1980 Mercury 70HP goes half speed.

[WesE]

I bought this boat last May, and the engine ran fine all summer after changing out all the hoses. I could cruise along at 32-34 mph. I was recently on the river when the speed dropped from 30 to 15 without the sound of the engine really changing. It just wouldn't get back up to speed. Someone told me that it sounded like the high speed jets had gotten clogged. Does that sound like reasonable place to start? If so, can someone direct me to a diagram that identifies those jets and some instructions on best how to clean them out?

Thanks for any info....
wme
waco, tx

 

[Georgesalmon]

Re: 1980 Mercury 70HP goes half speed.

Before you start taking things apart its barely possible you spun your prop. Though usually the engine will increase speed when that happens. Cheap to check, just mark the prop and run it then see if the marks are still aligned.

 

[Maxz695]

Re: 1980 Mercury 70HP goes half speed.

Here is a diagram of a 1979 70 HP like mine should be the same . This will help yo identify the parts and the part numbers. You can look through ebay andsome of find the parts cheaper. Mercury Marine 70 HP (3 CYL.) CARBURETOR ASSEMBLY Parts Also check your plugs and fuel line hoses. Yes it is true the prop may be spun and should be checked. After dismanteling the carbs soak in cleaner over night and then blow them out real good with an air compressor to clear the passages. New gaskets to the intake are recommended. Check the mixture srews. Close to a light seat then back out 1 1/4 turns then start the engine and adjust to the best idol. turn out about 1/8ths more and should be close to where it should be. Check the fuel tank vent is not clogged at all which would restrict the amount of fuel to the carbs from the fuel pump. Check all fuel lines for cracks loose fit pin holes etc. Check to see if any of the plugs are whitish indicating steam cleaning by water intrusion.