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Possible bad ignition module

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briang0

Regular Contributor
Hi I'm looking for a reality check on this. I have a 1983 Mercruiser 898 305 with a Thunderbolt IV Ignition. I have been having a lot of trouble with my boat this year since I put it in the water. So far I have pumped 15 gallons of fuel out of the tank replaced the plugs, wires, distributor cap, rotor, coil, rebuilt the carburetor, and replaced the ignition senor. I'm still having trouble. Through all this I have had back firing through the carburetor, random loss of power between 1500 and 3000 RPMs. Now I have the idle will vary a couple of hundred RPMs and when I put the timing light on I can watch the timing mark move when the idle changes sometimes more than 8 degrees. I'm thinking that the ignition module is the problem but if anyone has any other suggestions I'm open. I've tried to adjust the carb at idle but it keeps shifting 3 - 400 RPMs again with the the timing. I'm really getting frustrated with this thing all I want to do is take it for a ride where I don't have to limp back to the dock.
 
You need to check the spring loaded power valve in your carb to make sure it's working smoothly. This is the rpm range that would cause a problem if it's frozen in place.
 
Alrighty then,

I've read a few times about issues like you're having with the T-Bolt IV, & T-Bolt V ignitions lately, so I do know it's something to check, but I'm not sure how to diagnose them. They're supposed to be Plug & Play?
 
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