[cariboumarkt]

Hello everybody. Hoping somebody can help me with a detonation problem. I've got a 1969 Chevy K20 four-wheel drive pickup with a high mileage and basically stock 350 engine. It has an Edelbrock 1406 with an adapter, headers and an HEI distributor I got from the parts store. Not sure the brand of the HEI. I initially started trying to time the thing by reaching for a total mechanical advance of around 32 degrees, so I needed an initial advance of around 18° with the HEI adding another 14°. I figured I'd deal with the vacuum advance after I got the MA sorted out. Anyway, the thing pinged like crazy. I started backing off the initial timing a couple degrees at a time and ended up all the way back at 6° btdc with the thing still pinging. I verified the timing mark on the crank with a piston stop so I'm confident it is reading correctly. I then determined where my mechanical advance was starting and ending with this aftermarket HEI. Turns out the advance was starting to come in around 1200 RPM and was all in by 1800 RPM. I got an aftermarket spring kit and started swapping in for heavier springs to push the onset of mechanical advance off and ended up doing multiple spring changes. I did some calculations on the Mr Gasket spring kit that I got and figured out foot pounds per inch for these springs. Even the heaviest spring, the gold one in the package (which came in at 13.8 ft pounds per inch, by the way) barely made any change in the onset of advance. The stock springs in the HEI and the silver springs in the kit had my mechanical advance starting at 1100 RPM and fully in by 1600 rpm. The heaviest Gold spring in the kit started the advance at 1200 RPM and was all in by 1800 rpms. At this point I went rogue and started buying springs of different pound for per foot readings that were above the gold (13.8 pounds per foot). I got springs that were 16 and a half pounds per foot 22.8 lb per foot 36 lb per foot up to a whopping 54 lb per foot! Those 54.7 lb per foot springs gave me a mechanical advance that began at around 1800 RPM and was fully in by 2800 RPM which I felt was more appropriate. Even so, when trying to run an initial advance of around 15° btdc, at the RPM where the mechanical advance starts to come in, I get detonation! Very low RPMs under a heavy load and all other situations no detonation but right when that mechanical advance starts to come in the thing starts pinging. It's not super bad but it is there nonetheless. I've now dropped the initial advance off by a couple degrees at a time down to eight degrees initial and I still get pinging when the mechanical advance comes into the picture. FYI, plug gap set to .045. Sorry for the long post but was hoping somebody might be able to explain to me what might be happening here? This truck will likely live most of its life under 3500 RPM, just weekly trips to the dump in the hardware store but I would like to A) not destroy the engine from detonation and B) get the most mileage out of the thing that I possibly can. Does anyone have any advice on how to avoid this pinging?