200 20hp gear issues

I called a few shops and took the boat out several times and the only solution I can think of is there is some shaft play, more that what I would assume to be normal, Is this causing the prop to lose grip when vibrating?

How hard/expensive is such a fix?

 

[Chinewalker]

Re: 1971 Merc 200 20hp gear issues

The spinning in forward when the motor isn't running - that's normal. The forward gear's shift cogs are ramped to the clutch dog, allowing the prop to spin on a rapid decelleration (so the motor doesn't kick up into the boat). The no spin while the shift is in reverse (also not running) - that's normal, too as the cogs and dog aren't ramped to each other.

If you drain the lower unit oil and it has chunks of metal - then it may be gears. Otherwise, it isn't likely to be anything internal. You've eliminated the prop as a possible issue. That leaves the driveshaft. Mercs of that era had carbon steel shafts that were prone to rust. If a previous servicing didn't include a new o-ring and grease in the upper splines it is entirely possible that the upper driveshaft splines and/or the crankshaft are spun out. If this ends up being the case, then both the crank and driveshafts must be replaced - probably not recommended on a nearly 40-year old motor...

 

[Duck_Hunter12]

Re: 1971 Merc 200 20hp gear issues

Thanks for the response,

I figured that was normal but wanted to ask in case that cluth was getting worn. after the last run it appeared that the prop would slip as soon as the load was coming off the motor, like right before the boat wanted to plain.

I took the LU off afterward and inspected the splines of the driveshaft and prop shaft and there was no signs of slipping or worn metal. I drained the LU as well and no metal in lube.

I am pretty sure there is too much shaft play in the prop shaft causing it to vibrate when there isn't enough load on the prop, is the enough to cause the prop to completely lose grip until the RPM get back down? If so how hard and expensive is it to press in a new prop shaft bearing?

 

[jake505]

Re: 1971 Merc 200 20hp gear issues

just a quick thought is it possible that motor is to short for the boat its on and when you start to come on to plane and the boat starts to lift out of the water the motor starts trying to push air instead of water. The would cause it to rev up when it starts to come on plane because there would be no water resistance on the prop

 

[Chinewalker]

Re: 1971 Merc 200 20hp gear issues

Can you actually wiggle the propshaft side to side? What is the condition of the oil? Worn bearings would likely be burning up the oil - making it very dark and smelling like... well... it would smell bad.

In any case, I've never heard of a worn bearing causing the problem you're having. A slipping clutch would be "chunky", not "smooth". The motor would jump heavily as each lug of the clutch slipped. Also, have we eliminated the prop for sure? Your "new" prop might not be so new after all. Mark the hub and prop nut and see where the marks are after a run. Some "new" props have been gathering dust for 40 years...

 

[Duck_Hunter12]

Re: 1971 Merc 200 20hp gear issues

Its a shortshaft motor on a shortshaft boat, i had a '61 18hp SS johnson on the same boat without an issue. The merc shaft is actually slightly longer.

I did check the hub to see if it was shot, I made a mark and it was good. I will recheck just to make certain.

I can move the prop shaft slightly, maybe .010" or more back and forth, more than what you would think a good bearing would be able to. The oil seems fine, no burnt smell. The shaft can slide in and out some, I was assuming there are some shims in there so I didn't pay any attention to that.

It is runs smooth until the prop slips, then engine revs high and motor vibrates more. There are definatly no clunking, banging noises like a gear issue would cause.

Like I said I'll have to recheck the prop hub again, this thing is bugging the crap out of me.