1981 Merc 90hp looses power when in lake

[rikster411]

Hi everyone,<br />I have a 1981 merc 90 hp, that has had the carbs rebuilt, a leak in the lower unit fixed, and one coil replaced. The engine runs fine on dry land, with the hose hooked up to it. Ive had the prop off and put it in gear and can run it for hours, no problem. It does however run at 1000 rpms at the slowest in gear out of the water. put it in the lake and the rpms are right where they should be at 550 in forward. Give it some gas and it starts to take off gets up to about 3500 rpms then just dies down to 1000. It'll stay there for a while, then may kick in and take off at full power for a short period before falling back down again. I took it back to the mechanic and he runs it with me standing there and it runs fine out of water. Does anyone have any ideas? it sounds electrical to me, is that right?<br />Rick.

 

[schaiss]

Re: 1981 Merc 90hp looses power when in lake

Running the engine on land with muffs on does not simulate the load on an engine when it is in the water trying to push your boat. When were the carbs worked on? If I were a betting man I would look at the jets in those carbs. If the engine were not stored properly after last season and untreated fuel was allowed to sit in them, the fuel would have broken down and left a varnish residue. Does not take much of this stuff in the jets to ruin the performance of that engine. Also, when was the last time you replaced the spark plugs?

 

[rikster411]

Re: 1981 Merc 90hp looses power when in lake

Thanks,<br />Carbs were rebuilt 1.5 months ago, as well as replacing plugs. At least thats what the shop said. Wouldn't varnish be a constant problem, instead of kicking in and out? or could dirty carbs run ok then loose power, then run ok?<br />Thanks for the input.<br />Rick.

 

[jim phillips]

Re: 1981 Merc 90hp looses power when in lake

I just walked in the door from having the same problem, Put a rebuild kit in the fuel pump on my 60 hp merc $12.00 for the kit 30 min time = boat runs great!

 

[rikster411]

Re: 1981 Merc 90hp looses power when in lake

Thanks,<br />That's great to hear. I'll buy a fuel pump kit this week. I'll let you know.<br />Rick