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Engine pre ignition knock


Oldcarbuff
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What engine? 70s Ford engines were prone to that.

 

What weight oil and what is your oil pressure? Just to make sure it's not a wrist pin or rod knock.

 

If determined it is a detonation problem, make sure engine is not overheating. If it's an EGR engine, make sure the valve functions.

 

What octane fuel are you using? 87 octane in an engine over about 9.5:1 compression can cause it.

 

Run a couple cans of SeaFoam or Chevron Techron fuel system cleaner thru the tanks. Sometimes that will clean up carbon buildup. 

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I agree that checking timing should be the first step.  I recently looked closely at the timing on a 1909 2-cyl Maxwell using the setup shown.   I used a ‘whistler’ to find TDC for each cylinder, and setup a pair of plugs on a piece of angle iron bolted to the engine so that I could see exactly when the spark was occurring for each cylinder.   Then with the working plugs removed, I was able to turn the engine by hand thru the cycles and mark just where each spark was happening relative to TDC (at a fixed position of the spark advance.)  What I learned was that my timer was imbalanced, so to speak:  There was an offset of about 7 degrees of timing between the two cylinders.  I was also able to mark on my spark advance lever exactly where is TDC.

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By the way, it could be carbon buildup as you say.  Carbon retains heat, sometimes enough to ignite the air/fuel mixture prematurely.  I have noticed that many of the very old owner’s manuals recommend once-a-year removal of the head to scrape away the carbon.

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     I have an old  head carbon cleaning outfit which resembles dentist hygienist tools, only heavy duty. Enough different shapes so scraping could be done through the spark plug hole. Probably a must for non-detachable heads.  It all folds up in its metal box. I have used it in the past on non-detachable head Chalmers and others.  If anyone is interested in it get in touch.   JIm43

 

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Do you want to remove carbon from a single cylinder engine, just use a spray bottle with water in it. Running at 2/3 throttle fully warm, just keep spraying the water into the intake until you put two or 3 ounces through it. It’ll be perfectly clean in minutes. 
 

The odds of it being carbon causing a early fire are about zero.

 

10 to 1 says it a lean burn misfire or timing issue.

Edited by edinmass (see edit history)
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Yep, water poured with proper precautions through the carburetor works on many engines, even Chevrolet V-8s. Always a good idea, but never remembered in time, to do the water trick before disassembling an engine. Makes the heads cleaner during disassembly. It also cured the spark knock of my aunt's '65 283 PG Chevelle that was just city driven.😉

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We used the hose with the nozzle at a fine mist. watered the carb for a minute or so until it (the cool engine) stumbled and quit. Only do on a cool engine. Then we went one step farther and let it sit over night. Dont stand behind the car when it is started the next day. I learned this in auto shop from a good mechanics teacher!

Edited by JFranklin (see edit history)
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Not knowing what sort,of engine it is, nor the conditions under which detonation occurs makes it a bit hard to guess what’s causing it. Detonation, spark knock, pinging or dieseling is normally caused by early explosion of the fuel in the compression stroke. The 87 octane gas we get now is a problem, but the removal of Boron, or Lead, has a lot to do with it too. You haven’t said whether the detonation occurs during unloaded engine acceleration, normal acceleration or heavy acceleration, such as up a hill. Usually this is due to overly advanced spark, and if your car has a steering wheel mounted spark advance lever, fixing the problem may be as simple as retarding your spark adjustment there. If you have a car with a vacuum and centrifugal advance, check the condition of your vacuum hoses and assure that the weights in the distributor move freely and the springs are present and in good condition. The primary culprit in vintage engine detonation is high compression and low octane fuel. Try a bit of “instead of lead” or a similar lead substitute additive. My experience dictates that carbon accumulation in the combustion chamber is as likely to cause engine after- run as it is to cause detonation in a otherwise normally operating engine. Last, but not least, a overheated engine which causes the fuel to vaporize in the intake manifold, and has already reached detonation temps prior to getting to the intake valves can cause vapor lock https://www.carparts.com/blog/vapor-lock-symptoms-causes-and-solutions/ and, if coupled with a bad piston pin (double-rapid) knock and engine loading could easily be misinterpreted as detonation or spark knock.

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