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Old 07-10-2017, 03:05 PM   #1
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Default Thunderbolt IV Wiring Question

Hello folks! Just bought a 1993 Maxum 1900SR with 4.3L + Alpha One gen 2 and I'm going over general maintenance to make sure everything is in order since i don't know the history of how well it was taken care of. My next thing to do is check the timing at idle. Boat idles in nuetral fine at 650 RPM. So from what I've read about the Thunderbolt IV ignition, I don't have to use a jumper to check timing, just hook up the timing light, point and shot. However , i did find a purple and a Black (possibly gray) wire that is zip tied to the carb and not connected to anything. The end of the black (possibly gray) wire has a ring looking connector. The purple one has a plastic end that looks like it should snap to another connector. Long story story short, what do those do? I suspect the black/gray is the tachometer...? No idea what the purple one is. I'll try and post a photo when i get home. Thanks!
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Old 07-10-2017, 04:25 PM   #2
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You are correct the TB IV does not require the jumper to set base timing that the TB V uses.

Does your carb have an electric choke as it these would be the wires for it from what you have described. A picture would help.
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Old 07-10-2017, 09:27 PM   #3
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I'll try to get a photo up asap to confirm your hunch. Thanks Mike.
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Old 07-24-2017, 09:45 PM   #4
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Finally got a chance to get some photos. Also was able to confirm I have a TB IV.

https://photos.app.goo.gl/HgG5anOH4HK2zrqf2
https://photos.app.goo.gl/z7lw3iF0EC4fAJFI3
https://photos.app.goo.gl/MPgwlncsgiaPPVtx1
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Old 07-25-2017, 02:48 AM   #5
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Hmmm...My last post didn't save for some reason.

So here's that purple wire I was talking about. I think you're right that it's for the carb. I was also able to confirm it's a TB IV. I was not a 100% certain until i took a photo of it today.
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File Type: jpg 20170724_172155.jpg (208.0 KB, 4 views)
File Type: jpg 20170724_171805.jpg (223.6 KB, 4 views)
File Type: jpg 20170724_171758.jpg (160.0 KB, 5 views)
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Old 07-25-2017, 04:53 AM   #6
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Those wires are more than likly for an electric choke (or unchoke as some call it )!
You have a manifold heated unchoke! ( Spring type coil on intake manifold ) that opens the choke as the engine warms up!
Please fill out your signature so we can help you better!
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Old 07-25-2017, 11:35 AM   #7
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The first picture is hard evidence it's a TB IV.

Like Don said you have the older non-electric choke. It uses a bi-metal spring that sits in a pocket on the intake manifold. As the engine heats up so does the spring which causes it to unwind as one metal expands more than the other. This pulls the choke rod opening the choke plate.
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Old 07-25-2017, 02:26 PM   #8
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Thanks for the info guys. Just to sum it all up, my timing was all jacked up. Once it was confirmed that I have a TB IV, I hooked up the light and found i was running 8 degree ATDC. I looked at the bolt on the distributor, it still have the factory paint as if this was a factory setting. No signs of anyone tinkering with it. I adjusted it back to 8 degree BTDC at idle (~650 RPM). I don't have a fancy advance timing gun to check total advance. I might rig something up to extend the current timing indicator to go up to 22 degrees BTDC to check total advance timing. I'll report back once i get a chance to work on the boat. Hopefully this will help someone in the future.
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Old 07-25-2017, 04:03 PM   #9
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You can get a timing tape from places such as Summit Racing just make sure you order it for the same size balancer.
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Old 07-31-2017, 02:40 PM   #10
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Default Timing Marks

Finally got a chance to check the total advance. Didn't have timing tape so I measured the distance of what 10 degrees would be and traced that on the harmonic balancer from the original line. When I was running at 2500 RPM and 3500 RPM, I just made sure that my new line is at the 12 degree BTDC mark. That would put the original line at 22 degrees. Glad that is over! Thanks for the help everyone.
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