I have a knock that i can't figure out what it is.

The motor is 350 out of a 89' chev.. bored .30 over open plane intake, mild cam, holly 670 off road carb, 2 1/4" ram horn manifolds, HEI Distributor off a 86 camaro with a high output coil.
Only suppose to have 5'000 miles on it since the rebuild.

Just noticed it after it came back from the exhaust shop. Was running open headers before and just putting around the yard.
It sounds like a diesel between 1200 rpm and up.
Except when in a high gear at WOT driving up a hill (so low rpm under a load)
Does it cold and hot. Clutch in or out.
Can't hear anything at idle. Can't tell if its top or bottom. Sound more to the rear of the motor.

I have been running 91 octane fuel at 6-8 degrees at 800 rpm. But I've retarded my timing and its still there.
Ive changed distributors and verified my plug wire are not touching for cross induction. Running accel 8.8 with brand new ac delcos plugs for that year motor(plugs are a dark cardboard color).
Carb is running on out of the box settings.

Im stumped :confused:

check oil for metal flake.

pull intake and inspect cam lobes.


pull pan and inspect cap bearing.

Have someone revv it and with a stethoscope see if you can find where it may be coming from. Or cardboard roll thing from the paper towels for the moving parts.

check oil for metal flake.




pull pan and inspect cap bearing.

caps are fine and no metal flakes.. May pull the intake on the weekend

Broken flexplate maybe?

Broken flexplate maybe?

Standard.. So a flywheel. Ive wondered if the bolts came loose though They were torque and loctite though. And i maybe have 4 hours on this truck since i put it together

do you have tall roller rockers and tall ENOUGH valve covers?

Flat tappet or roller cam? Hydraulic or solid lifter? If it's hydraulic lifter, did you set the valves static or running? I'd start by checking valve adjustment while running.

Sure it's not an exhaust leak?
I know it sounds dumb, but double check the firing order to be sure.

It would be factory rockers. I never Did any of the Work. I bought the motor.
and i have no clue what kind of lifters are in.. I would assume hydraulic.

So ill check my valve adjustment..

but one of my buddies is set on it being Piston slap?

Stethoscope or some type of listening device will seperate piston slap from valve noise. Piston slap will be louder in the block and valve noise will be louder on the covers or intake. I use a car solid radio antenna to listen to stuff, works pretty good.