Author Topic: Reading The Plug  (Read 1968 times)

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hpwaco

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on: June 24, 2018, 04:12:20 pm
Classic method:
    Warm up engine
    Ride short distance at full throttle (don't get caught)
    Pull in clutch lever
    Kill engine
    Coast to stop
    Remove plug (without burning fingers)


Bert Remington

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Reply #1 on: June 24, 2018, 04:36:59 pm
Due to the risk of thread damage to hot aluminum heads, I concur for race engines like I have in my Mustang but don't recommend for street engines, especially those with iffy production quality.
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Bmadd34

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Reply #2 on: June 24, 2018, 05:01:28 pm
Due to the risk of thread damage to hot aluminum heads, I concur for race engines like I have in my Mustang but don't recommend for street engines, especially those with iffy production quality.
Agreed. When I did my plug chop, I coasted to round about my destination and let it cool a bit first. Not 100% accurate, but at least 75%.
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Rattlebattle

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Reply #3 on: June 25, 2018, 10:53:22 am
That used to be the method for checking the main jet size in a carb. Not really necessary in an EFI engine really. Anyway, with all the crap in modern fuels it’s hard to assess unless extreme at either end.
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Richard230

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Reply #4 on: June 25, 2018, 01:56:21 pm
That used to be the method for checking the main jet size in a carb. Not really necessary in an EFI engine really. Anyway, with all the crap in modern fuels it’s hard to assess unless extreme at either end.

Plus, with a modern Euro 4 FI engine, what are you going to do about a plug that doesn't look perfect, other than replace it with a new one?   ::)
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hpwaco

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Reply #5 on: June 25, 2018, 03:03:20 pm
Change the heat range.

Don't use a resistor plug (bpR) with a resistor cap.


Rattlebattle

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Reply #6 on: June 26, 2018, 11:59:52 am
The suggested method is fine for WOT but doesn’t cater for partial throttle conditions. Changing the plug grade to adjust for a fuelling fault isn’t as good a solution as addressing the fuelling. Whilst this is possible with PCV, Dobeck etc not a lot can be done with the standard setup (unlike pre l/c Triumphs etc on which the fuelling map can be adjusted throughout the range using free software). Ideally the fuelling should be spot-on before adjusting spark plug heat range IMHO. Ultimately though, these days it is by no means as simple as it used to be reading a plug. Fuel containing 10% ethanol will differ from that with none in it in terms of plug colour.
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