[ggrims]

I figured I would start a new thread as I think I have the problem isolated. My #2 and #4 cylender are not firing or mabe a spark every now and then as I saw a few marks on the side of the electrode on the plug, but they were new looking and wet. I replaced the wires cap and rotor. It is a thunderbolt 4 ignition. I have not worked on many of these and when I did they were easy fixes 10 years ago. The engine is a 1992 350 mag. What in this system would cause 2 cylenders not to fire. I was wondering if it was the sensor in the distributor, but I assume that only sends the spark to the rotor when it passes. I dont know much about the modules on the exhaust manifold. I have worked on outboards and there is a powerpack that tells the cylender when to fire, does this work the same way?? I really miss the basic points ignition..

[m&mmariah]

How about checking all of the basic stuff first.....the things that need to be present for the cylinder to fire are are fuel, compression, and spark. Fuel can be ruled out, since it's getting to the other six cylinders, so that leaves only compression and spark to check. So, make sure the compression is within a few pounds of the other cylinders. If not, then it could be due to leaky piston rings or a leaky intake or exhaust valve. Once compression is ruled out, then we need to check for spark. An easy way to check for spark is to just hook a timing light up to the plug wire...if it's flashing, you have spark. If there's no spark, then it's got to be a problem with the plug wire or spark plug. Is the plug's gap set correctly? Is the insulator broken, or is the plug fouled with oil? Is there a piece of gunk stuck between the electrode and the ground tip? You should be getting spark, since the other cylinders are. After all, the rotor in the distributor should be sending the spark to each of the eight contact points inside of the distributor, right? If all else fails, take it to the boat shop and have then diagnose the problem. Good luck!

[mikeyt]

You might want to go back one step further and confirm that you reattached the ignition wires in the correct firing order.

[ggrims]

I will try and check the wires again but it was the same two cylinders that were not firing before replacing the parts ( The previous owner said the boat ran great but who knows). I replaced the cap , rotor, plugs (Gapped) and wires. That is why I was trying to figure out if the other components of the ignition are responsible for firing individual plugs or just generating the electricity for the distributor to send the spark to the individual cylinders. What function does the pack for the ignition that bolts to the exhaust manifold do besides the coil, does it just create a hotter spark or is it more like an ECU. Thanks for all the help.