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Thunder bolt IV timing question

Old 05-24-2011, 08:04 AM
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I believe I have the same advance unit on mine - boat started life with the 350 mag. Does the engine have to be under load to attain the max advance, or is it just a measure of RPM? Since it is a TB IV, there is no knock sensor to retard the ignition. In other words, is it possible to set total advance running on the hose, or do I need to be actually under way in the boat on the water?

I was thinking about starting out at around 10* 12* initial with mine, but now I am thinking 36* total advance may be too aggressive. It is a 489 with about 8.7:1 CR, forged pistons, stock iron heads, relatively mild cam.

Last edited by Budman II; 05-24-2011 at 08:08 AM.
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Old 05-24-2011, 08:55 AM
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The reason to consider changing the module is to allow more intial timing at idle without exceeding the total timing at WOT. Larger camshafts and performance combinations need the extra timing for consistent idle quality while shifting and varing loads. To test the theory adjust the timing up at idle, and see if the engine likes it. If your shifting rpm drop is better and the overall quality is better then look for a module that will allow the extra initial without exceeding the total. Do not run the boat under load with the intial timing set that way, it is a diagnostic only. You may not gain any drivability with the extra intial timing, thus you dont need the module change. Always set the timing by looking at the total timing advance, not the initial. A set-back timing light or specific marks on the balancer can be used.
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Old 05-24-2011, 09:37 AM
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Originally Posted by Boat1 View Post
The reason to consider changing the module is to allow more intial timing at idle without exceeding the total timing at WOT. Larger camshafts and performance combinations need the extra timing for consistent idle quality while shifting and varing loads. To test the theory adjust the timing up at idle, and see if the engine likes it. If your shifting rpm drop is better and the overall quality is better then look for a module that will allow the extra initial without exceeding the total. Do not run the boat under load with the intial timing set that way, it is a diagnostic only. You may not gain any drivability with the extra intial timing, thus you dont need the module change. Always set the timing by looking at the total timing advance, not the initial. A set-back timing light or specific marks on the balancer can be used.
Can the total timing be set on a TB IV in the driveway on the hose, or does the engine need to be under load?

Another option might be an ignition module that has adjustable advance like the Crane Hi-6 or something similar. More expensive, but the Merc modules are getting harder to come by.
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Old 05-24-2011, 09:40 AM
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I have also had issues getting the motor to spin over easily while starting it with a lot of initial advance. I have had to go as far as pulling the lanyard switch to get the engine spinning, and then snapping it up to fire it up, especially if the battery was drawn down and the engine was hot from heat soak.
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Old 05-24-2011, 11:09 AM
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I personally dont rev an engine above 1200 rpm on the hose. You can leave it on the trailer and back it down a ramp, the engine does not need load, but does need rpm for timing advance. Your example illustrates my point. Some engine combinations do not benefit from excess intial timing, I'd say yours does not like it. Some engines are so lazy at idle they stall when loaded, sometimes more timing will help that situation. The Thunderbolt ignition has limits to the flexability of the advance curve when compared to aftermarket, but it is a good system. Your job is to determine what your engine likes at idle, re-start conditions, and at WOT for timing, and then try and connect the dots as evenly as possible. Hope this helps.
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Old 05-24-2011, 12:52 PM
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Originally Posted by Budman II View Post
I believe I have the same advance unit on mine - boat started life with the 350 mag. Does the engine have to be under load to attain the max advance, or is it just a measure of RPM? Since it is a TB IV, there is no knock sensor to retard the ignition. In other words, is it possible to set total advance running on the hose, or do I need to be actually under way in the boat on the water?

I was thinking about starting out at around 10* 12* initial with mine, but now I am thinking 36* total advance may be too aggressive. It is a 489 with about 8.7:1 CR, forged pistons, stock iron heads, relatively mild cam.
34-36* total timing is fine with 8.7 CR and 89 octane.

The engine does not have to be under load to set it.

You should not have problems with the engine turning over with only 10-12* of initial timing. Maybe a weak battery or starter going bad.

Last edited by Griff; 05-24-2011 at 12:54 PM.
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