1979 Merc 115 two cycle - Slow at WOT

[TheGreenHornet]

New boat guy here. Really like this forum! Learning some good stuff.

I had my boat out yesterday and it was running great and then I ran out of gas on one tank so I switched tanks and forgot to open up the gas tank vent. The motor started to die like it was running out of gas again so I opened the vent which helped the motor get back going again but then I could never get WOT again.

The motor starts right up and idles fine and when applying just a little bit of throttle it runs as expected but when I open it up it just never gets to higher RPMs. It sounds like its going to start picking up but never does. At first I thought it was a fuel issue but it really smelled of gas and used up a lot of fuel as I limped back to the dock. I also tried spraying a little starting fluid in the carbs to see if it maybe was starved for fuel but it just made it bog down even more and almost die.

More background info:
* New fuel line and bulb, bulb stays hard once pumped
* New gas tanks, with fresh gas and oil
* Cleaned fuel pump screen before going out
* Sparkplugs are new this year and when I removed them they appeared ok, very light grey and wet with gas. Not fouled up just a little back dot right next to the tip.
* Carbs are opening wide up at full throttle
* Opened the gas tank and checked the pickup tube and it looked fine


While out in the water I also tried letting it sit for 30+ minutes and it started right back up and idled fine but did the same thing once I applied more throttle.

Any ideas on what I should try next?

 

[Dreddrs25]

Re: 1979 Merc 115 two cycle - Slow at WOT

When something similar to me happened, it ended up being poor fuel pressue. My Merc would fire right up no problem and easily get up to 10-12 mph, then fall on its face.

I had a bulb right by the fuel line connector and one just under the engine. Had to pump each a few times to get good pressue in them. Then the boat ran like a champ? GL

 

[Tadd_P]

Re: 1979 Merc 115 two cycle - Slow at WOT

it was most likely flooding itself out. Let it set a day or so and take it back out. Note to self: Clean the plugs first though. If it is still doing the same thing then you have either an air leak in the fuel line somewhere and or you might have sucked some chit up from the bottom of that tank and clogged your carbs up. If it does act up again get yourself some carb gaskets and carb cleaner and a bottle of compressed air. Clean the carbs out real good and try again. Also those engines like to eat gas anyways. I have a 1979 115 outboard without a distributor myself and it loves to suck gas!

 

[TheGreenHornet]

Re: 1979 Merc 115 two cycle - Slow at WOT

ok thanks for the tips, I will check for fuel pressure and line issues.

Also I should mention its actually a model year 1979 1/2. What is different on the 1979 1/2 vs the 1979?

 

[dinikin]

Re: 1979 Merc 115 two cycle - Slow at WOT

If you have vacuum and pressure gauge you can check fuel pump.
Not sure if you have internal or external, to see if daphram is busted, or line restricted.

 

[TheGreenHornet]

Re: 1979 Merc 115 two cycle - Slow at WOT

I checked all my lines, tanks, bulb put new fuel in and new spark plugs and took my boat out again and it did the same thing.

Starts up fine and sounds fine and goes as expected until about 2200 RPM and then it stays right there even as you continue to open up the throttle.

I tried pumping the bulb to see if maybe it was not getting enough fuel and all the did was flood it out. The motor doesn't sputter or pop and the carb butterflies are all the way open. Whenever I have had carb issues (especially multiple carbs) on cars it has never been this perfect where it just goes to a certain speed and then does not go further. Is that typical of carb issues on outboards? Should I just go for a complete carb rebuild or should I try something else first?

 

[Plainsman]

Re: 1979 Merc 115 two cycle - Slow at WOT

I'd rebuild the carbs myself. Clean em real good.

 

[Chris1956]

Re: 1979 Merc 115 two cycle - Slow at WOT

If your carbs are open and you do not have any more than 2200 RPM, you either have a spark advance issue, some cylinders are not firing or a total power issue.

Check compression on all cylinders. it should be at least 90PSI and even on all. This will eliminate the total power issue. If a cylinder has zero compression(usually no 1 or 3), you need to rebuild the motor or junk it.

Now look to see that the trigger under the flywheel is moving as you advance the throttle, with the motor off.

Now put a timing light on each spark plug wire in turn, while the motor is idleing and see that you have spark to all six.

If it passes all these tests, perhaps you are overpropped.

 

[TheGreenHornet]

Re: 1979 Merc 115 two cycle - Slow at WOT

Trigger is moving and the prop choice was fine earlier in the day so I think I can rule those out.

I will check for spark and compression before going in to the carbs just yet.