OK.....its become a matter of pride now as there is no reason why I can't just live with this minor but (for me) frustrating problem.
I have a 1988 low hour 20 hp Mercury 2 cycle that has always had a stutter between 1100 and 1600 rpm that I just cannot eliminate.
By stutter I mean it seem to run rich and "4-cycles" in the small off-idle range. At about 1600 rpm it smooths out all the way and including WOT
It starts very well. It idles well. At WOT is runs very well. I have changed out all the ignition components from a donor motor (stator - trigger - coils - switchbox - wires). Carb has been swapped with a known good one. The typical carb primer check ball has been checked and is not the problem (I wish it was). Motor crank seals changed both ends. I changed the reeds to Boyesen reeds with a new reed cage seal and gaskets....which improved my idle but did not eliminate the off-idle stutter.
Compression is 125 each.
Exhaust manifold gaskets were updated as per a recall notice.
The 2 bleed check valves are working and the hoses to them are good.
Timing has been checked and is correct at WOT and at idle pickup.
New Plugs and Plug wires did not help.
Plugs are the recommended NGK BP8H-N-10 gapped at .040 I don't dare run hotter ones for this motor.
I am pretty sure my problem is with Carburetion not with ignition.
The stutter that I hear indicates the motor is running rich in that range.
no amount of LS adjustments helps the off-idle stutter ...but the LS adjustments do properly affect the idle is all the typical ways.
The carbs for these small mercs do not have (that I can see) any way to adjust the mid range. I have tried a smaller HS jet with no change
Main Question--------------
Is there any way that anyone knows to affect the off-idle or mid range of a WMC carburetor ? The carbs for 18 20 and 25 hp Mercury / Mariner from 1983 to I think 2004 of are designed about the same for that 2 cycle motor.
Is there a carburetor diagram or schematic available than anyone knows of ?
I am not completely sure this problem is all that unusual for my motor but this bugs me while trolling.
I am tempted to try--------------
There is a small non-replaceable brass air-jet that leads from the top of the carb to the main jet emulsion tube that I believe is responsible for the mid range. I am getting to the point where I might slightly open that up to see what happens. I sure would like someone else's opinion however before I do it. Its not designed to be screwed with.
Any opinions are welcomed.
signed
Frustrated in East Winthrop Maine
I have a 1988 low hour 20 hp Mercury 2 cycle that has always had a stutter between 1100 and 1600 rpm that I just cannot eliminate.
By stutter I mean it seem to run rich and "4-cycles" in the small off-idle range. At about 1600 rpm it smooths out all the way and including WOT
It starts very well. It idles well. At WOT is runs very well. I have changed out all the ignition components from a donor motor (stator - trigger - coils - switchbox - wires). Carb has been swapped with a known good one. The typical carb primer check ball has been checked and is not the problem (I wish it was). Motor crank seals changed both ends. I changed the reeds to Boyesen reeds with a new reed cage seal and gaskets....which improved my idle but did not eliminate the off-idle stutter.
Compression is 125 each.
Exhaust manifold gaskets were updated as per a recall notice.
The 2 bleed check valves are working and the hoses to them are good.
Timing has been checked and is correct at WOT and at idle pickup.
New Plugs and Plug wires did not help.
Plugs are the recommended NGK BP8H-N-10 gapped at .040 I don't dare run hotter ones for this motor.
I am pretty sure my problem is with Carburetion not with ignition.
The stutter that I hear indicates the motor is running rich in that range.
no amount of LS adjustments helps the off-idle stutter ...but the LS adjustments do properly affect the idle is all the typical ways.
The carbs for these small mercs do not have (that I can see) any way to adjust the mid range. I have tried a smaller HS jet with no change
Main Question--------------
Is there any way that anyone knows to affect the off-idle or mid range of a WMC carburetor ? The carbs for 18 20 and 25 hp Mercury / Mariner from 1983 to I think 2004 of are designed about the same for that 2 cycle motor.
Is there a carburetor diagram or schematic available than anyone knows of ?
I am not completely sure this problem is all that unusual for my motor but this bugs me while trolling.
I am tempted to try--------------
There is a small non-replaceable brass air-jet that leads from the top of the carb to the main jet emulsion tube that I believe is responsible for the mid range. I am getting to the point where I might slightly open that up to see what happens. I sure would like someone else's opinion however before I do it. Its not designed to be screwed with.
Any opinions are welcomed.
signed
Frustrated in East Winthrop Maine