Irregular spark advance Thunderbolt IV

[Uncle Binky]

My early 90's 250 HP 5.7 Mercruiser (carby) with Thunderbolt IV ignition has a slight, but regular miss throughout RPM range. Put the timing light to her, and confirmed a steady 8 degree advance at idle. On the muffs, at operating temp, the engine bogs when I add throttle to her, and there is no advance from 8 degrees through about 1900 RPM. Somewhere around 2100 RPM the advance jumps to about 20 degrees (can't measure that far with my basic light), and the engine runs smoother and responds to throttle input much more freely. I installed a new module in the distributor last year, so I would hope it's not that. As a test I also swapped out the amplifier with a (used) spare I bought for a backup. Any thoughts? Thanks.

 

[Speakrdude]

Re: Irregular spark advance Thunderbolt IV

you probably already have but verify a good ground from the black wire of the TB module to the engine block/battery.

 

[Uncle Binky]

Re: Irregular spark advance Thunderbolt IV

you probably already have but verify a good ground from the black wire of the TB module to the engine block/battery.

Yes. I have both confirmed that all grounds have zero resistance, and combined the ground wires from the sensor module and the amplifier to the engine ground, just to add redundancy. I thought I had some info laying around that detailed how to test the amplifier module, but I can't seem to find it, if it did exist. I am not one to diagnose electrical problems by throwing new parts at it, but I'm at a loss as to what I should do next. I hear that this is a robust system, but it should be for what they want for the parts...

 

[dubs283]

Re: Irregular spark advance Thunderbolt IV

just for fun, separate the grounds for the sensor and the module, i.e. leave the module ground where it is and put the sensor ground on the same stud as the battery ground

i know you have a carb but merc recommends that efi equipped engines use the same ground as the battery

keep us posted, i have seen some strange behavior from t-bolt ignition modules, mainly the older large square box styles when used with the new style, three wire sensors