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Definitely check compression. There is not much testing that can be done on the switchbox other than substituting it with a known good one. You can test voltages in and out but you need a DVA (direct voltage adaptor) to do it. Even then the voltages can be within tolerances but the box still be bad. FYI the motor is a 1995 and made by Mercury.
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1 Attachment(s)
Another FYI. You can make your own DVA. Its pretty easy but the cap (which is important) may be hard to find.
Attachment 298774
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Thanks all. So the general consensus is brainbox?
I'll pass this on to the boss, see how he wants to proceed. Other than a prop, this is the most issue we've had with this engine.
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Did you install a diaphragm kit in the carb when you rebuilt it?
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Yes, everything in the kit, that I could have replaced I did. Given the above symptoms, could I have done something wrong?
One person even sprayed starting fluid in the carb, still nothing.
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Possibly. The two most common problems I see with recent carb jobs on that design will lead to a very rich running problem. If the motor is saturated with fuel, it won't run right if at all.
Both issues involve the diaphragm check ball. It's located under the 11mm brass cap on the bottom of the carb bowl, to the side.
The most common issue with the newer kits involve the spring. The new springs can allow the check ball to be pushed down into the spring. If that occurs, the ball becomes trapped and will not return and seat. This will leave the primer circuit wide open. The engine will flood out in short order. (This is very common) The fix is to slightly crush the end of the spring that contacts the check ball. The very last loop of the spring can be bent to the point that it will not allow the ball to be pressed down into the spring.
The second issue is less common. That is the loss of the check ball during disassembly. If you aren't ready for it to drop out of the carb, it will and it will run for the closest hiding spot, never to be seen again. Again, same problem with the primer circuit.
I still think the fuel needs to be checked for the presence of water but the above issue is so common that it needs to be checked before anything else.
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Fuel is always the first thing to check but if fuel is good and clean it would be electrical
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Check you key for the flywheel. Not sure about the Mariner's, but the small Merc's and Johnson's have a keyed flywheel. The larger motors are usually splined.
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Thanks to all for the replies and help!
I've passed most of this on to the boss, will get this last one to him Monday. When I relayed the others to him, I asked how he wants to proceed, never got a response.
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One issue that can cause weak spark is bad ground --- I am not sure how Mercury builds motors but Jap engines have a valve plate between Carb and cylinder that usually is never the problem but if it is it will cause back pressure into the Carb