Elusive engine miss

[purpleRac3r]

Okay, only a member a few days and already need some assistance. Good timing! Here's my problem: After a recent auto cross, my 1974 350 small block has developed what sounds like an ignition miss along with a metallic ticking sound. The engine runs as if one cylinder isn't firing. However, the plugs are all clean and get great spark. The timing is correct. There are no faulty vacuum lines. The plug wire have no signs of crack, burns, etc. No plug is cracked. For all intents and purposes, I cannot find any fault with the ignition system. The metallic ticking is somewhat difficult to pin down. I've narrowed it to the upper portion of the engine, but can't seem to get any more specific. Even a stethescope isn't helping me locate it. The ticking seems to come from everywhere. The sound is kinda like a cross between a metal rod tapping the valve cover and an intermittant vacuum leak. It varies with the rpm of the engine. I'm almost certain it's mechanical in nature. I've been told locally that a broken valve spring couild cause the symptoms I'm experiencing. I've also thought a failing hydraulic lifter could also be the case. Valve springs I can readily check, but how does one check the lifters without tearing dowm the engine? My service manual is rather lacking in such details. Any other thoughts? I'm going into driving withdrawals. Please help!

 

[waterboy1976]

Coming from experience on a "broken valve spring" you would definitely notice it running rough if you broke one, especially an intake valve....Thats what caused mine to go through a two year resto thats almost complete. Are you sure you dont have a header of manifold leak. I had one on my collector that sounded like it was coming from the right side valve train but the car ran stout. Good luck...

 

[jdp6000]

These problems are sometimes tough to figure out. Does it do it at all speeds? You can try unplugging one plug at a time until you find one that doesn't make a difference. At least you will narrow it down to the right cylinder...thats if its a lifter or spring. A compression test may also tell you.

Do you have an EGR? Is it working?

Maybe try changing the oil.

Jim

 

[goertz]

sounds to me like a rocker arm issue. pull off a valve cover and look inside to see if everything is as it should be. if you really wound up the motor, it's possible that a rocker arm came loose, a pushrod bent, or came out of adjustment. that's what I would suspect. It would give the sound you are describing, and would also cause an engine miss.

 

[purpleRac3r]

Hmm, I don't think exhaust is the culprit - no signs of leaks and all bolts tight. No EGR on the car. Vavle springs and rockers do make sense. I replaced all the valve seals back in the early spring, so it's possible something could be amiss there. I'll look into it. I'll also try to pull plug wires as it's running to narrow down which cylinder is not pulling its share of duty (though that can be a bit of a challenge with block-hugger headers!). Thanks to all who offered suggestions! I'll post again once I've figured out the problem.

 

[AKRAY4PLAY]

pull the plug wires from the distributor and save yourself some burnt fingers. make sure to used insulated pliers or you might get shocked. best thing is to remove a wire and then start the car, unless you like tazers. also if the spark trick does not zero your problem in, pull the valve covers and measure rocker travel. easiest to do without running the engine, just turning by hand. measure the pushrod side, you're looking for lobe lift. this will narrow your search to a lifter or flattened lobe. good luck, Brian.

 

[purpleRac3r]

Okay, well, after some more head scratching and plug wire-pulling and a new battery, still no go. Thanks, Brian - good call on pulling distributor end of wires. I have verified without a doubt it is not an electrical ignition problem. There are also no broken valve springs, though I suppose there could be some weak ones in there. I need to purchase a dial indicator and magnetic base to measure cam lobe lift, so, as might be expected, I'll have to post more later. In the meantime, any other suggestions will be more than welcome! I do suspect a failed hydraulic lifter/flattened cam lobe is the root of my problem. Time will tell!

 

[chevyaddict]

I'm just responding to this thread so I can keep apprised of how things progress! Very interesting! Good luck there Plum and Welcome to CAC!

 

[Jack]

I do suspect a failed hydraulic lifter/flattened cam lobe is the root of my problem. Time will tell!

Pushrod ends can & do fracture and collapse ... especially if you flail on OE pushrods that're 30 years old. Lifters can & do collapse ... especially if you flail on OE lifters that're 30 years old. Check pushrods first. Then check valve lift followed by lobe lift.
JACK:gap