I have a '68 Skylark with the 350-4. It's powered by a 1979 M4MC Quadrajet and Buick HEI. The engine was rebuilt prior to myself owning the vehicle. It has what I believe to be a TA 212( .475"-223'/.475"-230',110'). Carb runs a little on the rich side currently(to eliminate lean possibilities) and timing is all in 3000 around 34 degrees. At 4000 rpm and up the engine produces a rattle. It can occasionally be heard while in park and sounds like it is coming from the driver's side rocker/valve area. While driving, the rattle occurs every time the engine hits 4k or above. Valve lash is close to spec, I have an adjustable rod coming to check the correct rod length. The noise is more of a rattle however and not the normal lifter tick from lash. I have inspected the rockers, shafts show no cracks, all of the pads and cups are in place.
The heat riser was in the passenger manifold. It was making some noise and was removed. The driver's side does have the "heat tube chimney" used to run from the exhaust to the air filter housing. I believe that is just an adapter though, no valve there?
Check flexplate for cracks and loose bolts. .check harmonic balancer bolt. Turn timing down and see if its spark knock 34 may be too much for your combo or fuel. Really want full timing in sooner. 2600 or sooner . Did you know if cam was degreed in ? I would think its spark knock, which can cause bearing damage or stretched rods, again causing bearing issues.
Thanks, No cracks in the flexplate, bolts are tight. No signs of rubbing. Same for the harmonic balancer, rubber hasn't twisted at all. Marked it a month or so ago, still lined up. Had timing down to 28 at one point, in sooner, and in later with no change except for performance. When the noise occurs it is running rich. I do not know if the cam was degreed. No obvious signs of heat or anything abnormal on the cylinder walls or pistons. -If I don't find anything prior to winter, I am pulling the intake and timing cover and inspecting cam timing, lifters, etc.
Could be a resonance with something loose that only vibrates at those higher RPMS. Had a fuel line that was not clipped down that drove me crazy trying to find out the rattle at certain RPMs.
Oblong rod big end or piston slap. Was it rebuilt or ring and bearings.? And lean it out, running it too rich causes wall wash of cylinder
When you bring it up to 4 grand does it rattle then, or when you release the throttle and the rpm comes down? Most times too loose (worn) rod bearings will be quiet at idle, even light throttle cruising, but as rpm increases, the rod starts slapping around on the crank journal. Same thing will happen in park/neutral, slowly bring the rpm up, then release the throttle, if theres excessive clearance, when the throttle is closed and the downward load is lessened from the piston pushing downward on the rod, (that's why most rod bearing wear is on the upper shell) the rod is allowed to dance around the rod journal on the crank from too much clearance. Worn rod bearing/s typically doesn't show up in oil pressure readings.