That may be my next step. I thought I had the problem fixed with the pick up sensor. Did not have the timing light with me.

i,m not saying it can,t be an ignition problem but surging is almost always caused by a fuel problem.

A lot of time surging can be fuel related. With that said it has to be a process of limitation. And of course timing / or ign related related to. Ign related is more of a miss and can get worse.

I would put a fuel pressure gauge on this first and make sure that is not a problem. (fuel delivery problem) Next I would run a true intake vacuum source test with a Vacuum gauge. Vac testing can tell you alot.

I would also run the engine on a new portable fuel supply with fresh gas and new fuel line with a primer bulb inline. Squeeze the bulb if the surging is still taking place . Anything change

A layman's simple test also is while surging spray some carb cleaner down the carb did the rpms pick up or did the engine want to stall or did your surging go away ? very basic test but can pin point a fuel problem or a very lean condition.

Merc has a T bolt ignition flow chart for testing the components that can help as well.

Looks like the Thunderbolt V has modual and knock sensor. This is most likely where the problem is.

Ordered the knock sensor today. I will report back on the results.

Make sure you use the correct torque value.

What made you determine the knock sensor is the issue ?

If he has the OEM service manual or google it they Merc have a testing procedure for knock sensor and knock module right after for the T bolt V ignition testing flow chart.

Knock sensor install is 12 to 16 ft lbs no more and nor less installed per Merc specs. I just did one on a 350 HO mag.

If the knock sensor is bad it can retard the timing dropping about 300 RPM. And it is the cheapest part to try next.

Thanks for the tip on the torque value.