Easy way to check spark while running?

[STV_Keith]

I think my motor has a coil or wire problem. Seems like it will go ~7500 or so, then feels flat. Sometimes (wear a helmet most of the time), I think I can hear it with a dead hole. At times, I can accelerate up to 6500 or so, then when I back out the port pyro drops like 400 degrees, and sits there until I bring the r's down and slowly get back on the throttle. You can definitely hear and feel it clean up and come back - and the EGT comes back up too.

I did find the #2 plug boot with a crack in it last week while checking compression, and swapped that for the weekend - no difference. I've ordered a new complete set, so maybe that will fix it.

If it doesn't, is there an easy way to kill the signal to each of the three port cylinders individually so I can see which one may be misfiring?

Any other way to troubleshoot this?

[sosmerc]

Honda, Yamaha, Suzuki, Mariner......ah, Merc? It's got a port and a starboard....so I guess we're talking V block? V-4, V-6, V-8?? One way to do a "universal" test.....pull the plug leads off one at a time at 7500 and see which cylinder is dropping.....probably should have someone drive while you lean out back and do the plug wire pulling!!
Come on.....give us a model, serial number, etc. so maybe we can give you a decent answer!

[inspectorlance]

I can just picture this. Running an STV at 7500 rpm, likley 90/ 100 mph, and leaning over a motor to pull spark plug wires. Watch out for the flywheel !!!!

I want a picture !!!!

Lance

[STV_Keith]

2.5/260 MERC EFI. 0G205239 if that helps.

Exactly Lance, can't exactly get to it on an STV, especially at 90mph. Timing light works fine, but the problem only happens at WOT up in the R's.

[sosmerc]

I was kidding of coarse, but jeez, it would be nice if everyone would start a technical post by giving us a little basic info about their rig.
Most ADI ignition Mercs with kind of problem can best be tested by monitoring voltage at each coil. Merc (and CDI/Rapair) make a tester that can be hooked up to ALL coils at the same time with a DVA meter and then the driver (or passenger) can view the output voltage to each coil when the problem occurs...the instrument has a dial so you can quickly move from one coil to the next to catch the "culprit" cylinder. If output voltage stays constant on all cylinders, then you have to start checking the coils themselves, or plug wires, or spark plugs.
But if the engine is a PCM 555 or o38 equipped engine, you can use the DDT to drop cylinders one at a time to isolate power loss to a particular cylinder.
But heck, there are other procedures as well, but if we don't know what engine were talking about, what's the point?

[sosmerc]

OK, so that is a conventional ADI Merc ignition. I'd start my monitoring output voltage to each coil with a DVA meter. The CDI tool that I was referring to is part number 511-9776 and it has a 15ft. harness so the driver can switch from coil to coil to monitor readings. You can also hook it to stator and trigger and do the same thing...in conjunction with a peak reading voltmeter.

[sosmerc]

Keep in mind that your injector firing is dependent upon proper ignition on 1, 3, and five. Injectors fire in pairs, so if you lose spark on say #1, you are going to have 2 cylinders without fuel....and you are going to drop more than 400 rpm. So it is more likely you are losing fire on one cylinder...probably 2, 4, or 6.
I'd start with new plugs, and new wires. Ohms test your stator and trigger.
Check the bias circuit on each switchbox.
Pressure drop test your injectors and monitor your fuel pressure at WOT.