Plug chop, opinions please...

I did a proper plug chop on the 920.

This is with Shell V Power unleaded and Silkolene Pro Boost octane booster (it was on a track day).

I’m think it looks good, but perhaps a touch on the lean side of ‘safe’ for a carburettor engine with no engine management system.

Thoughts please...

I would say the mixture looks good but the plug is a bit too high of a heat range.

It is good practice to use a plug which is one range higher than normal when setting the main jets. It keeps you safe from jetting too lean. However in both of those photos what needs to be seen cannot be seen. You need to shine a light and use a magnifying glass to look at the porcelain, right down inside the plug where it meets the metal. There should be about a 2mm black ring there which remains when the soot burns off. From the look of the rest of the plug, the mixture is probably too rich everywhere else. I always run with main jets which are slightly too rich, but getting the needle jets and needles right is critical. If they are even the slightest bit too rich, your motor would be noticeably sluggish. You only usually start burning pistons and valves when you are using the main jets, which on most circuits is not that often.

There are two ways of setting the needles and needle jets. You can raise the needles until the motor 8-strokes, then lower them one notch. Or you can lower them until the motor coughs as you ride the bike, then raise them one notch. I use the latter - it makes a difference. The shape of the taper on the needles can be important for good acceleration as you come up through the gears. Main jets are usually not a problem. By he time you really start using them, you are probably very much towards the ends of the straights. Doing plug chops on most race circuits is not easy. I have only ever done it on long public roads while keeping an eye out for the cops.

Hard to tell because modern unleaded burns so clean and best indication of heat range is erosion of sharpness of edges of center electrode and the point of using iridium electrode is that despite its sharp small diameter is that it won’t melt. Ground strap does appear to show some erosion though but a cooler plug can’t do much to help that.

A proper plug light/loop that allows you to see the mixture ring at the base of the insulator is useful

Is the Pro Boost mainly methanol? If so, then it bears restating what you already know - it leans mixture while it suppresses detonation by cooling the charge. It’s like a pillow - push in here, it pops out over there

acotrel said:

There are two ways of setting the needles and needle jets. You can raise the needles until the motor 8-strokes, then lower them one notch. Or you can lower them until the motor coughs as you ride the bike, then raise them one notch. I use the latter - it makes a difference. The shape of the taper on the needles can be important for good acceleration as you come up through the gears. Main jets are usually not a problem. By he time you really start using them, you are probably very much towards the ends of the straights. Doing plug chops on most race circuits is not easy. I have only ever done it on long public roads while keeping an eye out for the cops.

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Al, that “raise the needles till it 8 strokes then drop ‘em one” approach , or the other way around, isn’t really good e ought I’m afraid.

On the dyno there was a distinct change in AF ratio and power / torque curves way before there was any 8 stroking or coughing. But, on the FCRs I think the needle has a greater impact across a greater range than it does in an Amal.

comnoz said:

I would say the mixture looks good but the plug is a bit too high of a heat range.

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Well Jim, I used to run #9s on track days, these are #8s (NGK) so perhaps I should stick with the #9s !

Fast Eddie said:

Well Jim, I used to run #9s on track days, these are #8s (NGK) so perhaps I should stick with the #9s !

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I used 11's when I raced.

Or the equivalent Champion C53

I run 9's in my streetbike.

comnoz said:

I used 11's when I raced.

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Blimey !

Guess I’ll start by putting #9s back in it...

Is that at WOT?

acadian said:

Is that at WOT?

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Is what at WOT? If you’re referring to my plug chop, then yes.

If you are using #9 plugs for racing, tuning using #8 plugs keeps you safe. The different heat ranges determine the rate at which the soot burns off the porcelain in the plug. If you are using a dyno to set the needles and needle jets, you probably need to work backwards and find the air/fuel ratio which gives best performance in your particular motor with it's own ignition advance settings. I would not use a 'one size fits all' approach.

I learnt a few things after I'd fitted my O2 sensor and AFR gauge. One was that Nortons tend to run quite well across a wide range of jetting. The sniffer that my dyno man put up my exhaust showed that my mixture was quite lean down low and richened up as it went up the rev range. When I fitted my gauge, it showed the opposite almost. A very interesting exercise which I would recommend to anybody.

Fullauto said:

I learnt a few things after I'd fitted my O2 sensor and AFR gauge. One was that Nortons tend to run quite well across a wide range of jetting. The sniffer that my dyno man put up my exhaust showed that my mixture was quite lean down low and richened up as it went up the rev range. When I fitted my gauge, it showed the opposite almost. A very interesting exercise which I would recommend to anybody.

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I just ripped the O2 sensors off my KTM 1290, so it may be a while before I get around to putting one on a Norton!

comnoz said:

I would say the mixture looks good but the plug is a bit too high of a heat range.

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Hi Jim,
It seems like sorcery to me that you can differentiate between proper fuel mixture and proper plug heat range by looking at the photos. How do you do it? What are you looking for/at?

Thanks,
Ed

Danno said:

I just ripped the O2 sensors off my KTM 1290, so it may be a while before I get around to putting one on a Norton!

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Why?

APRRSV said:

Hi Jim,
It seems like sorcery to me that you can differentiate between proper fuel mixture and proper plug heat range by looking at the photos. How do you do it? What are you looking for/at?

Thanks,
Ed

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The color of the center electrode [not the insulator] shows the operating temp of the plug. That vs the color of the carbon at the base of the threads which gives a good indication of the temperature of the burn.

His burn temp looks good. His center electrode looks quite hot.

Once those look right then fine tune the mixture by looking at the burnoff line on the side electrode. Ideally it should be at the bend.

Colder plugs usually use a shorter side electrode also. That helps keep the side electrode from becoming too hot and becoming the un-timed source of ignition. That is why I do not recommend an extended tip plug in an air cooled race motor.

Race plugs generally use a very short side electrode or none at all. [particularly for air cooled] This is so there is less chance of a glowing side electrode on a highly tuned motor.
If the side electrode glows it will mean instant destruction of the motor.

Water cooled motors will have much lower head temps so they can cool the plug more effectively and will have more leeway in the plug choices.




Notice also the insulator is nearly flat across the face on a race plug. That provides a conduction path for cooling the center electrode.

He's also got about a 3/4 ring of carbon on the end of the threads which says the mixture is pretty close.

Fullauto said:

Why?

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Because they keep the low-rpm fueling a bit too lean (their only function, an emissions trick) for smooth rolling and removing them made 6th gear more usable as well as smoothing out the low-rpm running.

The Pro Boost probably contains a lot of aromatics - benzene or toluene. My plugs never look so black. The trouble with running a mixed fuel, is that once you get the jetting right, you need to always strictly control the fuel composition. With petrol, it is worse than with methanol for losing power, if you run slightly rich. And with methanol, you jets flow twice as much fuel, but a thou difference in needle jets, is the difference between sluggish and fast. It is beyond my imagination how the Brits tune Manxes and G50s to go fast using GP carburettors. They must have the patience of Jobe.