merc problems

Post by **1lastcast** » Sun Oct 05, 2008 6:49 am

I purchased a use 20hp merc that we use for waterfowl hunting. Im running it on a 15ft tracker grizzley flat bottom style jon boat. The boat is rated up to 35hp. Im running 3 men a dog and gear.

My Problem? The motor starts fine and also runs fine. But I can't seem to throtle past half with out the motor over reving and the boat almost stoping all together??? Its almost like when you have a motor with weeds around the prop. But there are no weeds???

I have tried adjusting the motors trim by placing the steel bar in differnt adjusting holes. It does not seem to make any differnce. Could the motor be to short for the boat? Or too much weight in the boat? Or is this a sign of a failing motor???

thanks guys.

Post by **fishmaster** » Sun Oct 05, 2008 7:17 am

If this is an older motor it sounds like it could be a prop issue. I had an older 15hp one time that did the same thing. It ended up that the old props had a rubber core inside them and the rubber would get sand in there or it would get old and as soon as the prop got any sort of load on it it would just spin inside. Other than that I can't think of anything else.

Post by **1lastcast** » Sun Oct 05, 2008 7:48 am

Im unsure of the age? Id guess mid 90's?

Its a 20 hp 2 stroke with gear and throtle controls on the tiller arm.

I did have the prop fixed as it had some wear and a large chunk out of it. I would assume the prop shop would check the rubber bushing inside also? Maybe not? I have had that happen before on a differnt boat. Thanks for the reply. I'll check into that for sure.

anyone else with input?

Post by **orrsey** » Sun Oct 05, 2008 11:38 am

I would also guess it to be a spun prop....had it happen to me a couple times as well.

orrsey

Post by **Seabass81** » Sun Oct 05, 2008 12:46 pm

does the transom size correspond to the proper shat length? 15" / 20"

john

Post by **Cyber** » Sun Oct 05, 2008 5:51 pm

I'd be very interested in the outcome of this diagnosis too!!

I have a 1980 50HP Merc & I believe my symptoms may match yours 1LC.

Sometimes, but not always, when I try to 'Pin' her from a slow speed the engine overrevs like crazy

If I back off and slowly increase the speed it'll reach full speed no problem.

Best way I have to describe it is it reminds me of a 'slipping' transmission and it'll rev so high I think she's gonna blow up!!

But when you guys mention the 'spun-prop' scenario that makes much more sense.

My props are the type w/ the rubber core. Next spring I'll try another prop & see if this alleviates the problem, that sure would be easier on the pocketbook than rebuilding the 'drive-train'!!

Post by **1lastcast** » Sun Oct 05, 2008 6:23 pm

Yes Its 20" at the transome and the motor is also a long shaft.

Wouldnt prop spin happen at any throttle speed? THis only seems to be happing at half to 3/4 throttle?

Thanks for the input guys. Im going to pull it off and have a prop shop take a look. Unless there is some way to test this myself?

Post by **Terry_F** » Sun Oct 05, 2008 6:43 pm

I also agree with the prop slip theory.
I just had this on my boat after an engine rebuild.

Boat would seem to rev high at top end (Wide Open Throttle WOT), but the speed wasn't as good as specs said it should be.

Changed the prop and voila, WOT was a reasonable value and top speed went from approx 35 MPH to 43 MPH.

This was on a Johnson with a rubber core prop.
I still have the old prop and there is no way I can make it slip on the bench, but I guess at 5000 RPM and under load that is exaclty what was happening.

Check with a dealer, they should let you try before you buy, just to prove that is the problem.

Post by **orrsey** » Sun Oct 05, 2008 6:45 pm

1last,
I have read somewhere that a spun prop will not start acting up until over 1500rmp usually. I have had it before where I could run normal on calm water but in any kind of chop (it would push harder through the waves) it would start acting up.

orrsey

Post by **1lastcast** » Sun Oct 05, 2008 8:03 pm

Great input

SOunds like I need to purchase a prop.

This leads me to my next question.

Should be changing the pitch to help with a heavy loaded boat? Or just stay with the stock pitch. I think its a 13p?

Post by **xcal** » Mon Oct 06, 2008 5:25 pm

1lastcast:
I have an older 20hp merc (1978) There is a rubber bushing inside the prop that wears out and starts slipping. I have to chang it 3 times in the last 8 years. Sometimes when you have work done on the prop, they heat it up. Not long after the rubbers slutch lets go. They usually test iy by putting the prop on a merc spline and see if they can spin prop. If they are able to spin by hand, you need a new bushing. Hope this helps.
xcal

Post by **1lastcast** » Thu Oct 09, 2008 6:51 pm

OK I hope you guys are right. I purchased a new prop today at $140

I will report back after my next trip out. wish me luck

Post by **1lastcast** » Sat Oct 18, 2008 4:19 pm

Well after installing a brand new prop. Im still having the same trouble???

Im starting to think that the motor is to short for the boat?? The transome is 19". Is there such a thing as a 20hp short shaft???

I also noticed that the steering was very hard to the left. I have adjusted the skeg to make up for the torqe. Could this be causing the engine to over rev?

I have to note that this trips boat load was less with 2 people in place of 3. Also trimed the motor up 1 hole or seting from the bottom.

Post by **Haybay Larry** » Sat Oct 18, 2008 4:38 pm

It seems like you might have a cavitation problem. It's possible that the motor is mounted too high on the boat. This would cause the prop to spin with no forward movement because it is hardly moving any water.
Try moving the motor down one notch and see if there is an improvement.
...Larry

Post by **beachburger** » Sun Oct 19, 2008 7:06 am

Yes there is such a thing as a short shaft (15") 20HP.

Try taking it on a solo run or have a 2nd person sit towards the back. This will keep the motor as far down in the water as possible and let you know if you have a cavitation problem.