A friend has a Starfire w. 425 4 bbl that has high-ish miles on it getting tired perhaps, has a "rattle" at cruise speed (not accelerating). Do these have a plastic cam gear? loose chain? could have jumped time? maybe a poor gas spark knock (10.5 : 1 comp ratio?). Any speculation as to what may be going on?.

 

I would go with "maybe a poor gas spark knock (10.5 : 1 comp ratio?)". Make sure he is using at a minimum 91-93 octane.

The first thing to check is dwell and timing. If they are off that by itself could cause spark knock.

If those are OK. Let them try to get the highest octane available on a nearly empty tank. Some places sell racing gas. Make sure to add enough so that at least 80% of the fuel is very high octane.

See if it goes away. If that's the case and he can't get better fuel than he is using, then he could back off the timing from 7.5BTDC to 5BTDC with the result in less pinging and slightly less power. Perhaps the knock would go away.

There is also a chance it's his idle speed mixture is set too lean. If everything else doesn't seem to help, let me know and I'll tell you how to adjust the idle mixture.

You don't really hear the timing chain slapping at speed. If it jumped a tooth, it would run terribly and a small rattle at speed would be the least of his problems.

 

Yes good to try that stuff first, though the fact that it is happening at steady cruise makes me wonder.
I'll check on the gas he's using.
Curious: do 425s have a plastic or fiber timing gear? If so when do they typically wear to point of needing replaced?

 

The idle screws only control the mixture at idle.
The mixture at cruise is controlled by the jets.
The mixture may indeed be lean at cruise if ethanol laced fuel is being used.
It has been recommended that the carb be re-jetted one or two steps larger when using ethanol gas.

 

They are made of metal with plastic coated teeth (maybe 1/8" or less thick). As they age the plastic breaks off and falls into the pan, clogging the oil pickup. That can cause oil starvation and low oil pressure. As far as the effect of the plastic no longer being on the teeth. There could be a rattle at idle, but the biggest thing is that the valve timing and ignition timing become irregular as the cam position is no longer so tightly tied to the crank position. Which obviously reduces power. It also probably makes it more likely to jump a tooth, although I never saw that.

That said, they should be replaced, but on an old tired motor with nothing done to it, I would wait until at least a partial rebuild to replace it. It's not worth fixing if you are not rebuilding the original heads, and replacing, camshaft, lifters and the rest of the valve train. Especially if you don't have the money to invest.

 

Nylon toothed aluminum timing gears replaced iron gears in 1964.
Any nylon toothed gear that is over 50 years old should be replaced.

 

Yes plastic teeth. If the gear is original I'd change it now. Teeth will break off and plug the oil pump intake or worse yet a valve and piston get introduced to one another on a HC motor if it jumps. Pinging due to low fuel quality will damage the pistons.

Good luck!!!

 

Define "rattle". A lot of things rattle -- the first thing that comes to mind is a PCV.

Can you post audio?

 

Check your dwell and timing to include the vacuum advance. There would also be engine surging if it were timing related.

 

Heat risers can also rattle.

 

^x2^...do they ever!

 

It's 600 mi away so I'm not able to hear it first hand.

 

Is your friend capable of making a recording?

 

No the car is put away in storage now. He asked for my opinion after putting it away.
I'm suspecting bad/lo-octane gas.

 

It’s easy enough to check timing chain wear with little tear down work.

Remove the distributor cap. Slowly rotate the engine in a clockwise direction until the timing marks read zero. If you go past zero, rotate the engine in a clockwise direction until they line up.

Once they are lined up, slowly rotate the engine counterclockwise until you see the rotor move, then stop. Read the timing marks, whatever the marks read is the amount of slop in the chain. If it’s more than 5-10 degrees (I don’t recall the actual spec) I’d replace it.

 

I don't know how you can make that diagnosis on a car you have no access to that is 600 miles away.

I don't think any of us can be much help to your friend without at least getting a better description of the sound, e.g. does it sound like a coffee can full of marbles? A spray paint rattle can? A baby's rattle? A rattlesnake?

See if he can narrow it down a little for us.

 

Coffee can full of #2 split line sinkers.

 

That'll melt a piston crown right there.

 

Thought lead was good for the earlier engines...

 

A rattling heat riser does have a unique sound, though.

 

If you have never heard one, well, that’s a good thing!

If you have, you know an automobile does not make a sound that will freeze you mid step, accelerate your heart rate, and make you immediately evaluate your choices in life.

 

Heard my fair share of them (rattlers)…drop dead stop.

 

Winter fuel?

 

Apparently, you've never driven a Viper.

 

Another possible culprit would be old fuel (as in at least one year) .
When a car sits for a long time and is used rarely the fuel in the tank deteriorates.
The lighter molecules in the gasoline evaporate first. Lowering the octane.
Eventually, it deteriorates into something resembling kerosine.

The use of fuel stabilizers helps slow this process.