I thought I would post my recent experience with spark plugs in my 1943 M20.
Since purchasing the moto a few years ago, the engine has experienced pinging and knocking on occasion, although more often and more severe since replacing the piston and rings. The valves were also replaced and lapped in, per M20 specs. Often, the moto would stop completely when riding, and easily restart after a minute or two of rest. I am using 92 octane gas commonly available in the US. After a lot of reading I came to the conclusion that the spark plug I was using was too hot, causing pre-ignition. Also, research by motor oil manufacturers has shown that motor oil properties might contribute to pre-ignition (Refs 1-3 below). SN+ is the manufacturer's notation for their anti-pre-ignition formulation. Look for the round label on the oil container that lists this specification.
I purchased a series of NGK B*HS) spark plugs to see the effects of spark plug heat ratings, after changing the motor oil to a SN+ 10-40 formulation sold by O'Reilly auto parts stores in the US. NGK sells a range of plug heat ratings, starting at B5HS (hottest) to B9HS (coldest) with B6HS as their standard (Ref. 4). I was using their standard plug #6 heat rating, and experiencing pre-ignition ("pinking") and knocking. For now I have settled in on #8, which stopped the pre-ignition. Hotter plug ratings all resulted in pre-ignition. The manufacturer recommends choosing a heat rating that provides the performance you need, while also operating with a high enough electrode temperature that provides self-cleaning. Too cold will soot up with black deposits and adversely affect long term plug reliability (Ref. 5).
The moto runs now without pre-ignition, and when the weather warms up in WA state, I expect road testing will confirm this.
My belief is that modern gasoline formulations are far from what the M20 was designed for, and is the root cause of the pre-ignition in my vehicle, as premium gasoline likely ignites easier and burns hotter than internal combustion vehicle fuels in the 1940's.
Ride Safe, and ATGATT = All The Gear All The Time
Dave W
References:
(1) https://www.pennzoil.com/en_us/education/know-your-oil/what-you-need-to-know-about-api-sn-plus.html
(2) https://www.castrol.com/en_us/united-states/home/motor-oil-and-fluids/new-api-sn-plus-specifications.html
(3) https://mobiloil.com/en/article/why-the-mobil-advantage/mobil-1-modern-engines/lspi-protection
(4) https://www.briskusa.com/spark_plug_cross_reference_heat_range_chart
(5) https://www.enginebuildermag.com/2011/05/understanding-spark-plug-heat-range/
email (option): dwdiak4@gmail.com
Interesting stuff...I'm planning to experiment with plugs and ignition settings at some point this year...Ian
email (option): ian@wright52.plus.com
I always do 14 mm instead the 11mm pre ignition because the modern fuel burns slower than the fuel that was used at wartime.
I using a champion L10 sparkplug.
Have good results with this combiation.
L10's are ok if you can get them, they were replaced with L86c.I managed to find the original spec KLG which is working fine!
email (option): Richiewhite765@gmail.com
quote.
My belief is that modern gasoline formulations are far from what the M20 was designed for, and is the root cause of the pre-ignition in my vehicle, as premium gasoline likely ignites easier and burns hotter than internal combustion vehicle fuels in the 1940's.
You have to remember that octane ratings are counter-intuitive, the higher the octane the less volatile it is. High performance engines or some modern car engines require a higher octane to prevent pre-ignition. In high performance engines it's most probably the high compression causing pre-ignition, in modern cars it's the tighter production tolerances required for emission control.
Your WDM20 should run fine on standard unleaded, You state that your engine would stop while riding which sounds like the engine is over heating and partially seizing. Maybe look at your ignition timing, it could be too retarded or the carb mixture at that throttle opening is too weak.
Hope this helps.
I managed to purchase a couple of NOS KLG L777 plugs and running one at the minute without any problems at all.
email (option): deepsea@vol.net
After the change to ceramic plugs ('Corundite' at the time), Nortons ran a cold KLG M80 and BSA's an F70 so there probably is scope to run a cooler plug without fouling problems. Oddly, Green Spark plug list an NGK 6 against the KLG70 and an 8 for the KLG80...Thinking back to my 2-stroke days, NGK 6s were a seriously hot plug, guaranteed to hole pistons if thrashed.
I believe that mica plugs were prone to lead fouling but that is no longer a problem so perhaps the L777 is a good choice again. It certainly looks the part.
Matty
quote: "You have to remember that octane ratings are counter-intuitive, the higher the octane the less volatile it is. High performance engines or some modern car engines require a higher octane to prevent pre-ignition. In high performance engines it's most probably the high compression causing pre-ignition, in modern cars it's the tighter production tolerances required for emission control."
For pre-ignition, a dominant factor in Spark Ignition (SI) gasoline engines is the ignitability of the gasoline vapor/droplets in the combustion chamber that results in an inverse relation between Octane and ignitability, that is, the easier to ignite (higher ignition temperature) the lower octane rating (Refs 1 and 2). I did not refer to fuel volatility in my original statement above, rather, fuel ignitability. The research cited states that pre-ignition characteristics are affected by hot projections in the vaporized fuel, such as hot fixed carbon particles, spark plug electrode temperature, hot valve edges, etc., as well as fuel ignitabiity, fuel injection timing, fuel air mixing and turbulence, valve settings, ignition timing and fuel composition (Ref. 2). Fuel vapor pressure and / or volatility may play a role in pre-ignition. In my case, the ignition timing is per M20 spec, substantiated by timing light inspection. Fuel / air mixture is per a brand new Amal 276C/1B carb, set up per the Amal instructions. The spark plug heat rating (electrode design) was the dominant factor in eliminating my pre-ignition issue, because the NGK B6HS standard plug and B7HS were providing hot ignition sources to the fuel. No other factors were varied in my solution.
Recent research by oil manufacturers has shown evidence that oil formulation can also be a factor in pre-ignition, presumably by oil droplet ignition, even at low engine speeds.
Regards
Dave W.
References:
(1) Characteristics and Control of Low Temperaature Combustion Engines, RK Maurya, Springer Press ISBN 978-3-319-68507-6, section 3.1 Auto ignition Characteristics, Section 3.1.2 Impact of Fuel Molecular Structure [gasoline formulation and Octane Number]
(2) Comprehensive Handbook of Mechanical Engineering, J. Sirinivas, Laxmi Pub LTD, 1st Ed., 2004, pg.37
.
email (option): dwdiak4@gmail.com
Error in prior message
"(higher ignition temperature)"
Should be
"(lower ignition temperature)"
Apologies for any misunderstanding.
Regards
Dave W
email (option): dwdiak4@gmail.com
B8's will work well but only if you run the carb dry to stop the bike if the engine has not been run for about 1/2 hour.
Same when you put the bike to bed, run the carb dry and the final 30 seconds of lean burn will clean the plug.
On the days when we will be riding 300 to 500 kms I run the BP8HS.
For riding down to the shop, short club runs and the like then it is BP6 or BP7.
Standard B plugs tend to foul really easy and there is plenty of space to run the projecting electrode which is the P in BP7HS
The biggie on side valves is opening the inlet too early as the fresh charge will run across the exhaust valve .
The edge of the exhaust valve is between red to yellow hot and has no problems igniting the fresh charge.
It is a big problem with racing lawnmowers
We delay the inlet till the piston is around 10 deg past TDC and the exhaust valve has closed
So it is late & high for the inlet grind
email (option): bsansw1@tpg.com.au