[CaptRonR]

My 1999 175 Mercury SportJet was running fine up until the last time out on the river. Engine started fine and ran good at high speed but after shutting off the engine for a while to fish. It was hard starting and kept stalling after starting and leaving in idle. Replaced the anti-siphon valve, fuel filters and tightened all the clamps on the fuel lines.Engine will idle while it is cold but after it is warm stalls within a couple seconds of slowing down to 1000 rpm. While testing I removed the carb cover and the engine ran fine even after being warmed up. Any suggestions as to what to check next will be appreciated greatly.

[jetdriven]

"While testing I removed the carb cover and the engine ran fine even after being warmed up."Was this testing done with the boat in the water or out of the water hooked up to a water hose?How about starting with running a compression test. Note the condition of each sparkplug as you remove them.

[CaptRonR]

When I removed the carb cover it was out of the water, hooked to hose. I had put in new spark plugs not to long ago. Would the fact that the engine ran better with more air be an indication of a plugged jet??

[jetdriven]

I don't think that you can assume a clogged jet by what you have observed. I really recommend that you test for a spark at each cylinder to be sure that all are firing. If you have a timing light, you can just move the inductive pickup connector from one spark plug wire to another and observe the flashing of the timing light. Watch for intermittent firing or lack of plug firing. If all wires are carrying a spark, your ignition is probably OK.Check the compression on all cylinders next. If compression on all cylinders is good and you have good spark to all cylinders, then your problem may be with a carburetor.