bwiencek

FlyFast - It's Vortec headed, and I'm running a Holley street dominator manifold - very similar design to a performer RPM intake.

John B

bwiencek,
You seem quite knowlegable, have you determined the cause of the burned valves? Are they going again? What type of ignition timing advance do you have? I also have had coils that don't work when hot. What is the voltage at your coil when the starter is running? Is there an resistance wire in your start circuit? Do you have an electric choke that may be shorting or causing a voltage loss?
Let us know, I'm sure one of us has had your problem before.
Regards, JB

bwiencek

Burnt valves was from detonation and too much timing - long story short - mechanic that set the timing didn't think the advance was working so he cranked it up and up and up.... ended up at about 25* base.

Ignition is stock Mercruiser Thunderbolt IV module, stock replacement coil (from NAPA), I hadn't checked for a resistor in-line... Didn't think they had one. The timing is set to 8* base and the advance curve adds +20 deg (Whatever stock is). I'll doublecheck voltage at coil, but spark is good and strong. Electric choke is working and not shorted.

P.S. anyone know the if there's an RPM limit on the stock ignition?

fstboater

If it is rich it will spit and slober when it starts black smoke you shoud have a good blue spark at the plug wire and good compression or late timing? Any questions drop me a email good luck.

fstboater

Set the timing to 33 at 4k rpm and lift the carb up and look for a drip after you get it hot from a good run.

bwiencek

Spark is nice and blue at the plug wire so I really don't think that's it.

How the heck do you "lift the carb up after a good run" ? Mine's bolted to the intake I've pulled the flame arrestor after a hard run looking for dripping after a hard run, but found nothing.

Also - exhaust is currently thru-prop (waiting on a thru-hull setup to be shipped from another member )

Thanks for all the responses - I'll doublecheck everything hopefully this weekend!

bwiencek

OK - checked everything out this weekend again. There is NO fuel smell or visable fuel dripping from the carb. The accelerator pump gives a good shot when it's in the situation where it is hard starting. What I did find is that the coil had only about 2.x Volts between the "+" connector and ground (engine block).

Does anyone have the wiring diagrams for a 1994 Mercruiser with the thunderbolt IV ignition system? Is there a resistor anywhere in-line with the coil?

- Brian

Gary Anderson

Should be full 12 volts at coil. No resistor or bypass wire is needed with the thundebolt ignitions. Disconnect the coil and check for 12V from the purple wire (with key on). If that's OK, just get a new coil. BTW a standard $11 auto coil works fine.
If you dont get 12V, it may be a problem at the ignition switch or with a short (voltage drain) dropping the voltage, or a partially broken wire. Also check voltage at coil with tach lead (tan or brn wire).
If you still need a diagram let me know, I can scan one tomorrow.
Gary

Gary Anderson

Look what I found online.
http://www.marinemechanic.com/site/page250.html
Gary