1974 Evinrude 70hp on full throtle but still goes slow?

Question

I just bought this boat and took it out on the lake today and i got it up to about 25-30mph which was good, all of a sudden it slows down when the throtle is still pushed all the way foward, it seems like im not getting enough gas or something. I called the guy and he said to change the gas tank because there was soemthing wrong with the gas tank. What do you think is wrong with the outboard?

Answer 1

check to see if throttle linkage is still conenected, also may have leak in fuel hose

Answer 2

Sounds like it's sucking air either from the fuel hose, a connection, or the adapter where it plugs into the tank. Also, you could have a hole in the fuel pump diaphragm. The fuel pump on that one may be the little square shaped thing you will see on the side, it's very cheap and easy to rebuild.

Answer 3

do you have water sepperator, change it   is there water in the bowls on the carbs  check it  also is the vent clean and not blocked

Answer 4

A cylinder is dropping out.

DON'T run the engine until you fix it - any cylinder starved for fuel is also not getting any lubrication.  It's being dragged through a dry hole - not good.

It does sound like fuel starvation, but before you start tearing into the carbs, there's a couple things you can check first, might save yourself some trouble.

Compression test.  No sense spending money on a motor with bad compression.  Compression gauge, about $25 at any car parts store. Need help with the testing, just ask (in a separate question).

Spark test. Not a "spark plug" test. Use an inline tester (couple bucks at a car parts store) to test whether the ignition is capable of generating adequate spark, set the gap for 7/16", clip it to the engine block, connect the spark plug leads to it (one at a time), and look for a sharp blue spark while someone turns the engine over with the key.

Does the fuel line hold pressure?

Is the fuel tank vent open?

If that all checks out, then what you've got is a classic symptom of clogged hi-speed jets.  Motor sitting idle for a few months, gasoline turns to varnish in the carbs.

I would probably just go ahead & rebuild 'em now, but you might want to try a new fuel line & fittings -  get a name brand bulb (BRP, merc, yamaha), the after market bulbs have a poor reputation for reliability.  

If you're using the original 6-gallon metal fuel tank, it should be replaced anyway, the finish is about to start flaking off the inside and those little particles will get past the screen filter and clog your carbs (don't ask me how I know) - they really don't last much longer than 25 years.

Otherwise, unless it leaks, a tank is a tank is a tank, get a new one if you want.  I use two plastic six-gallon tanks.

Next time that happens, squeeze the primer bulb.  If it collapses (doesn't open back up) then you've got a restriction upstream of the bulb.  Debris in the tank, faulty bulb, or your tank vent is blocked.

Use only champion QL77JC4 plugs gapped at .030 - not just a personal brand preference, that motor really does better with the champs.

Use only 87 octane gasoline mixed 50:1 with a TCW3 certified oil. Higher octanes will just give you a less complete burn.

If you don't know the age of the water pump impeller, or if it's more than two years, this would be a good time to change it.  Basic maintenance, but the downside of pump failure is catastrophic.

Be a good idea to change the lower unit oil, too - needs to be done annually.