HELP

[shartwolf]

I have a 1980's mercury 9.9 and it won't start. It was running fine then as i was under full throttle and it just died and won't start back up. What is wrong

 

[galamb]

Could be many things so you will have to start with some basic trouble shooting.

Unless you heard some really strange noises before it quit it's probably not a cylinder/piston issue, but to be sure a compression test would rule that out. You should have somewhat north of 100 psi in each cylinder with no more than 10-15 psi difference between the two of them (115 to 125 would be factory new).

If the motor "quickly" died - simply stopped firing and wound down (just like when you kill the motor to stop it), you could be looking at an ignition issue.

After the compression test I would put a timing light on the wires and see if I detected spark. If the timing light shows activity I would test the ignition by seeing if it is capable of jumping sufficient gap - usually 1/4 to 3/8" (most discount auto places sell a little (tool) that looks like a spark plug where you set the gap for the purpose of testing.

If no spark is detected then there is a whole series of test to determine what "isn't" working (including making sure that you didn't accidentally trip the kill switch from run to stop and didn't notice - no spark at all would normally indicate a bad stop circuit, bad stator or switchbox - weak spark could be a bad stator - no spark on one cylinder could be a trigger, switchbox, coil, spark lead etc - but there are no "absolute" rules when dealing with an ignition issue. However, if you lost one cylinder the motor will normally continue to run (like crap), especially if at full throttle.


IF it was running at full throttle and kinda died out (slowly dropped rpms and died) I would suspect a fuel issue. So, since I don't know the history of the motor or what you did/did not check here are things "I" would check if it showed up at my garage -

Is the vent on the fuel tank "open", is the fuel tank to motor fuel hose/bulb serviceable?
Is there gas in the fuel filter - if so then the issue would probably be "down stream".

A toasted fuel pump diaphram, clogged carb, busted reeds could be the issue - so could bad main seals, powerhead gasket, intake gasket, manifold gasket and a dozen other things could cause it to quit.

As you can see there is no "simple" answer to your question. It's like calling a mechanic on the phone and saying "my car won't start" - no way of telling you what is wrong without going through a process of elimination....