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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10977
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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1996 Mercury Force 120 HP motor cranks but will not start.

Customer Question

1996 Mercury Force 120 HP motor cranks but will not start. Ran yesterday
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: Turned off motor yesterday to fish for 30 minutes. Motor would not start afterwards. Fuel gage doesn't appear to be correct. Not sure if spark is getting to plugs.
JA: Is there anything else the Marine Mechanic should be aware of?
Customer: No
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Marine Mechanic about your situation and then connect you two.
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.

Hello. My name is Tan.

Could I get the engine serial number please?

The first thing we need to check is the quality of the spark. Do you have a spark gap tester?

Customer: replied 1 year ago.
0E1761681996 120 ELPT
Customer: replied 1 year ago.
Don't have a spark gap tester. I have plugs without typical gaps. NGK R RESISTER PLUGS.
Expert:  Tan replied 1 year ago.

Do you have a spark gap tester or a timing light to check for spark? We have to verify you have spark first. Then we can move on to the fuel system if you do have good spark.

Spark gap tester has nothing to do with the spark plugs.

Here is a spark gap tester.

Customer: replied 1 year ago.
I pulled 1st plug, grounded the side to the motor casing and then tried to crank motor. Saw no spark.
Expert:  Tan replied 1 year ago.

Ok that isn't an accurate test but it does tell us you have no spark. Ok what we need to do now is disconnect the main engine plug # ***** just pull the plug apart. Now locate the start solenoid. Jump with a piece off wire or jumper wire from the battery cable connection on the solenoid to the yellow/red small terminal of the solenoid. Do you have spark now?

Customer: replied 1 year ago.
Disconnected plug #11. Using jumper as described. I see spark on the top plug.
Expert:  Tan replied 1 year ago.

Ok we have a bad key switch or a bad kill switch. Plug # ***** back together. Now locate the black/yellow wire on the back of the key switch disconnect that wire. Connect a hose to the engine turn water on see if engine will start. Does engine start? If no connect black/yellow disconnect one of the wires on the kill switch. Now start engine does it start ?

Customer: replied 1 year ago.
Lifting the blk/yell wire at the key did not fix it. Clipped one of the blk/yel wires on the throttle but still doesn't kick attempt to run. Did prove that reconnecting #11 prevents spark.
Customer: replied 1 year ago.
Did not connect water yet. Must I have water to even see a sputter from the motor.
Expert:  Tan replied 1 year ago.

Ok that means you have a short in the engine wiring harness or at the plug it self. Look inside the plug do you see any kind of burnt tracks? We know the engine isn't the issue as when you disconnect the plug spark returns, we have eliminated the key and kill switches leaving the harness.

Customer: replied 1 year ago.
I'll look for spark again with #11 connected and while using jumper at starter and with one blk/yel clipped at throttle and while blk/yel landed At key.
Expert:  Tan replied 1 year ago.


Customer: replied 1 year ago.
Update. I have spark with kill sw connected and in run mode. No spark in off position which appears normal. In my opinion the spark is weak, not bright even garage lights turned off. I will connect water now with boat outside of garage and try to start again.
Expert:  Tan replied 1 year ago.

Here is where the spark gap tester comes in. You set the gap at 7/16" crank engine it should jump that gap.

If the engine doesn't start then we will ahve to test the output of the stator. The stator is bad when you have spark on all cylinders but it is weak.

Customer: replied 1 year ago.
The spark plugs are resister plugs so I guess the tester will still work the same way? The spark appears weak to me on all plugs.
Expert:  Tan replied 1 year ago.

The spark plug is not in the circuit when you use the tester. The plug wire connects to the tester and then the tester is connected to a ground. The spark plug has nothing to do with the spark test. The way you are testing for spark is not accurate.

Weak spark on all plugs is a bad stator.

Customer: replied 1 year ago.
Understood. Thanks.
Expert:  Tan replied 1 year ago.

Please don't forget to positively rate my help as that is the only way I get credit for helping you.

Follow up questions are always welcome even after rating at no extra charge.

Thank you.......Tan

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