1995 Mercury 2+2 Help needed

[NITRO DAN]

I have a 1995 Mercury 115hp 2+2 that I am fighting through an issue. The motor has an erratic unstable idle. It runs good at any power setting above 1/8 throttle. I have had the carbs off twice and I believe they are not the issue. New Plugs. When at idle, it will run almost like its on one cyclinder, but with no change in throttle it will sometimes speed up and idle normally. When placed in gear under a load, it runs erratic and sounds like a miss. As soon as I input throttle it speeds up and runs normal. At 3/4 throttle across the lake it runs normal. I am wondering if this sounds like a bad reed valve? I don't have much experience with 2 stroke reed issues. Can someone provide me with any advise. Thank you.

 

[racerone]

Steps to take.----Nodody can see or feel your motor.----Need to know the compression values.----Post the actual numbers.----Does spark jump a gap of 3/8" on all 4 leads on a test device, yes or no ?----Replaced water pump impeller every 5 years ?

 

[richw46]

Lisle has an inexpensive spark gap tester, about $11 on Amazon

https://www.amazon.com/gp/product/B0002STSC6

When you replaced the plugs, did you put any dielectric grease on the plugs or wires? If so, clean all of that off. Dielectric is non-conductive. It goes on the boot and plug ceramic, not on any electrical connection.

Are you the original owner? (just to eliminate the possibility of hidden issues)
You've had the carbs off, but this was before or after your idle issue? Only the top 2 carbs have idle mixture screws. Are you sure the carbs were installed in the correct order? Did you put in new carb kits or just close them back up? Properly torqued? There is a sync process to go through if you moved any of the linkage.

The engine should idle at about 800 RPM. Mine can run rough when it's damp or raining but smooths out when it warms up.

Have you tried swapping the plug wires or replacing them? One could be bad. Do you have the ignition coils or CDI modules?

Have you replaced the fuel filter? Opened the fuel pump? Replaced the fuel line and squeeze bulb? These lines can break down and flakes of polyethylene can get in the fuel.

Above 1800 RPM it's normal at any speed, correct?
Below 1800 it's consistently rough, like 1 cylinder firing or just the old 2-stroke cough-spit-sneeze rough.

 

[racerone]

This motor only idles on the top 2 cylinders.----The bottom carburetors kick in at about 1800 RPM.

 

[NITRO DAN]

I just ordered the spark tester earlier. I did not use dielectric grease on them.

I have owned it for 10 years and I am familiar with how it runs and its short comings. The carbs are in the correct locations. I did not adjust any linkages during this.

The boat hasn't been used much in the last few years. One day it wouldn't idle correctly no matter what. I attributed it to bad/old gas. Brought it home and drained the tank, removed the carbs and cleaned them and replaced plugs. Next time at the lake, same deal. Removed the carbs(2) and inspected them more thoroughly. Figured out how they work and cleaned all the little holes in the upper portion and verified the pick-up tube was clear. Did not replace any parts or gaskets. No improvement this weekend.

I haven't, but will do a compression test later this week.

I have regular coils instead of CDI's. I haven't verified base timing and I see where this may be critical at low engine speeds? I also will do the spark intensity test as mentioned.

The Tachometer doesn't work. I bought a digital tach off of Amazon, however it doesn't have a 2 stroke/4 Cylinder option. It seems to run well as soon as you get out of idle and probably 15-20% throttle(just guessing)

Thank for your responses. I will get better data later this week.

 

[flyingscott]

Have you Looked at the fuel pump diaphragm? The accelerator pumps and or primer.

 

[richw46]

I think our motors are pretty much the same. Mine is an early 1995, made in fall 1994. I took delivery about September 1994. I have the coils and not CDM.

If your tach isn't working and the gauge itself is good, it's possible the voltage regulator is bad. Check the voltage output at the battery at idle. If it's above 15 then it has a problem. Mine went to 20 volts and the connecting wires were burnt and discolored. I don't think that would affect your idle but a non-functioning tach usually points to the voltage regulator.

How about the fuel lines on/to the carbs? Are they clear? Maybe something is lodged in one of them or maybe it's cracked and needs replacing

Here's the coil test from the service manual.

 

[NITRO DAN]

Update. I did the compression test. All 4 are between 136-139psi. Did the spark intensity test. Top(1) cylinder would not jump the gap. Decided coil was bad. Ordered new coil. Still no spark. Replaced switch box/CDI unit. Boom, have good spark. Water tested it yesterday evening. Ran as good as ever. I that cylinder was sparking sometimes, just not at low idle. Anyway, learned alot more about this motor. Thanks for all your help.