115 tower of power- won't go over 1/2 throttle and doesn't want idle on the lake

[Atoyot1031]

I've been lurking this summer as I've been working on my first boat. I've picked up a lot of great tech via search, and I'm greatful for the knowledge base. So thanks! Now I've got a couple questions.

I've got a 1987 Mercury 116 on a 17' 1977 Lund. When I bought the motor, it ran on muffs. Before I started it, I did the following PM: rebuild carbs (also plugged economizer & opened port), rebuilt fuel pump, rewired mercontrol, replaced mercontrol cable, replaced both control cables, replaced R&P steering, replaced spark plugs, replaced starter pinion gear, link and sync, removed idle stabilizer, and removed mercury kill switch.

I live at 8,000' and the lake is at 9,000'. I got it running well on muffs, and took it out on the lake. It didn't want to idle, and if I gave it over 1/2 throttle it would bogg out. I messed with mixture and timing to no avail. I did not try squeezing the bulb. After I got5 home, I broke out the FSM and tested the stator and trigger plate with my multimeter for ohms. Trigger plate was fine, but stator tested out bad. I replaced it, adn took it back out on the lake. Small improvement, but problem was still there. Again I didn't squeeze the primer bulb when running on the lake.

After I got home again, I tested the voltage rectifier using the FSM matrix and my multimeter. It tested out bad, so I replaced that. To ensure that fuel delivery wasn't an issue, I also replaced all fuel lines back to the tank with 3/8" up to the tee (after the fuel pump) between carbs #1 & #2. Line between carbs is 5/16". I also followed some online instructions and build a DVA adapter for my multimeter. Last night, I tested the switch boxes and coils according to FSM, and received the following results:

Coils should read 150-250VDC (<1k RPMs), 180-280VDC (>1k RPMs). Mine all (1-6) read 50-60VDC (<1k RPMs), 90-105VDC (>1kRPMs).

Switch Box Stop Circuit should read 200-360VDC (<1k RPMs), 200-360VDC (>1k RPMs). Mine read 120VDC (<1k RPMs), 220VDC (>1k RPMs).

Stator Low Speed should read 200-360VDC (<1k RPMs), 200-360VDC (>1k RPMs). Mine read 120VDC (<1k RPMs), 215VDC (>1k RPMs).

Stator High Speed should read 20-90VDC (<1k RPMs), 130-300VDC (>1k RPMs). Mine read 120VDC (<1k RPMs), 215VDC (>1k RPMs).

Switch Box bias should read 2-10VDC (<1k RPMs), 10-30VDC (>1k RPMs). Mine read 12VDC (<1k RPMs), 21VDC (>1k RPMs).

Prior to testing, I also hooked up a timing light and observed the strobe on all 6 cylinders. It appeared to me that 1-3-5 were a little erratic, but the motor sounds great when it is running. Following that observation, I ordered a switch box as a precaution.

Do the test results mean that both switch boxes are bad? I'd like some opinions before I go spending any more cash.

Thanks again!

Nick

 

[exciterfan]

Re: 115 tower of power- won't go over 1/2 throttle and doesn't want idle on the lake

I have a 1984 90HP tower and have done the stator & switch box too.

You could have a dirty carb causing the motor to run on 4 cylinders. After running the motor check the teperature of the spark plugs. If two are warmer than the others you could be running two cylinders lean. Premium fuel & SeaFoam might fix the problem or you might need to pull the carb and clean it manually.

You could also have a reed valve issue (broken reed). To fix this you will need to pull the power head apart. I have an old 50 HP merc that was doing weird things on the RPM front and it turned out to be a piece of the fogging oil straw stuck in the reed cage (previous owner). Every now and then it would stick in the reed valve and keep it open and cause the motor power to drop off. That was hard to diagnose!!!

Exciterfan

 

[Chris1956]

Re: 115 tower of power- won't go over 1/2 throttle and doesn't want idle on the lake

Nick, Don't change the timing. That can cause the engine to blow up. Smaller main jets are called for at that altitude, but the idle mixture should be settable. What did you do to "plug the economizer and open up the port"?

What do the spark plugs look like?

 

[Atoyot1031]

Re: 115 tower of power- won't go over 1/2 throttle and doesn't want idle on the lake

The economizer bypass is laid out here: http://www.sidewinder-boats.com/portal/windertalk/showthread.php?t=1666 (hope its ok to link this mods).

The jets I believe are correct as good mixture is achieved around 1.75-2 turns out from lightly seated. The plugs are ok. They had a little build up and I cleaned them up (as mentioned they are new), but I haven't de-carbed the motor yet.

Tonight, I rechecked the stator & trigger plate according to FSM, everything is good.

I think I need to order a 2nd new switch box as my test results were so far off spec.

Still looking for more input.
Thanks!
Nick

 

[exciterfan]

Re: 115 tower of power- won't go over 1/2 throttle and doesn't want idle on the lake

I think Chris is hitting the nail on the head. The only adjustable jet on that Carb (I belieive) is the Low speed Jet. The high speed Jet is a fixed jet that needs to be physically replced based on temperature and altitude, altough temperature is not the issue on an outboard motor). There should be an altitude/Jet table somewhere which would give you the right jet size for your altitude. You are likely running a very rich mixture with the jet sized for 1000 ft. Where did the motor come from?

Of course if you have electronics issues then you ned to also sort those out.

 

[Chris1956]

Re: 115 tower of power- won't go over 1/2 throttle and doesn't want idle on the lake

OK, so you disabled the backdrag feature. At 8000 feet, you would have had to put a smaller backdrag jet in anyway to have it work properly. Check the main jet size. See if it ia appropriate for your altitude.

 

[Atoyot1031]

Re: 115 tower of power- won't go over 1/2 throttle and doesn't want idle on the lake

According to the FSM, I should be using a .070 jet or .072, if I'm a little lower. I bought the motor at ~5,000', so I can't say for sure what the jetting is. When I rebuilt the carbs, I was only able to remove two of the jet plugs, and none of the jets. I guess I will order the appropriate jets, and new plugs. I'll have to destroy what's in there to get them out.

FWIW, the vent jet never changes, just the main.

Thanks,
Nick

 

[Atoyot1031]

Re: 115 tower of power- won't go over 1/2 throttle and doesn't want idle on the lake

I thought I should wrap up loose ends here. Last fall, I ordered the appropriate jets and jet plugs. I ended up having to drill one of the caps and use an extractor to get it out. I also had to use an extractor to remove two of the jets. Only one jet was removed intact. It was the factory jet for sea level!:facepalm:

I replaced all of the jets with my highest altitude jets (+7,500' IIRC) and have a 2nd set for Lake Powell (~3,700'). Unfortunately, I got into the hunting seasons and didn't have a chance to put the boat in the water after this work.

Our ice is just now melting, so hopefully before May is out I will be able to properly setup the boat in the water.

I'll report back with the results.

I always need to remember that assumption begins with "***"...:embarassed: