sometimes spark sometimes no spark

i have an 91 chapparral with 2 bbl 4.3l mercruisier, thunderbolt IV ignition. it will start when its cold or if it has set awhile but if u run it and shut it off it wont start i check for spark at the coil wire to the distributor cap no spark while crankin on it but as u let off of the key it will spark. so i replaced ignition switch still no spark so i checked juice on positive side of coil has good volts while crankin and while settin with key on. checked and bypassed engine safty switches didnt make a difference. i replaced plug wires, distributor cap , distributor sensor, coil, and the ignition amplifier i am at a stand still cant figure it out but when its runnin it will run fine just dont shut it off or it wont start again for hours or next day. does anyone have any ideas that i can try? thanks

Re: sometimes spark sometimes no spark

http://www.marinepartsman.com/Mercury-Marine-service-bulletins/MerCruiser/1983/EN_24.PDF

Check out page 3. It has a step-by-step troubleshooting process for the Thunderbolt IV ignition. My first guess would be the ignition amplifier (just went through mine and found that problem), but since you replaced it already, I'd guess the ignition sensor in the distributor is bad. It's about $40 or so to replace. Go through the steps - it takes about 15 minutes. After I found that I needed a new amplifier, I decided to just switch over to a newer EST system:

http://www.michiganmotorz.com/delco-voyager-marine-electronic-distributor-p-119.html

Re: sometimes spark sometimes no spark

thanks i did replace that sensor to begin with but i did the trouble shooting and it comes back that distributor sensor so im going to get another new one and hopefully it will fix it i will let u no.....................

Re: sometimes spark sometimes no spark

well im back to were i started the coil is good, spark amplifier, rotorand cap, and the distributor senser still cant figure out the problem. ive checked grounds and power to all ignition and switches all good. my boat will start mainly when it is cold and it will run just fine but if u shut it off after it has been ran it wont get spark while u crank it over sometimes if i just quickly bump the key to start it will fire up or spark and most the time just no spark? and i have replaced the ignition switch and bypassed it as well, if anyone has anymore ideas id be greatly appreciative thanks.

Re: sometimes spark sometimes no spark

this one sounds like you need to look into more wiring, something is shorting out, when warm. check and change your main power lines.