Do I Have A Bad Lobe On My Cam, Or A Bad Valve

Posted in Internal Engine Parts | Asked on Jun 18, 2010

I have a brand new 427 Big Block Chevy motor in a 67 Camaro. Recently on my last drive, I heard a ticking sound from the valve cover. Having concern, I turned around and headed home, 15 miles back. On my way back the ticking got louder and it sounded like machine gun fire from the carb, motor was running 215 degrees water temp. I pulled over and messed with the timing until the popping went away. I nursed it the rest of the way home with the loud ticking still going, but no more popping and it ran at normal temp of 185 the whole way, seemed to run fine, but like it had a dead cylinder. I figured I had some rockers to adjust and a plug to replace.

When I got home, I pulled the valve cover and found a rocker arm sideways chilling with no damage, and a bent and broken pushrod (intake) on number 8.

After pulling the intake, I inspected all 16 rocker arms, pushrods, and lifters to find no other damage. I did notice the intake lifter from the broken pushrod cylinder was slightly smaller then the rest of the lifters. No discoloration, and the lobe on the cam seemed totally fine. I replaced the pusrod and lifter, adjusted all the rockers, and reassembled the motor.

I ran a compression test on all cylinders. They were all equal at around 126, except for number 8, the one that had the broken pushrod, came in at 119, still within reasonable limits but slightly low.

With the coil wire off, and no fuel or spark to the motor what so ever, when I turn it over, I can hear air coming out of the carb, and when I run it with spark and fuel, it pops continuously through the carb as well.

From what I know ive narrowed it down to a slightly bent valve or a worn lobe on my new cam. Its odd because I still have good compression at 119, but shouldnt it be much lower compression with a bent valve? and the only reason I think it may be the lobe on the cam is because when I pulled the lifter, it was smaller in length then the rest of the lifters and had a slight outer bulge on the bottom (wobbles when you set it on a flat surface), but the lobe on the cam looks and feels perfect. No metal shavings or anything anywhere in the oil filter or the motor. I have a 500 lift cam with 280 duration at 0.50, and 1.72 roller tip rockers. The motor has 54 miles on it and was built completely with all new parts. Im a little stumped and could use some help on what to do next. Im still leaning towards the bad valve but that small lifter has me worried.

There are 4 Answers for "Do I Have A Bad Lobe On My Cam, Or A Bad Valve"

  1. kelly_f_1999 says:

    good sign you need to look into it farther valve or cam or even worse

  2. oldmanarnie says:

    i would say bent valve it takes a lot of wear to ruin the lobe, the valve still moves even the pushrod isn’t the right size

  3. cussos58 says:

    i think your going to need to take the heads off and inspect the valves and guides and if all is ok there then pull the cam out for inspection but it would need be unusual to damage a cam lobe in so short a time did you have hydraulic lifters or solids maybe when you assembled the motor the lifter was bad then but you missed it who knows but i still suggest pull the heads and inspect all the valves and guides maybe think about having brass inserts fitted if you havent already done it and when you put it together use new lifters on both intake and exhaust in 8 with new rods and take extra care when you adjust the clearances good luck with it just bad luck what happened but i know how you feel i have blown a few motors myself most recent was a 5L injected in a commodore i rebuilt it and did 1500klms and grabbed 2 pistons and we still cant work out why good luck hope all goes well

  4. Country Boy says:

    Roll the crankshaft up to TDC on #8 cylinder and remove the #8 spark plug. If you have one of those valve retainer/spring removal lever tools, remove the spring and retainer off the intake valve on #8 cylinder. With your fingers see if the valve slides freely down the guide till the head of the valve hits the piston. This just checks to see if the valve stem isn’t bent. Put the retainer, split locks and spring back together. Hate to say this but the intake’s gotta come off again. If there isn’t a slight crown on the bottom (cam side) of the lifter toss it and get a new one. Inspect both cam lobe wear as far as width pattern down through the lifter hole. Don’t forget the special cam lube on the face of the lifter and the lifter hole. *Without a bearing ball the same size as a pushrod ball placed in the top of the lifter you can’t accureately check lifter length. McMasterCarr.com has precission steel bearing balls.

    If you’re sure the rocker arm geometry is OK on the intake of #8 and the push rod length is OK, put a dial indicater on the end of the intake push rod and roll the engine over by hand with no plugs in it using a ratchet & socket on the nose of the dampener pulley. Jot down the # and compare the cam lift with another intake cylinder’s cam lift. Uou’ve got to have all good crowned lifters for this test.

    If the intake lobe lift is less on #8 intake I guess we know the cam is shot. If replacement is necessary look into special soft break-in valve springs and let the engine high idle @1,000 -1,200 for an hour to break in the cam and lifters.

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