Driving the 350 skylark on the highway today, just put the hammer down to pass a vehicle and is started making a noise with a miss, got immediately nervous and let off the throttle. As I continued on toward home I noticed the noise was not noticeable under light throttle but more and more noticeable as throttle is applied. It is definitely a valve train noise, not bottom end. I can even hear it through the carburetor so I’m thinking it’s a valve issue. Could be rocker arm, lifter, pushrod, or worst case cam lobe. Am I missing anything? Just wondering if I should start by yanking off the valve covers or should I be able to narrow it to a certain cylinder by pulling the plugs first?
Doing the following in any order is neither "wrong", nor "more" correct. All will provide info needed for a good diagnosis and eliminating causes Pull the plugs, might need to replace them, might only need to evaluate them. But, are they the correct plugs? Pull valve cover and run while observing push rods for some rotation, any thing that is loose or broken, any metal, and if all rockers are moving about the same amount. A stethoscope to isolate any particular noise in only one location Compression check, and if possible, leak down. Is there an exhaust leak? Can sound like a miss and or valve train issue. Plug wires can also cause a miss, bad distributor cap. Ruling out what you can before spending money. Post back what you know, want you've tried and if anything changes.
MAYBE a leaky exhaust manifold/header gasket. Leaky gaskets will act like that, quiet at idle and progressively louder as you apply gas. Valve train noise generally will be VERY noticeable at idle as well as off idle, sometimes it’s a tad quieter at higher rpm
I thought that was the issue at first, but when you get on the throttle under load there is a miss so that rules out an exhaust leak to me.
I can hear a tap at idle when I’m under the hood, couldn’t hear it while driving under light throttle. When I rev the engine in neutral it backfires and when I’m under the hood and rev I can hear it in the carburetor, hard to describe, but I’m guess an intake valve would allow me to hear ignition back through the intake?
Plugs are correct yes. Engine complete rebuild has about 4,000 miles on it. I did just have the intake off to reseal an oil leak from the front end seals, is this coincidence? I don’t know. Wonder if a piece of debris could have gotten into a lifter?
motor is a 72 350. custom JE pistons with 9.5:1 compression. heads ported and polished. stock exhaust manifolds. Quadrajet carb set up by Ken at Everyday Performance. TA 284 cam 475"-223'/.475"-230',110'. 2800 stall converter from JW. TA stage 1 dual plane aluminum intake. Pertronix points eliminator.
The first thing to do is to pull off both valve covers and look for loose or bent push rods, or broken valve springs or factured rockers as you bar the motor around so each cylinder comes up to TDC. If that all checks out then with the motor at a rpm where the noise is being heard at you pull the plug wires off one at a time and if the noise stops then you have a Rod bearing issue going on. if this proves to not be the case then when your foot got heavy and the rev’s where hi weak valve springs may have allowed valve float and a lifter plunger retaining clip might have popped out and now you have a valve not working right. If a valve is hanging open such would show up with a compression test.
You might see something by using a vac gauge it should be steady no matter what vac the engine is at. I used Comp Cams pro magnum lifters when I put my last cam in they were just better and would not get mushy when motor was off. I miss could be any thing from cracked dist cap to bad plug wire or plug to a bad lifter or valve leakdown, compression test, start without valve cover on just don't rev it and it won't splash too much oil lol. look for broken valve spring. Everything that was basically said here already. If you got points it could be a bad condenser, a bad coil will let a car run but barely, worse when you step on it. If you have a MSD it should shoot about 1/2 inch lightning bolt to ground when going thru their test procedure. I have heard stuff about that Petronix check that out too I didn't think they were all good.
It sounds like you have an exhaust valve that is not opening, could be a bent push rod, flat cam load etc. Any pressure that builds in a cylinder that the exhaust valve isn't opening tries to escape when the intake valve opens, causing a popping sound and miss, under acceleration, the more you accelerate the louder it gets. If its ticking at idle it probably isn't a cam lobe. Good luck with it! Bob H.
I have seen this. At idle the engine sounded fine. At 1500 rpm and above it would miss and pop out of the top of the carb. Pulling valve covers revealed a broken exhaust rocker arm.
I pulled the valve covers and ran the engine, second cylinder back on drivers side, so that is number three cylinder I believe, it’s the fourth valve back not sure if exhaust or intake but it is not opening all the way. Next step is to pull pushrod and have a look, if it is straight I guess pull the intake and look at the lifter right? What should I be looking for as far as a bad lifter goes? Seeing as how I just had the intake off last week to reseal is it possible that I got some debris in the valley that screwed up that lifter?
You can turn the engine until that valve is closed, valve stem side of the rocker arm all the way up, then see if you can push the rocker arm down on the pushrod side, if you can it is probably the lifter, if not it might be a rounded or flat lobe! Bob H.
So I just pulled the lifter that was not opening, the cam lobe is wiped out, ****! I guess I need to pull the motor and have a look at the bearings huh? What’s the chances they survived?
What could have happened? I did the proper break in procedure, I use vr1 high zinc oil, change it every 1,000 miles. I can’t remember who manufactures that 284 cam but my engine builder was not real excited about it I remember, could it have been poor quality parts? It ran 4,000 miles then failed, what could have happened? I sure don’t want to repeat this!
Your cam bearings are fine, so no need to fret about them. break in is critical ! The cam needs to be Parkerized. Break in lube and the motor filled with a NON detergent oil so the break in lube stays in place for as long as possible. A can of break in additive should be used. And above all the motor needs to fire right away and be taken up to 2500 to 3000 for 15 minutes. No cranking away for 2 minutes just to get the thing to even kick! Anything less then 2500 rpm will not kick up enough oil to the cam. a big big plus is to use lifters with the lobe oil hole added to the bottom . With this oil hole feature you have to work at failing a lifter and or lobe!
The crank and rod bearings are what I was worried about but thank you. What lifters have this lube hole you mentioned?
