Engine Knock Diagnostic Help Needed

smurph

smurph

Senior Member
I need some help from the FordBarn experts.

My 1929 “A” has developed an engine knock.

At idle I can hear a steady tapping of the valves which sound normal.

But, there is also a steady knocking sound at idle, which stops as revs increase to 800-900rpm, and then resumes again as revs increase to 1500ish.
I would estimate the frequency of the knock at idle at 3X per second, and the
frequency increases at higher revs.

At idle, if I short out #2 spark plug the knocking sound gets louder. This does not happen when #’s 1,3, and 4 are shorted out.

I’ve compression tested all cylinders and they all read the same at about 60-62 lbs.

I think the electricals are working OK. I was having carburetor problems which I’m not sure are fixed yet, but I think that is separate from the knock.

So, I would appreciate hearing your thoughts on what the underlying problem may be.

Thanks in advance for your inputs.
 
smurph said:
I need some help from the FordBarn experts.

My 1929 “A” has developed an engine knock.

At idle I can hear a steady tapping of the valves which sound normal.

But, there is also a steady knocking sound at idle, which stops as revs increase to 800-900rpm, and then resumes again as revs increase to 1500ish.
I would estimate the frequency of the knock at idle at 3X per second, and the
frequency increases at higher revs.

At idle, if I short out #2 spark plug the knocking sound gets louder. This does not happen when #’s 1,3, and 4 are shorted out.

I’ve compression tested all cylinders and they all read the same at about 60-62 lbs.

I think the electricals are working OK. I was having carburetor problems which I’m not sure are fixed yet, but I think that is separate from the knock.

So, I would appreciate hearing your thoughts on what the underlying problem may be.

Thanks in advance for your inputs.
Click to expand...


If it sounds like it is coming from the lower end of the motor, you may have to drop the oil pan and inspect. I recently had a motor rebuilt due to a motor knock. In my case the motor had a cracked Rod Cap form the bolts holding the cap on were over torqued.. I am sure there are a multitude of other things that can cause a knock, this was wha happened to me.
 
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You're going to get a ton of wild guesses, maybe a few educated guesses. None really worth much I'm afraid. I'd just pull the oil pan and check it out. If nothing else, it gives you a chance to check bearing cleaarances and clean out any sludge. Could try to take out the timing pin, then insert something in the hole to apply pressure to the cam gear, see if it makes a difference.
 
You should get a mechanics stethoscope ( I like to replace the metal end with a rubber hose), and try to isolate where it’s coming from. Do you have any clubs nearby or know someone that can help you diagnose before disassembly. It would be a bummer to tear a bunch of things apart to realize it was not necessary to go that far.
 
Most likely not the issue, take the fan belt off and see if the knock changes. Also if you do not have a stethoscope, use a large long screwdriver, handle against your ear as a stethoscope, old mechanics trick. I've done this to find bad distributor bushings, and a bad generator bearing.
 
Vaguely familiar to the knock I developed driving the Model A on the Mid-Cape Highway at 55.

Came home and dropped the pan to find one of the connecting rod nuts had dropped off. The connecting rod cap was held by one side.

Joe K
 
Joe K said:
Vaguely familiar to the knock I developed driving the Model A on the Mid-Cape Highway at 55.

Came home and dropped the pan to find one of the connecting rod nuts had dropped off. The connecting rod cap was held by one side.

Joe K
Click to expand...

That is why I use locktite on every nut everywhere along with lock washers or cotter pins.
 
J Franklin said:
That is why I use locktite on every nut everywhere along with lock washers or cotter pins.
Click to expand...
These were cotter pinned - but admittedly this motor was so worn if you carefully set the nuts "tight", the shaved bearing caps would "bind" on the egg shaped rods. So one backs off a flat and inserts the cotter.

Thus the vibration of the engine was transferred to the cotter and eventually wore through it. In the bottom of the engine pan besides the nut I found the THREE pieces of cotter pin.

BUT - the end of the story is pleasant as I finally had motivation to replace that engine with a newly rebuilt.

This one uses nylock nuts which the rebuilder recommended over cotter pins and their possibility of wear. (They do move with the engine he explained - which technically makes sense.)

Joe K
 
You have done the right thing by trying to isolate the noise. I'd recommend a little more like has been said by trying to isolate it a bit more with a stethoscope.

It does seem like there is a problem with #2, but, a little more looking might be worth while before dropping the pan or side cover.
 
Have not experienced it, but have read that if the screw that holds the distributor in the block is over torqued/tightened a knock can occur.
 
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