454 thunderbolt 4 timing

TBarCYa

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I followed the procedure in:

http://www.boatfix.com/merc/Servmanl/16/16B4R2.PDF

Which doesn't specify setting it I to base timing mode. I have her running about 650rpm on the dash and the timing sits about 16 degrees but sporadically drops down to 8 which is right where it should be. The engine is responsive in neutral and returns immediately to 650 when I close the throttle. I can't take her out tonight to test but since there's no way I can set it into base timing mode am I right to assume that it's already advancing and where it jumps down is the base timing? The manual doesn't mention anything about it
 

alldodge

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There is no base timing mode with a TB4, its point and shoot. If rpms are going up and down then you have a carb or other problem
 

TBarCYa

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The RPMs are staying dead where I set it at around 650 RPMs. It's the timing that seems to change. 80% of the time it's up around 16 degrees but then the other 20% it's at 8 degrees.
 

achris

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If you have a 2 wire sensor in the distributor, then make sure the distributor body has a ground wire on it.

Please post an engine serial number, as there are 3 different Thunderbolt IV module generations and without know which one you have it's difficult to offer advice.

Chris...
 

TBarCYa

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There are only 4 wires from the module. White/red and white/green that go to the distributor and two that go to the coil... One of which is grounded.

I'll post the serial number tomorrow when I'm at the boat.
 

alldodge

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The newer replacement design distributor module has an additional wire coming from it for an additional ground. This helped the module for causing issues
 

achris

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There are only 4 wires from the module. White/red and white/green that go to the distributor and two that go to the coil... One of which is grounded.....

That's not right. Depending on the module (large black box mounted on the exhaust elbow, small 'triangular' module on a plate on the exhaust elbow, or small rectangular module on the distributor body), it will have a white/red and a white/green to the sensor inside the distributor (later distributor sensors also had a ground wire), a purple that goes to/comes from the coil+ and a gray that goes to the coil- (which also has another grey for the tacho feed). Some modules have an additional black ground wire, some are grounded through the module body... Neither wire on the coil should be grounded.

I've looked at some of your other posts, and your engine has varied in year from 1994 to 1997... So, not sure what it is...

Here's what the ignition wiring would look like for an exhaust elbow mounted generation II TB-IV module...

9474.JPG

Also, you might want to have a read of this to see how it all works -> http://forums.iboats.com/forum/engin...n-systems-work

Chris....
 
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TBarCYa

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When I bought the boat I was told it was a 92 but the books said it's a 95 and the title says it's a 94. The engine I assumed to be consistent with the hull but it appears to be a 95.

I have the ignition module that's attached to the distributor but it's the earlier version of this module without the ground wire. Since the module appears to get it's ground via the mounting screws could / should I run a ground wire from one of those screws to the block? I don't appear to be having an issue now other than not knowing what the base timing is set to other than when it jumps down to 8 degrees BTDC.
 

achris

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When I bought the boat I was told it was a 92 but the books said it's a 95 and the title says it's a 94. The engine I assumed to be consistent with the hull but it appears to be a 95.

Which is why we ask for serial numbers... With an engine serial number we can tell you exactly what the engine model year is...

I have the ignition module that's attached to the distributor but it's the earlier version of this module without the ground wire. Since the module appears to get it's ground via the mounting screws could / should I run a ground wire from one of those screws to the block?

Most definitely...

I don't appear to be having an issue now other than not knowing what the base timing is set to other than when it jumps down to 8 degrees BTDC.

​​​​​​With TB-IV, there is no 'base timing mode'. You just set the initial timing at the specified idle rpm. Make sure you use a calibrated tacho for that too. I've seen dash tachos off by as much as 200rpm at idle...

Chris...
 

TBarCYa

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​​​​​​With TB-IV, there is no 'base timing mode'. You just set the initial timing at the specified idle rpm. Make sure you use a calibrated tacho for that too. I've seen dash tachos off by as much as 200rpm at idle...

And this is the confusing part because the timing is mostly at 16 degrees right now but sporadically drops to 8. My assumption is that it's at 16 due to the timing advance and that 8 would be the initial. When it shows 8 it's typically a single flash of the timing light that is followed by 4 or 5 at 16. If I'm truly at 16 right now I can't wait to see how it runs at 8 because she's very smooth and responsive but I haven't tested under real load. So far since setting the timing where it is I've only been able to put it in gear while tied up in the slip.
 

achris

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Are you saying that by my description I'm set to 8 BTDC or are you saying that's where it should be set?

He's saying that's where it should be... Check your timing light on another engine... See if it behaves the same. Also check the trigger lead isn't across a different spark plug lead.
 

TBarCYa

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Last time I used the light it worked just fine... I really don't have anything else that has a distributor to test on.
 

alldodge

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If the think bounces around from 6 to 16 but the rpms are not changing, your light is bad. Go borrow one from the local parts store

If it bounces around and rpms do change, then there is a bad component or loose connection.

Once timing and idle are adjusted, it should stay rock steady on 8
 

TBarCYa

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We took the boat out and after tweaking a little on the mixture screws was able to get her to run really well below WOT but she would bog down past 3/4 throttle. She was able to run 3500 rpm (27.5 mph) without issue.

When we got back to the slip I pulled out the timing light and this time did a better job of making sure that the pickup clip was not near the #3 wire and the indication was much more clear that the timing was running 16 degrees BTDC so I adjusted it to 8 degrees. I won't get to take her back out until Saturday but hopefully that will allow her to run past 3/4 throttle since the mixture seems to be as good as it's going to get. I'll be looking for threads on how to tune the Rochester carb before Saturday.
 

achris

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We took the boat out and after tweaking a little on the mixture screws was able to get her to run really well below WOT but she would bog down past 3/4 throttle. She was able to run 3500 rpm (27.5 mph) without issue.

When we got back to the slip I pulled out the timing light and this time did a better job of making sure that the pickup clip was not near the #3 wire and the indication was much more clear that the timing was running 16 degrees BTDC so I adjusted it to 8 degrees. I won't get to take her back out until Saturday but hopefully that will allow her to run past 3/4 throttle since the mixture seems to be as good as it's going to get. I'll be looking for threads on how to tune the Rochester carb before Saturday.

Where our serial number? ;)
 

aimlow

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We took the boat out and after tweaking a little on the mixture screws was able to get her to run really well below WOT but she would bog down past 3/4 throttle. She was able to run 3500 rpm (27.5 mph) without issue.

When we got back to the slip I pulled out the timing light and this time did a better job of making sure that the pickup clip was not near the #3 wire and the indication was much more clear that the timing was running 16 degrees BTDC so I adjusted it to 8 degrees. I won't get to take her back out until Saturday but hopefully that will allow her to run past 3/4 throttle since the mixture seems to be as good as it's going to get. I'll be looking for threads on how to tune the Rochester carb before Saturday.

A couple possibilities on "bogging" at high fuel flow. 1) blogged fuel filter. Don't forget the one in the carb inlet 2) Secondary air valve is opening too early. 3/4 turn of the control spring works fime for a big block inboard...anything less is not enough tension.
 
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