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Old 05-17-2021, 09:39 AM   #1
delco1946
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Default engine knocking 292

I'm not finding a ton of info on the matter but my engine makes a noise. I'm assuming its knocking for now. I haven't had my car for a while so I drove it all weekend, at first with clear premium gas during which I didn't recall hearing the noise. I now have ethanol premium and its knocking. the cost difference is almost $2 dollars per gallon and those stations are few and far between.

I just spent thousands on this car for other things, and have other cars also costing me money, so an engine rebuild, expensive fixes, etc is nowhere on the horizon. Besides the gas issue, what are other ways i can minimize knocking.

It sounds like a possible option is switching to a cooler spark pug? Ie instead of autolite 46s use autolite 45s?

My carb needs to be rebuilt as gas is leaking from the carb (too slowly to see but the manifold will be damp when parked). I'm sure it hasn't been done in decades so the gaskets are thrashed, no doubt. Can an overly lean mixture from vacuum leaks in gaskets also causing knocking? I sorta thought it was running rich all these years, but if it was it shouldn't be knocking, right?
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Old 05-17-2021, 10:04 AM   #2
Dobie Gillis
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Default Re: engine knocking 292

https://www.onestopauto.com/What-cau...0in%20altitude.

You're correct, a lean mixture can cause it Refreshing your carb and any vacuum hoses would be an inexpensive 1st step.
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Old 05-17-2021, 11:05 AM   #3
dmsfrr
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Default Re: engine knocking 292

The color of the spark plugs should tell you whether it's running rich or lean, overall.
A quick wipe with a paper towel inside the end of the tailpipe may be similar. Black is too rich, light brown/tan is 'normal'.

Spark knock/pre-ignition/pinging on acceleration can often be reduced or possibly eliminated by retarding the timing a couple degrees at a time until it goes away. Preferably not lower than the factory static timing spec.
Ethanol mixed fuel has a different flash/ignition point that is not the same as 'pure' gasoline.
Backing off the timing a bit can keep them from fighting with each other, creating the pinging.
Using an octane booster may be another possibility?
.

Last edited by dmsfrr; 05-17-2021 at 11:16 AM.
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Old 05-17-2021, 12:00 PM   #4
delco1946
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Default Re: engine knocking 292

Quote:
Originally Posted by dmsfrr View Post
The color of the spark plugs should tell you whether it's running rich or lean, overall.
A quick wipe with a paper towel inside the end of the tailpipe may be similar. Black is too rich, light brown/tan is 'normal'.

Spark knock/pre-ignition/pinging on acceleration can often be reduced or possibly eliminated by retarding the timing a couple degrees at a time until it goes away. Preferably not lower than the factory static timing spec.
Ethanol mixed fuel has a different flash/ignition point that is not the same as 'pure' gasoline.
Backing off the timing a bit can keep them from fighting with each other, creating the pinging.
Using an octane booster may be another possibility?
.
I haven't wiped the inside, but the tailpipe will get black soot on the outside rim, which was partly why i thought it was running rich. The sound is present at idle, and changes with engine speed. Not sure if its there at higher speeds; i think so but the general wind/road noise seems to eclipse the knocking at speeds.

Yesterday with my current gas, the noise was most-minimal when decelerating ( which i think makes sense) - it didnt seem to change with engine load (just rpms) or dropping into neutral.
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Old 05-17-2021, 12:35 PM   #5
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Default Re: engine knocking 292

the sound should not be present at idle if its fuel.
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Old 05-17-2021, 01:41 PM   #6
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Default Re: engine knocking 292

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Automatic or manual trans?
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Old 05-17-2021, 02:05 PM   #7
hotrodart
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Default Re: engine knocking 292

You do not give enough information for diagnosis of any value. When does it "knock"? What engine RPM? Under load? etc.....


Y-blocks are famous for piston slap at low RPM/idle. How many miles on it since a rebuild (if one was done)?
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Old 05-17-2021, 04:13 PM   #8
delco1946
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Default Re: engine knocking 292

It is present at idle and low speeds. As stated previously I can't tell if it's present at higher speeds as the "background" noise becomes too loud relatively. The noise doesn't change at all whether in drive or neutral. It definitely has a consistent tempo that paces with the engine speed.

Its a fordomatic, but I wouldn't think the transmission is involved if the noise continues when in neutral (?). The car is about to hit 100k, but its not the original engine. No idea how many miles are on the engine.
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Old 05-17-2021, 04:44 PM   #9
dmsfrr
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Default Re: engine knocking 292

At the risk of sounding paranoid...

With all those years & miles,
a loose weight ring on the crank damper pulley, or loose rivets on the flex plate???
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Old 05-17-2021, 04:57 PM   #10
Daves55Sedan
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Default Re: engine knocking 292

Consider this: The original '54/55/56 Fordomatic had a flex-plate which had two spring steel mounting tabs riveted to the flex-plate. The flex-plate itself was a heavy, thick (about 1/4") steel plate. Those tabs connect the heavy steel plate to the air-cooled torque converter. Many years of use and abuse can cause the rivets on the flex-plate to loosen up. When that happens, it will make a knocking noise at idle, but it generally goes away when you put it in gear and load the engine.
In 1957 and later engines, a one-piece flex-plate was designed for the water cooled Fordomatics, but you should be able to swap your old flexplate with the newer design. You can order a reproduction of that newer design from some of the repro part suppliers. Another remedy would be to remove your old flexplate and heat up the loose rivets red hot and punch them back down tight somehow.
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Old 05-17-2021, 06:07 PM   #11
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Default Re: engine knocking 292

By the way you described your problem in post #8, the noise you hear is not spark knock/detonation, unless you have your ignition timing set way too advanced. Ethanol in fuel would help reduce spark knock if anything. I guess it's possible if you have a later than '56 carburetor on your '55, it could be over advancing because of too much vacuum to the '55 distributor diaphragm.
I sounds like you may have some other issue like piston slap as mentioned in post #7, or some other noise.

Sal

Last edited by scicala; 05-17-2021 at 06:13 PM.
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Old 05-17-2021, 06:22 PM   #12
Gene F
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Default Re: engine knocking 292

Is this a 2 barrel carb?
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Old 05-17-2021, 09:59 PM   #13
Tom in TN
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Default Re: engine knocking 292

First thing I'd do is get rid of the blended fuel. It makes our old engines (and every non-computerized engine) run lean. I run nothing but ethanol-free in my engines, both cars and lawn equipment.
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Old 05-17-2021, 10:41 PM   #14
55blacktie
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Default Re: engine knocking 292

Depending upon your location, it might be easier to tune your engine than to find a convenient ethanol-free gas station.
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Old 05-18-2021, 04:45 AM   #15
darrell
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Default Re: engine knocking 292

anytime i heard a y block with a knock at idle that sounded like piston slap it was always a spun wrist pin bushing.they will run for years like this.ive seen it many times people always think its piston slap.
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Old 05-18-2021, 06:06 AM   #16
ponymare
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Default Re: engine knocking 292

That knock could be coming from a broken spring, or worn pin in the fuel pump.
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Old 05-18-2021, 09:32 AM   #17
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Default Re: engine knocking 292

Is it a knock, or a ping? Pinging sounds more like a rattle than a knock. Does it run on after shutting it down? I don't think the problem can be adequately diagnosed remotely. It will require hands-on attention.
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Old 05-18-2021, 10:55 AM   #18
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Default Re: engine knocking 292

We need more information. Since it makes the noise at idle, it's not likely ignition 'ping' or spark knock. Is the carb original? Is the distributor original? Has the valve lash been checked (.019" hot)? Try shorting outone cylinder at a time to see if th noise changes.
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Old 05-18-2021, 01:34 PM   #19
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Default Re: engine knocking 292

I asked whether you had an automatic or manual because I had similar symptoms with my 56 with automatic and 292. I was getting a "knock" at idle and at low rpm's. It turned out to be the flex plate. I purchased and installed a new flex plate and the problem was gone.
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Old 05-18-2021, 06:22 PM   #20
Daves55Sedan
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Default Re: engine knocking 292

Quote:
Originally Posted by delco1946 View Post
Its a fordomatic, but I wouldn't think the transmission is involved if the noise continues when in neutral.
Engine running at idle and trans in neutral is PRECISELY when a bad Fordomatic flex-plate will knock.
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