1986 Johnson vro70 will run on muffs in my driveway but wont start at the lake?

Probably the pump but could be a vacuum leak

Change lower unit lube, use fresh gas mixture with added Seafoam. Run as before, but longer. You can download manual from the web. Has P.M. instructions.

no
possible water pump impeller failure that is blocking the water flow to the head / block

https://www.youtube.com/watch?v=t33wdLqbUC0

Hi. If you just want to run the motor then you can do it in you driveway. Go to a sports store with boating equipment and ask for "Muffs" to run your boat motor out of water. You can see a pictures of outboard motor muffs on google images:

http://www.google.com/images?hl=en&source=imghp&q=muffs+for+outboard&gbv=2&aq=f&aqi=&aql=&oq=&gs_rfai=

Take the muffs and place them over the water intakes located above the propeller (look like vents on each side). Hook a garden hose up the muffs and start water flow. Make sure nothing is near the propeller. Make sure to pump the bulb in the fuel line to the motor until it stiffens. Crank the motor. When the motor starts, you should see water start to flow out of the back of the motor near the cover and out of the center of the propeller with the exhaust. You must keep water flow to the motor using the muffs while it is running or it will burn up.

Yes the motor must be running for tell tail to work.

My money is on the impeller since it will stick or just dry out and get brittle. The "fins" have to bend to pump water. I'm surprised the top seal is still intact after 10 years.

Is it a reconditioned lower unit?

What sort of oil did you put in the lower unit?

Sounds like there is some drag in the lower unit causing the engine to bog down. It will work fine on a stand in neautral however.

How does it go in reverse on the lake? Can you get full throttle?

I had the exact same problem. The problem was that there was a piece of dirt in main jet of the bottom carb. The engine would rev up aton fast idle but would not produce enough power to plane the boat due to the lack of fuel flowing through the carb. Cleaned all three carbs. Motor ran like a champ.