98 mercury 115 idle mystery

[ebear]

A buddy of mine recently picked up a sea ray with a 98 mercury 115. I runs 30-40 mph so I would conclude that off idle it runs fine. But.........
Compression, spark, fuel, fuel pressure are all good. #1 and 2 mixture screws are set at 1 1/4 and timing is set to 0 (spec -2 to +4) At an idle it sounds as if it is laboring. one thing that I noticed is; at an idle I can unhook #3 and 4 plugs (one at a time) and it makes no difference in the running of the motor. The carbs are spotless and synced. plugs on 3 and 4 are wet. What in the 2 stroke world can cause a motor to have spark, fuel and good compression and still not fire correctly. also, I swapped plugs around with no change.

 

[theoldwizard1]

... The carbs are spotless ...

So you have dropped the bowls and cleaned all of the jets ?

A fuel float that does not float properly will cause rich idle but probably run fine at speed.

 

[racerone]

That motor only runs on the top 2 cylinders at idle.------# 3 and # 4 kick in above 1800 RPM------The bottom 2 cylinders only get a wee bit of fuel for lube and cooling at idle, not enuff to fire the cylinder.

 

[Nathan Bond]

A buddy of mine recently picked up a sea ray with a 98 mercury 115. I runs 30-40 mph so I would conclude that off idle it runs fine. But.........
Compression, spark, fuel, fuel pressure are all good. #1 and 2 mixture screws are set at 1 1/4 and timing is set to 0 (spec -2 to +4) At an idle it sounds as if it is laboring. one thing that I noticed is; at an idle I can unhook #3 and 4 plugs (one at a time) and it makes no difference in the running of the motor. The carbs are spotless and synced. plugs on 3 and 4 are wet. What in the 2 stroke world can cause a motor to have spark, fuel and good compression and still not fire correctly. also, I swapped plugs around with no change.

As an owner of a 115, I can confirm what racerone said, carbs 3 and 4 don't run at idle. You mentioned the #1 and #2 idle screws. You probably noticed there are no idle screws for 3 and 4 to adjust.. that's why. For me, 3 and 4 don't kick in until about 2200-2400 rpm. Specs on my 115 are for 1 1/2 turns, maybe with yours at 1 1/4 it is too rich?

 

[ebear]

sorry it's been a while. The rough idle was due to two things. when he cleaned the carbs , he didn't sync the linkage back up. That was the big part. Also he broke the face plate of the primer solenoid. Not sure what it may have done sucking air or not. but problem solved. thanks guys.