'73 340 Cuda Predetonation Problem

[193rdsow]

I have a 340 4 speed Cuda with a Predetonation issue. If I set the timing at 5 degrees BTDC, whenever I hit the throttle hard it brings like crazy. At 10 degrees ATDC it's not so bad. The engine is a low compression '73 motor that's got a cam slightly more aggressive than stock. Is there anything else that can be causing the Predetonation other than the lousy 93 octane gas? Should I run cooler plugs? Anything else I should try besides Turbo blue and retarding the timing?

 

[70rr-brian]

Inspect the plugs 1st. They may be showing some signs of distress. I doubt 93 octane is your problem with that compression ratio. 8.5-1 if memory serves. Run some seafoam through it to maybe cleanup any carbon embers that are left burning in the cylinder walls. Been a while since I used that...but if memory serves after that you could change your plugs.

 

[Kern Dog]

I think that the term you are describing is called PRE-Ignition or simply detonation.
In every case I have seen so far, even a STOCK engine requires the intial spark advance to be BEFORE TDC to run right. The bigger cam will want even MORE advance than that to idle decently. Running the timing AFTER TDC will make the engine run much hotter and risk burning exhaust valves.
Maybe the harmonic balancer has slipped and is giving you an incorrect reading as to actual TDC. It has happened before and when it does, you can't be sure of WHERE the TDC is.
As stated, the 72-73 340 engines went to 8.5 compression so it shouldn't detonate unless the timing is way off or unless the carburetor is set too lean. Try setting the timing to where it does NOT ping withOUT the vacuum advance connected. If you find that it runs fine on pump premium with what looks like a strange timing setting, you may need to either replace the balancer or determine the actual TDC using a "piston stop" and marking a new TDC groove on the balancer.
Chances are though, that once a balancer starts to slip, it may KEEP slipping. You may just end up chasing your tail. The cast crank 340 balancers are NOT easy to find though.

 

[193rdsow]

Never thought about the possibility of a slipping balancer. I checked the plugs. They are black. I got sea foam and will use it tomorrow. Will also put in a new set of cooler plugs. Is there any way to check if a balancer is slipping through a visual inspection?

 

[Kern Dog]

There are probably a few ways. You could compare the distance of the TDC groove to the notch in the center hole.(Where the Woodruff key is in the crank.) Take that number and compare it to a few other balancers. You could make a piston stop out of an old spark plug. Finding TDC using a piston stop is the most accurate without pulling the head.
A useful tip: Check YouTube on "How to find top dead center". There may be several videos to filter through, but I have found that there is a bunch of info out there waiting to be discovered. Last winter my father in law asked for me to replace power window regulators in a Ford Expedition. I checked YouTube and found several videos that helped me know what to do. It isn't always going to have exactly what you need, but it is worth a try.

 

[rumblefish360]

You really answered your own question by presenting the facts of what happens on where the timing is at. Bump the initial up to 16*'s and limit the total to 32, then hook up the vacuum line. Recheck total is at 52 or less degrees.

 

[193rdsow]

Ok guys,

i just did some timing and here are some numbers:


4000 RPM = 36 degrees ( with vacuum advance)
@idle: 18 degrees and 800 RPM

shutdown and restarted after a few minutes:

idle timing was 25 degrees. Disconnected vacuum line and plugged- still 25 degrees.

The other odd thing was that the timing mark was a little jumpy.

A buddy of mine thinks the distributor needs replaced. The distributor is a Mopar distributor new with probably less than 1500 miles on it since he engine was rebuilt. Could a faulty OSAC valve be causing some of my issues? I'm skeptical of a bad harmonic balancer. A friend advised they normally develop a wobble when going bad and this one runs true. Any thoughts?

 

[70rr-brian]

I know you probably won't want to check this....but I am wondering if your timing chain is giving up the ghost. You could pop the distributor cap off, turn the motor by hand in the correct direction first until the rotor starts to move. Then turn it in the reverse direction it would normally turn while watching for movement of the rotor. See how far you can turn it backwards before the rotor moves. If its good and tight it should immediately move when you reverse the normal rotation of the motor. If you have to turn it back a ways before you get movement then your chain is probably stretched out too much.

I'm not convinced that is the problem by any means....but I would do that test just to see how much slack is in that chain.

 

[rumblefish360]

The OSAC valve is designed to delay the distributor timing IIRC by 15 seconds.

Second- the distributor should not have had the vacuum advance hose hooked up but instead plugged.

Third- total advance should be 32*'s without the line hooked up, a max of 52*'s with the vacuum advance hooked up.