pinging / knockin

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boeing7873
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pinging / knockin

Post by boeing7873 » Mon May 15, 2017 4:45 pm

Can please someone try to describe pinging / knocking? I can hear like popcorn exploding inside the engine under load, is that it? Or is it more metallic? Is it similar to valves clicking? Thanks!
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63rover
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Re: pinging / knockin

Post by 63rover » Mon May 15, 2017 8:20 pm

The British call it "pinking" which I think imitates the sound better. Pre-ignition causes the intake valve to bounce closed which causes the noise. I would describe the noise as metallic and similar to rapid hammer taps on steel. Post-ignition or knocking is ignition after TDC. The knocking is the sound of the explosive combustion after the exhaust valve has started to be forced open. As the cam and push rod are acting to open the valve (push it up-hill, if you will) the resistance is different from the opposite condition of closing (going down-hill) which is why the different sounds and names.

In my experience when an engine is under load and throttle is quickly applied and pinking starts it can be stopped by backing off the gas and controlled by gentler acceleration. Knocking is experienced when advance fails and a well idling engine continues to perform well at low speeds but bogs down as throttle is applied. The worst case I experienced was with a V12 Jag that couldn't "pull the skin off a rice pudding". (my British father's favourite insult)

Hope this helps.

Cheers, Clive

PS Avoiding pinking is the main reason for newer and "high performance" cars requiring high test gas. Careful listening with different grades of gas can save losts of money over the life of a car. My 2003 Land Rover runs fine on regular even when loaded and towing. In contrast my 71 BMW R60 motorcycle only likes Shell high-test.

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rmel
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Re: pinging / knockin

Post by rmel » Mon May 15, 2017 9:15 pm

Pre-ignition case, possibly too much advance (at RPM) or running too lean.
I'd be more suspicious of the advance, and more worried about running lean :?
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boeing7873
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Re: pinging / knockin

Post by boeing7873 » Tue May 16, 2017 1:42 pm

What would make the engine run lean with everything stock? (I assume the carbs are stock).
Thanks for the answer, Clive.
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Andre
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Re: pinging / knockin

Post by Andre » Tue May 16, 2017 1:51 pm

Vacuum leaks.
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Haf-e
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Re: pinging / knockin

Post by Haf-e » Tue May 16, 2017 2:33 pm

Check the connection of the carb body and the throttle plate - you'll need to remove the air intake plenum to do this. The screws come loose frequently here and it causes air leaks around the carb part - resulting in a lean mixture. Do it now as it can result in serious damage to an engine rather quickly.
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boeing7873
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Re: pinging / knockin

Post by boeing7873 » Tue May 16, 2017 4:14 pm

Done it. No leaks. I had a year ago discovered the hose between the plenums had melted... fixed that and sprayed starter fluid everywhere. No leaks.
I don't think I am running lean.
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Jimm391730
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Re: pinging / knockin

Post by Jimm391730 » Tue May 16, 2017 4:45 pm

I don't think I am running lean.
Don't think, KNOW. Make, buy, or borrow a fuel level gauge and check that first. As Haf-e said, lean can be very damaging to the engine.

What main jets are you running? What altitude do you normally run at?

Another possibility for pinging is high (or at least higher than normal) engine temps. Do you get pinging right away after starting (when the engine is cold/cool), or after a few minutes when the cylinders have had time to heat up? Have you checked that the cylinder fins are not plugged with mud or debris and limiting cooling?

Lastly, a very dirty combustion chamber can hold hot "embers" that can cause pre-ignition. A hard drive can sometimes clean these out, but if running lean a hard drive can be disastrous. Prove that it is not lean before a hard, hot run.
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63rover
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Re: pinging / knockin

Post by 63rover » Tue May 16, 2017 5:26 pm

Running lean causes extra heat build-up.

On an older system like this there should be a little carbon build-up on the exit of the tailpipe. Check by wiping it with your finger. No black -lean, Dirty finger-good, black finger -too rich. This is a good test for an engine that is running reasonably.

The level of the floats can certainly effect mixture and are the easiest to check. This may be a good place to start. Also partially plugged jets can lean out mixture. (a good 2nd step) Jets can be cleaned and seldom need replacing.

Remember this engine/truck was designed for simple field maintenance, (Motorcycle guys, especially the old ones, are still used to carbs and happy to help.)

Good luck.

Cheers, Clive

63rover
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Re: pinging / knockin

Post by 63rover » Fri May 19, 2017 9:11 am

Just another thought, if you are running lean the engine should settle down with a little choke which is a mixture enricher. It doesn't fix the problem but it does let you know if you are on the right track.

Cheers, Clive

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