1984 starcraft mercruiser engine

So, I have a 84 starcraft boat that was given to me by my grandfather. It has a 120hp 4 cylinder mercruiser engine. I have had the boat for 7 years now. I was wondering if if would be safe to put a can of seafoam cleaner in the gas. I also need a new prop, how would I go about finding a part number for it so I can buy one from ebay?

Re: 1984 starcraft mercruiser engine

On the hub of the prop will be a number, something like 48-78120P40 ... Then a number, like 15P... They are the part number and pitch of the prop...

Seafoam in the petrol?... Can't see any problem with that... Are you having problems? You can tell me, I won't tell anyone else, promise... Have you been doing the annual maintenance on the engine and drive? Or have you just been using it for 7 years?

Chris........

Re: 1984 starcraft mercruiser engine

I've changed the oil every year and do the general maintenance to the lower unit. I just took the boat out of the barn for the year and did the usual stuff I do every summer. Every two years I let my boat mechanic take it apart and grease something he said needed to be done every 2-3 years. I tried doing it one year but couldn't get this one part to back in correctly. After that ordeal, I decided to let a professional do it from then on. Not having any problems but I use seafoam in my truck and snowmobiles so I was thinking it would be good in the boat as well. Boat is in very good running condition(knock on wood) and the only problem with it, is that it needs a new floor. Some of the wood by the gas tank is rotting from getting in and out of the boat while swimming/tubing. I also have a fitch fuel catalyst in the gas tank to keep the gas from aging.

Re: 1984 starcraft mercruiser engine

achris said:

On the hub of the prop will be a number,

Chris........

Click to expand...

there "should be" a number on the hub....mine was only a resemblance of where the numbers where. made it a challenge to find the right prop . sometimes you get lucky and might find the pitch stamped to the side of the prop as well.

Re: 1984 starcraft mercruiser engine

My fathers friend is an amazing welder and I may just bring him the prop to have him fix it up. Anyone know if this motor is carbed? I know I have to move the throttle up and down to get it started which makes me think it's carbed.

Re: 1984 starcraft mercruiser engine

Yes, it's a carbed...

Re: 1984 starcraft mercruiser engine

brettinNY said:

I have had the boat for 7 years now.

Click to expand...

brettinNY said:

Anyone know if this motor is carbed?

Click to expand...

7 years and never had to open the cowl to do any work? i wish i had that luck....

Re: 1984 starcraft mercruiser engine

cedarjunki said:

7 years and never had to open the cowl to do any work? i wish i had that luck....

Click to expand...

What kind of work should I be doing? The boat sees the water 5-8 times a year. Just took it out for the first time today and it ran great.

Re: 1984 starcraft mercruiser engine

brettinNY said:

What kind of work should I be doing? The boat sees the water 5-8 times a year. Just took it out for the first time today and it ran great.

Click to expand...

Exactly what you're doing... Sounds like you have a mechanic who knows what he's doing... If all the regular maintenance items are being dealt with, just let him keep doing it....

Chris.........

Re: 1984 starcraft mercruiser engine

How often should I replace the spark plugs? Should I even bother if the boat runs well? What about the distributor cap?

Re: 1984 starcraft mercruiser engine

Also, at full throttle my boat only goes to 3000 rpms. Is that normal when it goes up to six? This engine blew a head gasket about ten years ago. The same mechanic that does all my work rebuilt the engine. Does anyone think he set the throttle to not go over 3000 rpms? It seems like the boat has more power but I don't want to mess with a good thing if I shouldn't be.

Re: 1984 starcraft mercruiser engine

You've probably got a couple cylinders not firing. Def NOT a case of mechanic limiting throttle to 3000rpm.

Welcome to the world of "boat fixin".

Re: 1984 starcraft mercruiser engine

New spark plugs would fix this problem?

Re: 1984 starcraft mercruiser engine

? ?

Re: 1984 starcraft mercruiser engine

Brettin, there are many things that could cause your issue, if it exists at all. I am not farmiliar with the specs on your engine, but 3K does sound a little low. Normal operating RPM should be between idle and 4500 safely, it really depends. If it were propped incorrectly that could limit the RPM, or if your prop is damaged. Also, if you like taking the bigger girls out on the lake, that could do it too.