Bent pushrods?

[Farmstore34]

Hi all,
Quick question- if the poor idle on my 72 350 is because of bent pushrods, would I be able to hear them ticking (from excessive rocker clearance)? Short of tearing the motor down I cannot find anything wrong, I have a rough, poor idle but it runs fine above 1800rpm. A couple of people have suggested bent pushrods but I don't hear any unusual noises from the engine. Thanks!

[El Jay]

I dunno for certain.
But I've bent push rods/spit out lifters in my flat bottom and it didn't run worth a crap at any engine speed.
That's just my .02.

[special-K]

The only way I know of pushrods bending is from a timing chain jumping.When they get bent it`s a clackity-clack sound,very loud.An badly stretched timing chain will make a ticking sound from slapping the timing cover,mostly at idle and when letting off the throttle.This will cause a bit of a timing lag and make a mostor run rougher.That`s what I suspect you may have.If you don`t fix it you will end up with bent pushrods and maybe valve stems when the chain hops a tooth.Depends on how much load is on the motor when it happens.

[LONGHAIR]

I would think more like a flat lobe on the cam than a bent pushrod. Do a compression test.

[old Chevy guy]

Bent push rod or worn lobe the lifters would tick or clatter.A worn lobe would cause a skip or popping out of the carb depending on intake or exaust at any rpm.Check and see if the vacum advance is working? I had tha motor in my truck and one in my van I used to drive wore the top off the pushrod till it was too short then it started ticking and popping like the cam was bad.I quess the rod must have plugged up with crap and didnt get any oil up to the rocker arm.

[LONGHAIR]

A flat cam lobe will not necessarily make any noise, other than the miss...and it will seem to smooth out some as the RPM goes up.
Having the lift area warn off just makes it so that the valve never/barely opens. The lifter still rides the same base circle and the pushrod can stay tight enough to remain quiet.

But it could be several things.
Bad plug or wire
Cracked distributor cap
Vacuum leak
Vacuum advance stuck
Carb float too high

[bigsnookmaster]

Might be lifters that are bleeding down at low rpms,then pumping up as it revs.Good luck

[special-K]

I got the impression it is ticking,which is what I based my response on.If you`re not hearing anything then I`m with Longhair on all those possibilities.Cams are known to wipe ahead of the bottom end.If you check the timing and it`s hard to pin down a stable setting I`d look into the valvetrain/timing set.First,I`d look in the carb/tune area.

[Slammed70]

Originally Posted by bigsnookmaster

Might be lifters that are bleeding down at low rpms,then pumping up as it revs.Good luck

Thats what I was thinking, lay some towels around and yank a valve cover while its running and check to see (if)how much oil is sqirting up though the pushrods to the rockers.

[89 crew]

Without knowing the condition of your engine, I would start looking at electrical problems. I would be guessing wires or something simple like that. If it is a cam, it comes on slow and continues to get worse. if it is a bent push rod you WILL know when you did it. Both will make noise. One odd engine problem I ran across was on a g van 350. Every so often it would break a rocker and the push rod would slide about 1/8" through the hole where it normally sits. It was a notched stud that caused the problem. It didn't make a lot of noise this way but it did have performance problems.

[68GMCCustom]

carburetor.... or dist. vac. adv. problems?

[Farmstore34]

Wow thanks for all the replies!
Here is what I have. 1972 Chevy 350, older crate motor with about 36,000 miles on it, mild street cam, edelbrock dual plane intake and 650 cfm carb. It has a new MSD 6AL box, MSD distributor, coil and plug wires. It has always run great until my son and some buddies took it out to the lake for an afternoon. I beleive one of his friends drove it and had the gas pedal pegged to the floor for quite awhile. when they came home it was idling poorly and wanting to die. It runs fine under acceleration and above about 1800 rpm, but as soon as I let off the gas it starts missing and wants to die. I have checked for a vacuum leak, swapped out the carb with one that was on a running motor, changed the plugs and checked the ignition system over. I found nothing wrong. I haven't done a compression test yet but I believe there is an internal problem like a bad cam or pushrods. I'll let you know what I find. Thanks!
Colby.

[89 crew]

Where kids ran it hard and broke it I would start with a compression test. There is no way a cam, or timing chain goes from good to bad in a day with 36k. I would guess any of these could do it for you. blown head gasket, cracked head, broken valve springs push rod through a rocker. was there any other damage to the truck when you got it back? any oil in antifreeze? or vice versa? body damage? broken springs? For my final question, did you beat the little punk that took your truck out and tore the hell out of it?

[Farmstore34]

Boy I wanted to! I didn't get the full story from the boys until a couple days later, after changing the fuel filter and plugs. Nothing else was broken on the truck ( hooray for American steel!) and I dont see any oil in the water or visa-versa as of yet. Compression test is next.

[Farmstore34]

Well I did a compression test, they are all between 120-135 psi. I pulled a valve cover and all the pushrods/rockers are moving properly. I have a strong suspicion that the engine sucked down a chunk of the intake manifold gasket, creating a bad vacuum leak. I will pull the intake tomorrow.
Colby.

[Farmstore34]

Well I pulled the intake off, but the gaskets look fine. However, I did discover a bunch of rust and crud on the reluctor and etc inside the distributor. Think this may be the problem?