1988 Merc 70hp 2 stroke, losing power at WOT

Hi everyone, I'm new to this page as well as being pretty new to boat engines as well. I have an older Merc 2 stroke that I just purchased and took out for the first time yesterday. Rode her out about 30 minutes to my job site, topping out at only about 3/4 throttle. Docked her up and went to work. On the way in we decided to open it up and it ran flawless at WOT for approx 2-3 mins and then died back to around 1/2 throttle. We drove it in at that throttle without ever taking it all the way back to neutral. A few times we dialed back the throttle and raised it again with no results. However, when we made it back to the landing, after putting it in neutral and then back into gear it ran full throttle for another 20-30 seconds before dying back again. The first time it died back I checked my primer bulb and it was soft, but when I squeezed it a little fuel leaked from where it connects to the engine. We only tried that once or twice and didn't force the issue, just limped home. What's weird is when the power came back it came back just as strong. The fuel is maybe a year old but the tank has always been kept full. My boss seems to think it's the switch box. He's had a few boats and was riding with me when it happened. Said he had something similar on another Merc two stroke. A lot of forum reading last nigh has me believing I should siphon the tank and change the fuel filters and lines and run everything fresh before buying a power pack. I will add that the guy I bought the boat from said he DID replace the fuel filters and fuel lines when he purchased the boat in august 2016. Thanks in advance guys, I believe I gave as much info as I have. I'll check the model number on the engine when I get home to make sure it is exactly what we think it is. I believe it's the 3cylinder big bore. Thanks again.

 

You'd be wise to buy a workshop manual before buying parts that you suspect might be the cause. You can consult the trouble-shooting procedures for clues. That fuel leaked from the connector when you squeezed the bulb, suggests it might be sucking air there.

 

Thank you very much, and I planned on replacing the lines before anything else. However, I consulted the previous owner today and he let me know they only filled the tank with Aeroplane fuel, i.e =~110, I agree with him fuel shouldn't be an issue. But i will replace the lines before draining the entire tank. My plan was to siphon the entire thing today but after he told me airplane fuel and 0 ethanol fuel was all he has added, I'm not so sure it's not the pack.

 

You can check the electrics out if you have a manual and a simple test meter, I would not be buying expensive components on spec.

 

No manual, have a few meters.

 

So your best best is to replace the filter and lines?

 

Check the filter for debris, you can also check the fuel pump diaphragm for holes, I would be suspicious of the snap-on connector if it was seen to leak. But you need a workshop manual, you may even find something downloadable on-line.

 

vent open on the fuel tank?

 

18gallon in the hull. Where would I check?

 

The breather hose inlet/outlet. Usually a fitting on the outside of the boat.

 

I thought it might be a plastic tank with a manual vent plug
Your seller said he changed the fuel lines. It is not right to see a leak at the fuel connector when you squeeze the bulb. I would borrow someones merc red fuel tank and fuel line and plug it in. If it still runs bad, then there is a fuel pump issue or motor issue. If it runs well, then work from the connector back to the fuel tank

Not sure on your tank but there is often an antisiphon check valve on the top of built in fuel tanks. The idea being that with suction at the engine, the valve will open and let fuel flow, but say you had a fire and the hose burned through, then the check valve would not open. ie anti siphon valve
In old boats these are notorious to pick up junk from the bottom of the tank
when you start to run, in a short time they pick up enough gunk to plug the opening

If there looks like a longer than normal fitting on the drawline at the top of the tank, perhaps it has such a valve. Then you have to take it off and clean it

BUT BUT, you have to be EXTREMELY careful undoing an old fitting off the top of the tank as if the tank fitting into the tank is a little corroded the fitting can snap off and you have to install a new tank.
So if you have a problem, you need to ensure that you hold both sides of the fitting, the tank side and the nut side to break the connection. I would use several applications of a penetrating fluid, not WD 40 but an actual penetrating liquid. Apply it say 4 times over a day to loosen the fitting. Did I recommend to get a grip on both sides of the fitting?
Also borrow someones fitting nut wrench to grab onto the fitting.

We had this happen on a pair of 350 crusaders years ago. Cleaned the anti siphons and never had a problem

 

I will be sure to check in the morning. Going to clean the carbs and give high tide another go.

 

That was in reference to the breather, trying to take in everything said about the anti siphon

 

I added some info on penetrating fluid

 

What would be a longer than normal fitting at the drawline?