Starting the investigation.

After checking the ignition timing and other common causes it's going to be necessary to dig a bit deeper. 

Air cleaner off and then the rocker covers to check the valve clearances.

Valve clearance check.

The hydraulic cam followers should have a touch of pre-load when the lifter is on the base circle of the cam.

A piece of tape on the ones checked to confirm it's been checked.

Something wrong with N04.

When rotating the engine by hand there was hardly any movement on No.4 exhaust rocker.

Will need to remove the rocker, push rod and cam follower to inspect their condition and have a look at the cam lobe.

Cam follower condition.

After removing the cam followers it becomes clear that they are severely worn and it's likely that the camshaft is also damaged.

No.4 Cam follower.

This was the most worn follower. The bottom face should be completely flat with a light circular witness mark from its interaction with the cam lobe.

Camshaft removal.

With the long nose, on the Corvette, it is possible to remove the camshaft without removing the engine.

Radiators for the cooling system and AirCon need to be removed.

Front pulley.

To get to the cam it’s necessary to remove the fan, belts and front pulley wheel then with the use of a puller, remove the damper from the end of the crankshaft so the timing cover can be removed.

Timing gear and chain.

After removing the timing cover it also became clear that the timing chain and gears were worn. 

The chain should be tight with almost no slack to ensure the cam timing is perfect.

Camshaft inspection.

The lobe of a cam should be egg shaped with a point. 

As it rotates the cam follower moves up, pushing the push rod, operating the rocker and then opening the valve.

Cam lobe for No.4 exhaust.

The lobe for number four exhaust valve is almost worn away which would result in not allowing the exhaust gasses from the cylinder to escape.

The noise was caused by exhaust and unburned fuel coming pat the inlet valve, when opened.

New camshaft and timing gear.

After consulting the customer we decided to fit a higher performance camshaft, than OEM part and why not? The camshaft upgrade will give a good increase in horse power and torque.

New high grade timing gears and chain also fitted.

Last job is to now put it all back together.

After re-assembly the most important thing to do is to break the new camshaft in. This requires some special break-in additive to the oil and ensure that the engine fires up first time and is held at a high tick-over for at least 20 minutes.

After this break in period the car is ready to hit the road again.