1982 Mercury 40 HP 1982 Star craft Boat

" I have a 1982 Star Craft Alu

" I have a 1982 Star Craft Aluminum Bass type fishing boat with a 1982 40 Horse Mercury on it. The boat seems really under powered. I've tried props and on it I think the motor is too small. I heard the newer motors were rated differently than older motors. The plate on the boat says max HP is 85. I'm sure the motor came on the boat, It runs pretty good,its possible that its worn out. Would it be better to replace the rings? What would be a good motor for the boat.
Thanks Gary "

" Gary,

I can't say how

" Gary,

I can't say how that motor was rated, but at one point some engines were rated at the powerhead, while others were rated at the prop. The difference (so I've been told) was 10 - 20%. You haven't mentioned what size, weight, or type that boat is, but if the max is 85 and it's got a 40, it sounds like it's underpowered. Also, it makes a difference whether it's tiller or console steering. Post up a little info and I'll give you my .02 on what size engine.

To determine whether the rings and clyinders are OK, you need to test the compression; gauge costs about $25 at any auto-parts shop, you want the type that threads in. Pull the plugs out and check compression, it should be around 100+ and within 15 psi on all cylinders.

The main reason to change props is to gear the engine correctly for the boat. The only way to determine what prop is right, is to use a tachometer. Run the engine at full throttle and determine what RPM it tops out at; with a light load it should be close to the max operating range. Every engine has an operating range, not sure exactly what yours is, but post and I'm sure someone will answer that for you. It's going to be around 4500-5500. You don't get peak HP out of an engine without having the correct prop, it also can be damaging; although, my guess is the engine is too small to have it perform right on that boat anyway,

Jon "