Help with a 50 hp mercury thunderbolt

[ChatWizard]

I have an early 80's (at least i think, has the double blue bands) 50 hp mercury thunderbolt. I have it on a 15 ft bayliner capri bowrider that I just bought this year. Was working great but last run out the only way I could put it into gear was to have the pre idle on (if shut off engine would die). We were out in the middle of the lake when this happened so really had no choice. It would of course make a clunk noise (kinda like when a car transmission does when parked on a hill) when putting into gear from neutral, by the end of the day when we started heading back i heard a pop sound a couple of times and then the engine would start to rev up to max rpm's without the boat going anywhere. This happened everytime i would start to accelerate past idle speed. The motor worked fine at idle speed with the pre-idle pushed up and got us home at about 10 kph. I think i may have damaged a bearing or gear with the hard engaging but am not a mechanic so could be wrong.
Any ideas would be greatly appreciated, before I go starting to tear things apart i might not have to.
Thanks in advance.
Here is a video of the motor if it might help http://www.youtube.com/watch?v=oK1r-4v6gqc

 

[Jake.]

Re: Help with a 50 hp mercury thunderbolt

Have you checked to make sure you didn't spin the hub of your prop?

 

[ChatWizard]

Re: Help with a 50 hp mercury thunderbolt

No I actually I never even thought of that. Will check it and get back to you.

 

[ChatWizard]

Re: Help with a 50 hp mercury thunderbolt

K checked prop,,,everything seems intact. Still doing the over revving thing and seems to be slipping into neutral as soon as I top around 2000 to 2500 rpm. even out of the water with muffs on. Have checked throttle cable and mech, it seems to be fine.
Guess I am mechanic bound cause I am at a total loss as to what it might be.

 

[hotrod53]

Re: Help with a 50 hp mercury thunderbolt

Sounds like 2 different problems. You're probably only running on 1 carb, thats why you have to keep the fast idle up. If you remove the carb bowls, you will probably find one that is dry. If I don't remove the fuel line at the end of each day and run the motor out of fuel, mine will do exactly what yours is doing (the idling problem). Apparently the ethanol in the fuel swells the rubber tips on the needle valve in the carb and causes it to stick in the seat. It has happened to me 3 times this year until I started running it out of fuel. There is no way to unstick it unless you remove the carb and pluck the needle valve out of the seat with needle nosed pliers.

Second problem, not sure about that one but it doesn't sound good, especially if you confirmed that the prop is tight. Recently another poster had this problem after hitting something and he had a broken driveshaft. The driveshaft fits right up in the power head, that turns a gear on the bottom that turns other gears, there are no clutches to slip or wear out. I have seen the inside of a lower unit as mine was all busted up when I got my boat, but I am not the expert on this problem.

 

[ChatWizard]

Re: Help with a 50 hp mercury thunderbolt

Thanks HotRod
I will definitely look into the carb problem. That's a really good tip.
As for my other problem, yeah it has me baffled, I will probably open up the lower unit next just to see if I can see anything broke. The fact that at low rpms the engine runs fine really has me confused but I guess boat motors are different then cars so who knows. Maybe I will be able to see the problem. Just hope I don't have to completely replace the lower end.

 

[hotrod53]

Re: Help with a 50 hp mercury thunderbolt

The lower RPM running OK makes me wonder even more about a driveshaft. If you need a lower unit, it may not be as big a deal as you think. I picked up a good used lower unit on Emay for less than $150 delivered.

If you do pull the lower unit... MARK the reverse lock gear with a paint pen or something. That thing goes back on as many different ways as the number of sides the shift shaft has, sometimes 4, sometimes 12, ect. Its probably the biggest thing that people have problems with when reinstallin the LU. You goof that up, your reverse lock will not work right.

 

[ChatWizard]

Re: Help with a 50 hp mercury thunderbolt

Thanks again.
I am not exactly new to boating, had a 16 ft StarCraft Aluminum bass type boat with a 40 hp Nissan on it, but it was under warranty so always had a mechanic work on it. This is my first older motor so these tips will come in very handy. I would never have thought of that, and by the sounds of it, it could have been a very annoying issue if done wrong.
I really appreciate your help.