Mercury 115 hp engine problems

[mac_in_mt]

I have a 1996 Mercury 115 hp motor. When I purchased the motor (used) it seemed to run well for me. Over the past year I have noticed only at WYT it would bog down or cut out, but if backed down slightly it would run fine. Last weekend this problem became much worse and I seem to have problems from 1/3 throttle and up. Occassionly when I went from idle to WYO, the motor would run well for about 1 second and then bog down. I pumped the bulb while at WYO which had no effect. I tried spraying carb cleaner into the carbs with no effect. Last night when I returned home I pulled the spark plugs which were new approximately 3 months ago. Plugs 1, 2, and 3 had some oil and gas residue, but no significant carbon buildup on them. Plug 4 was completely clean and had no residue whatsoever. I plan to test the compression on the four cylinders tonight. From my very limited knowledge, it seems I have a fuel supply issue, but I don't know where to start. Any suggestions?

Thanks,
mac_in_mt

 

[Chris1956]

Re: Mercury 115 hp engine problems

it the primer bulbkept filling up as you pumped it at WOT, you likely have carb issues instead of fuel supply issues. Continue with compression test. post results

 

[mac_in_mt]

Re: Mercury 115 hp engine problems

All four cylinders had between 115 and 120 psi if I let the engine crank for aprox 2 secs. If I just hit the start and let off, they varied from 105-120 psi. I checked spark and had a good solid spark on all four cylinders. I pulled the downstream hose off the inline gas filter and hit the startor. I had spurts of gas shooting approx 6 inches out of the gas filter I pulled two carbs 1 & 4 and disassembled. I blew them out and do not feel like I had any blockages. There is a difference however with the two carbs. Just above where the throttle linkage operates the fly valve each carb has a screw that goes into the top reservoir. Two of the carbs have a pin screw with a spring for adjusting the depth. Two of the carbs do not. Interestingly, on the two carbs I pulled from 1 & 4, there were definate differences in the number and sizes of holes coming from the top reservoir into the carb. Also, the carb from cylinder 4 had a 4 on the bottom. The carb from cyclinder 1 had a 2 on the bottom. I plan to check the other two carbs tomorrow. Any suggestions or ideas would be greatly appreciated.

Thanks!
Pete

 

[chrillolin]

Re: Mercury 115 hp engine problems

I suppose your engine are like mine -89 115 4 cyl, 2 stroke. They just goes on two cylinders for rpm up to about 1800-2000 rpm then the other two will chime in. That?s why you don?t have idle mixture screws on the two carbs in bottom for cylinder 3 and 4. (Apparently they don?t get fuel in the low rpm range).

I?m interested in the problem you got because mine behaving the same. Carbs cleaned, and nothing happens when using the primer bulb... Can there be something with the ignition? I had the same on the forth cylinder when been taking a test drive in higher rpm. That cylinder had almost a clean spark plug! when i recently test drived and whent in to harbour, thus going on low rpm for some minutes the forth cyl had colour on the spark plug. That i suppose indicating something like it doesn?t get spark (or fuel) on higher rpm but in lower. And that is just the same how it behave!! It cut out cyl for over ca 4000 rpm?

I called a merc mehanic and if there isn?t a fuel issue the first thing he mentioned was the stator.? Someone more with experiences with this?

regards

 

[Chris1956]

Re: Mercury 115 hp engine problems

Pete, is that one of those merc 2+2 motors? If so, the motor is supposed to idle on two cylinders only, and then the other two will kick in as you advance the throttle. The carbs would be of two different kinds. It still sound like your carbs are clogged.

 

[CharlieB]

Re: Mercury 115 hp engine problems

Clean #4 plug may be a slight water leak from the exhaust plate gasket, spraying back thru the exhaust port.

 

[mac_in_mt]

Re: Mercury 115 hp engine problems

If it is a 2+2 motor, I did not know it...is there a way to look this up easily?

By looking at the difference in the carbs and the idle adjust screws, it would make sense. I will pull the other two carbs tonight. As for a slight water leak through the exhaust plate gasket......the repair manual I ordered isn't due to arrive for another week so can someone give me a quick rundown on where to access the gasket?

Something I didn't mention in the original post is that when I throttle from idle to WYOT it seemed I would sometimes have full power for about a half second and then I could feel a change in the throttle cable when the motor bogged down. Almost a click or slip in the cable sensation. I have looked through all of the throttle assembly to see if I have any cracked plastic pieces but have not found any. But if the engine is a 2+2 and I have a partially clogged carberatuer this might make sense?

Thanks,
Pete

 

[mac_in_mt]

Re: Mercury 115 hp engine problems

Okay....I have finished pulling the carbs and cleaning them. I did not notice any significant problems with the carbs with these two exceptions:
1. Carb #3 may have had a little blockage in the main jet
2. Carb #4 has a smaller main jet...Is this supposed to be smaller?

The next step was to pull the exhaust plate and look at the gaskets. I may have done some damage to the gaskets while pulling the exhaust plate, however, it appears that the gasket was in poor shape, thus I will be ordering new gaskets. While the exhaust plate was off I looked into the cylinders and inspected the cylinder walls. It appears that cylinder #4 has some scarring on the cylinder walls. Is there anything else I should be looking at while I have the exhaust plate off? Should I attempt to chip/scrape the minor carbon build up from the exhaust or am I risking a big mess if I do that?

So lets review cylinder #4:
Spark plug is very clean...not oil or gas residue
Main jet in carb #4 appears to have a smaller inside diameter.
The exhaust gasket may have been leaking water into cylinder #4.
Cylinder #4 has minor scarring on the cylinder walls.

Any additional advice would be greatly appreciated!

Thanks,
Pete

 

[mac_in_mt]

Re: Mercury 115 hp engine problems

Motor serial number is OG039485.....
Is this the correct exhaust plate gasket:
http://www.iboats.com/Mercury-Marin...3578749--**********.120585165--view_id.464401

Thanks,
Pete

 

[mac_in_mt]

Re: Mercury 115 hp engine problems

Update.....
I pulled and cleaned all the carbs and replaced the fuel filter.

I tested spark and compression and both seem good although spark plug no. 4 looked completely clean (no oil or carbon residue). The spark plugs were new about 3 months ago and have at most 15 hours on them.

I checked to make sure that I had reasonable gas pressure downstreeam of the fuel filter. Gas would shoot 6-8 inches out of the fuel filter while cranking the engine.

I removed the exhaust plate and replaced the gaskets.

I replaced the throttle and shift cables and while investigating the cable and throttle arm assembly I discovered that the throttle shift assembly will not fully open the butterfly valves on the engine.

I plan to follow the tune up procedures in the manual when I can borrow a timing light this weekend.

Last night I took the boat out and I am still having power problems in the top 1/3 of the throttle range. About 1/2 of the time the engine will run with full power and the other 1/2 of the time it will not. While running at WOT and with the engine not performing, it will suddenly snap to full power. The engine will run like this for a little while and suddenly snap to not performing well. To me, it doesn't feel or act like a fuel supply issue....which leads me to ask what next?

Thanks,
Pete

 

[chrillolin]

Re: Mercury 115 hp engine problems

Hi!

As you can see earlier in your thread I had almost the same problem with my 1989 115 2+2 engine. I think I have solved the problem now and maybe my story can give you some help on the way.

My cyl. 4 spark plug was much more clean just as yours and i suspected it can either be going lean due to fuel starvation or it can intermittent be out of spark and therefore be washed clean by unburned gasoline mixture. I took the carbs apart for the second time in less than a year (before the winter I did have a fuel starvation problem and that time there was some debris in the lowest carb) So I didn?t bet so much it was the same case now, but I found it easier to do the carbs again than check the ignition at high rev. Thus it din?t solve the problem.

Next step was checking the ignition at high rev (as that?s when the problem occured). I took my stroboscobe lamp (for setting ignition timing) with me, so I could use that at each ignition wire for checking which and when (or if) a coil was falling out to giving voltage to the plug. (If your familiar with setting ignition timing you know the lamp would stop flashing in that case) This I suggest would be a good thing for you to do.

I also bought a set of new spark plugs, even if the ones in the engine was almost new. pretty cheap investment for troubleshooting. I changed the plugs and then i thought I give the ignition system a last look before setting the boat in water. I had earlier cleaned almost all the connections between the switchbox? and the coils with no result but in that case i had the boat in the water and now it was easier to check it all up. I found one point that I had missed. From every coil it goes a black cable to a common ground point. I unscrewed the bolt and start cleaning with some sand paper. One of the cable connectors was almost broken (the one to cyl 4) . Only a part thin like hair hold it together and in the broken area it had a green corroded colour. I fixed that and put it to sea. Now the boat performed as it should again!

Some day later I changed back to the plugs that had been in the boat earlier because this had the correct spark plug gap of 1,0 mm (don?t know what that will be in inch) and the new ones only 0,7 mm. So the problem came back! Put in the new plugs again and it worked again. I don?t think it only was the plugs. Perhabs the erratic ignition had wounded the spark plug?

I don?t say you have exactly the same problem, but hopefully this text can give you some help.

Cheers!

 

[steve glover]

Re: Mercury 115 hp engine problems

One thing and I may be wrong here but I had my Carbs rebuilt on my 1998 115 2+2 doint pretty much the same descrption as yours and at idle out of water was ok, put it in the water and it just died after carb build. I was told by my mech. it was the arm on the side of the motor for timing advance was hung up and not moving enough. It travels quite a bit when you push the throttle forward so make sure it is moving properly. It is the arm that runs under the flywheel and along side of the motor has the idle screw on it which actually moves that arm for low speed idle by advancing timing.

 

[Napoleon]

I’m coming in pretty late on this and this may have been addressed already. Mine is a 1996 Mercury 115 2+2 with the oil injector system removed. Runs fine except for the inherent problem of rough idling below 2000 RPM.
Has anyone considered replacing the two lower carbs (3 and 4) with carbs that have idle adjusting jets, like carbs 1 and 2 ?

 

[racerone]

Apparently that does not work as you might expect.-----Start a new topic as this thread will likely go ----BOOM

 

[Napoleon]

Ok Done.......the Subject is 1996 Mercury 115 2+2 carburator issues........
See you there

 

[Napoleon]

Correction.....Subject is 1996 Mercury 115 2+2 idling issues

 

[GA_Boater]

Closed. Napoleon started his own thread.