Symtoms Of A Bad Cam?

[Old School]

What are the symptoms of a bad camshaft? Here is what the car is doing. The engine just doesn't sound right,
almost like a miss. If you raise the idle to say 2000 to 2500 the thing just shakes like mad. It seems to have lost some
power as well. Yes there is some lifter noise, I have heard much worse. Plugs all look good, compression seems to be
alright for the age of the motor I think (120). All of the electronics are new. I haven't pulled the valve covers yet.
Not exactly sure what to look for anyway. It just sounds wrong. It does take a few extra turns for the car top start.
Any help would be greatly appreciated.

Thanks

[firefighter3931]

It could be the cam or timing chain going away but it could also be ignition related. What does the timing curve look like Brian ? How much initial timing ? Total timing ?

The 120psi is low....this obviously isn't one of your Indy motors.



Ron

[elacruze]

'Telephone Diagnosis' is a big part of my job, and I love to play. Kinda like gambling but without the downside so keep in mind this an entirely selfish response.  

A bad intake lobe will act as you describe, but it's not the only possibility.
A bad exhaust lobe will cause a backfire in the intake manifold.

I'm by trade a Diesel tech. One of the most common failures is fuel injectors. When they break, they overfuel and overheat the cylinder. To find which one it is, we'll start the engine cold and immediately administer the 'hand test'-feel the exhaust manifold for which cylinder gets hot first.
If you have headers you can do the hand test (gotta be fast on a gas engine) Looking for the cold cyl. Maybe run it for 10 seconds then shut off. Or you can spray them with plain water and see if one cylinder is dead or weak. Do this before you run the engine and get it all warm.
Once you identify the down cylinder, you can check spark to each plug with a timing light. If the plug's fouled, it will show as good on the light but not light the fire inside.
After that, you're inside the valve cover. If yours are easy to get off, I'd probably start there anyway while it's cold. Finding anything visual also avoids any additional run time if there's valvetrain damage.

Here's my guesses;
Ignition
Rocker arm stud pulled out (GM product)
Intake lobe/lifter collapsed

[Old School]

The base timing I think is about 8 degrees total timing no idea. I put the cam and new timing chain
in about 4,000 miles ago. Otherwise its pretty much just a stock motor. Once this started I replaced
all the ignition components and I mean all and nothing changed. I changed to three different carbs
no change. Its a 74 400.

[LeadfootBob]

Another good tip, elaborating on elacruze's technique: use a laser temp gauge. The neighbours '70 440 R/T was running weak and missed heavily on the top end, zapped the header tubes one by one as it was heating up (this gauge will only handle about 500 degrees celcius, might work on a hot motor with a better instrument) and found that the last two cyls (no. 7 and were about 100 degrees colder than average... A new intake gasket solved the problem!

On my '74 monte with a tweaked 396 the only signs of the cam going bad was a hesitation at cruise, otherwise it ran great... 10 lobes were going, 2 were so bad we had to turn the engine upside down to get the lifters out 

[Blown70]

It could be the cam or timing chain going away but it could also be ignition related. What does the timing curve look like Brian ? How much initial timing ? Total timing ?

The 120psi is low....this obviously isn't one of your Indy motors.



Ron

I think Ron you maybe thinking of oldschool this guys name is Old School, me thinks maybe two different people?

[firefighter3931]

It could be the cam or timing chain going away but it could also be ignition related. What does the timing curve look like Brian ? How much initial timing ? Total timing ?

The 120psi is low....this obviously isn't one of your Indy motors.



Ron

I think Ron you maybe thinking of oldschool this guys name is Old School, me thinks maybe two different people?

Yep....you're right Tom. 

As for the original post my suggestions still stand. Base timing needs to be increased to at least 14-16* at idle. 



Ron

[green69rt]

The base timing I think is about 8 degrees total timing no idea. I put the cam and new timing chain
in about 4,000 miles ago. Otherwise its pretty much just a stock motor. Once this started I replaced
all the ignition components and I mean all and nothing changed. I changed to three different carbs
no change. Its a 74 400.

When I start trouble shooting I always look at the last thing I worked on.  You said cam and timing chain were installed 400 miles ago, think about this.  Cam break-in can be a bugger, this is the prime time to wipe a lobe.  ALso the chain may be lose.  No solution to these other than to look and measure. Good luck.