1986 115 Merc boggs out at wot

[frozenokie]

Great out of the hole then after about 10-15 seconds, it "boggs" out like it is leaning out. I checked timing according to the shop manual - it checks out OK. All plugs looks good. Carb kits & fuel pump kit installed a few weeks ago with no signs of malfunction. This thing will start right up like it should and idle all day long. I've checked the stator, trigger, flywheel, and all electrical connections and everything looks good. I've tried adjusting the idle mixture screws way out and way in, even though there is a difference in performance, it doesn't remedy the problem. I'm really confused as to no matter what I adjust or try, it still does the same thing - leaning out at WOT. I tried coming out of the hole then pulling back to half or three quarter throttle, but it does it there too just takes a little longer for it to act up.

The engine wants to run - a seasoned person can tell and this one just has a little something out of adjustment or wrong and I can't find it.

Has anyone had the same experience with their inline 6?

 

[Chris1956]

Re: 1986 115 Merc boggs out at wot

First of all the idle mixture screws have no effect on high sped performance. The high speed jets are fixed.

The first test I would try is to have someone pump the primer bulb while you speed along. If that cures it, find the air leak in the fuel line or the fuel pump is not working perfectly.

Another possibility is that the floats are set too low. If all else fails, you might raise 'em up a bit by bending the carb levers.

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

I'll try that bulb squeeze this weekend, but in the meantime I'll recheck my floats. When you say set too low, do you mean "low" in relation to level or position inside the fuel bowl while mounted on the motor or when you have the carbs upside down while adjusting/setting the float tabs? I believe you mean that the floats are not allowing the fuel bowls to be filled sufficiently and need to travel higher up inside the bowl while running - Correct?

I've checked the main jet size per the Mercury charts and I'm using the right ones at .074". I'm also going to take the autoblend system back out of the loop and premix all the gas. I'm thinking maybe that the autoblend is not alowwing enough fuel to pass through at high demand.

Thanks for the help chris1956.

 

[j_martin]

Re: 1986 115 Merc boggs out at wot

I'll try that bulb squeeze this weekend, but in the meantime I'll recheck my floats. When you say set too low, do you mean "low" in relation to level or position inside the fuel bowl while mounted on the motor or when you have the carbs upside down while adjusting/setting the float tabs? I believe you mean that the floats are not allowing the fuel bowls to be filled sufficiently and need to travel higher up inside the bowl while running - Correct?

I've checked the main jet size per the Mercury charts and I'm using the right ones at .074". I'm also going to take the autoblend system back out of the loop and premix all the gas. I'm thinking maybe that the autoblend is not alowwing enough fuel to pass through at high demand.

Thanks for the help chris1956.

Naw, the oil injection isn't limiting fuel flow.

You may have either a too lean, or a too rich condition, or you may be dropping a cylinder or 2 due to an electrical problem.

Don't add to the troubles by throwing "fix's at it." Systematically troubleshoot it. The first suggestion was to squeeze the primer bulb when it's acting up. If it gets better, it's starving, find it. If it gets worse, it's flooding, find it. If nothing happens, it probably ain't a fuel problem.

I saw a long thread on a similar problem on a 115. Turned out the problem was the prop was one pitch too steep. The complaint was overheating. The owner didn't realize that it shouldn't take half way across the lake to get on top. It happens. What is your WOT rpm?

hope it helps
John

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

I don't know what my WOT rpm is since I can't get to that point before it boggs out. The max shown on the tach is about 4000 when it happens. Once it boggs, I'm able to pull back throttle and keep her runnin. It's only doing this at WOT or three quarter throttle.

I do agree with your logic on troubleshooting the fuel problem.

Thanks John.

 

[CharlieB]

Re: 1986 115 Merc boggs out at wot

You say that it lasts a little bit longer at lower throttle settings before it acts up.

Sounds like it is running low/out of fuel, like a restricted fuel supply.

These engines take VOLUMES of fuel, ANY RESTRICTION at WOT and they will burn down or otherwise NOT perform.

You need to start at the fuel pick-up in the tank, pull it out and verify the it is not plugged up with gunk, that the check valve flows freely and one-way, that all lines are good, NOT PINCHED and supple, sealed at every fitting, not weather or alcohol embrittled or cracked. Fuel quick connectors should be FACTORY, many of the aftermarket connectors are cheap imitations and do not seal or flow near as well as their factory parts.

Fuel line diameter, NOT 1/4, minimum of 5/16, 3/8 is preferred for 115 and larger engines.

Repost your findings or any further questions.

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

Hey CharlieB, I did notice on the "T" fitting where the fuel line splits to the top and the middle carbs that the fitting looked too small and was plastic instead of metal. It was much smaller diameter than the metal elbows that enter the fuel bowls. Everywhere else on the fuel line are larger diameter fittings, this is the only one that seemed too small.

Should I replace it before I take this boat back to the lake?

 

[Chris1956]

Re: 1986 115 Merc boggs out at wot

If it is smaller, it may be a replacemnt and too small. It makes sense to change it. I would like to hear the results of the pimer bulb pumping experiment, to prove it is a fuel delivery issue.

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

I replaced that "T" fitting today. It was 1/4" plastic with one taper. I thought it looked suspect as the rest of the fittings are all 3/8". I put in a brass 3/8" T with two tapers on each outlet. I'm curious myself and anxious.

I'll report back in tomorrow to let you all know the outcome.

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

If it is smaller, it may be a replacemnt and too small. It makes sense to change it. I would like to hear the results of the pimer bulb pumping experiment, to prove it is a fuel delivery issue.

Good news to tell - FINALLY. and some bad too...

I was able to run around the lake after changing that "T" fitting to a 3/8" dia. brass. Bad news is that I'm still only achieveing 4200 rpm. After speaking with a trustworthy mechanic, we determined with tests that the stator/trigger and rectifier are all good. He then "pointed his finger" at he coils and/or switch boxes. I've done EVERYTHING thing else including that bulb test that showed no difference when squeezed at high rpm. Man did I go thru the fuel though - WOW. Eating double what it really should.

But, I have the coils and switch boxes on their way. We'll soon see...

Thanks to all...

 

[Chris1956]

Re: 1986 115 Merc boggs out at wot

OK - Replacing the coils and switchbox imply that a cylinder or two are not firing. A timing light on all plug wires will tell if this is true. Of course an incorrect prop will cause the symptoms you see, except for poor fuel consumption. That motor will use 11gal/hour at WOT.

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

Hey chris1956, I would have used my light but I didn't like the idea of having to high rev the motor in the yard (hooked up) or at the lake. I could be wrong but it seems to only show problems at WOT. I'll probably get curious and run outside to test it the way you suggested with the timing light

Hole shot - great. Prop - 13x19 quicksilver SS light cupped & vented, brushed finish. Mid-range running - great. Idle - running a little rich (1 5/8 turns out; if I use 1 1/2 turns it hesitates). Temp - perfect. Speed on gauge - 38 at 4200rpm.

Instead of "cutting out", it acts like it's "cutting in" with intermittent split seconds of power - if that helps. Maybe the coils and boxes will fix??? I only paid $100 for all of it. So - you know I had to do it

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

Woops... didn't mean to hit enter. The boat is light around 900lbs (18' Laser). Idle speed - a little high for my taste at 1000 no gear and around 750-800 in gear.

A big oil ring left in my test tub after running it about 30 minutes. Is this excessive in your opinion

What's the best way to tell if I have the right idle settings? Could the setting "6P" on the back of the tach be incorrect? I was told by Mercury Marine that it had a 12 pole and to set it at 6P or 12P. My carbs are fully open so I know that's out... I'm to the point of shooting in the dark, hence the purchase of coils and switch boxes

 

[gregpro50]

Re: 1986 115 Merc boggs out at wot

You check the maximum timing advance at cranking speed not wide open running.

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

I already did all that and then some... I used my manual. She sets at 5 deg BTDC at primary and 20 deg. BTDC at secondary. That's with the idle stabilizer and heat sensor disconnected. Just has a problem going over 4200-4400rpm.

The wide open thing was only pertaining to an attempt in creating the condition at where the problem occurs. Therefore, possibly catching the problem on a timing light.

 

[j_martin]

Re: 1986 115 Merc boggs out at wot

I already did all that and then some... I used my manual. She sets at 5 deg BTDC at primary and 20 deg. BTDC at secondary. That's with the idle stabilizer and heat sensor disconnected. Just has a problem going over 4200-4400rpm.

The wide open thing was only pertaining to an attempt in creating the condition at where the problem occurs. Therefore, possibly catching the problem on a timing light.

The idle stabilizer is also an advance module (combination). If it's still disconnected, you need to set your WOT timing to 23 or 24 degrees.

The suggestion that was made is to hook up a timing light to one plug at a time, on the lake (probably have to run with the cowl off, don't get carried away till you lean out and burn somehting.) The light just has to be somewhere you can see it flashing, on the back deck or something. Then go run. If the light gets erratic, you've found the bad cylinder. The rest is cross swapping or trying new parts.

You just said the heat sensor is disconnected. Why?

hope it helps
John

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

I disconnected the heat sensor due to the advise of a seasoned mechanic. I also had the idle stabilizer disconnected. The manual said to set at 20BTDC to achieve 18BTDC at WOT. So I unhooked them in order to get the right reading.

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

The replacement of coils and switch boxes must have worked!!! I now get 5000rpm. I just got back from a week vacation on the lake so it was thoroughly tested.

However... I noticed another problem with fouling plugs or burning too rich. When I took the carbs to a mechanic, he noticed that one of the air holes in each of the shutters had been drilled out just a little larger then the others. He mentioned that it was to keep from burning pistons.

I have set all idle screws at 1 3/4 turns out (vs. 1 1/2) which proved to be the best performance setting. Could this be the problem with fouling plugs and burning too rich? Or do I have the floats set incorrectly?

Other than the fouling plugs and gulping down the fuel, it runs the best that it ever has.

Any suggestions???

 

[frozenokie]

Re: 1986 115 Merc boggs out at wot

I will address this in another thread...

FrozenOkie