Sad Story

I built this 880 motor a few years ago for someone. It had a single Mikuni carb on it and it ran very well for quite a few miles.

He decided to put the old Amals back on it for a bit more power. It made more power but then the plugs kept getting sooted up. He had been running b8es plugs as I had advised when I did the rebuild. Instead of working on the jetting he tried going one step hotter on the plugs and he said it helped a little but they still sooted up.

So he put the single Amal back on it BUT he didn't put the colder plugs back in. The head had steel spark plug inserts in it which made the b7es plugs act more like a b6es.

Here was the result after pulling a good hill at speed.

It melted the end of the spark plug side electrode and went into pre-ignition.

The plug shown is from the side that was still intact -except for the two cracks in the piston top. The plug from the other side was smashed.

What is the background with those Pistons?

illf8ed said:

What is the background with those Pistons?

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They are Dreer 880 kit pistons. [JE's}

Didn't it make a racket before it went fatal?

swooshdave said:

Didn't it make a racket before it went fatal?

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He was wearing earplugs -but once pre-ignition [not ping] starts, the damage happens in seconds so he probably wouldn't have saved it anyway. Jim

" The head had steel spark plug inserts in it which made the b7es plugs act more like a b6es."

This is an interesting effect, excuse an ignorant old man, but how does that work? Something to do with heat distribution caused by the steel inserts?

Tommy

Jim, my near stock 850 (Boyer MK3, open peashooters, .020"headgasket, b7es plugs) went onto one cylinder near the top of a seven mile full bore run up the Salmo Creston. After backing off the throttle for a few seconds it went back onto 2 cylinders and normal running. I assumed that the one cylinder cutout due to an overheated plug. This is the only time that it has ever done this, but it was also probably the longest and hardest pull it has done. Should I be changing to colder plug? It did occur at the time, then I forgot all about it until reading your post here.

My Vincent did the identical thing being thrashed on the mountain grade at the IOM course. Five Vincents that I had passed earlier on the Sulby straight, all of us full bore and laid flat on the tank, went by me in a flash on the mountain grade when the bike dropped onto one cylinder near the top. The mountain grade at the IOM is not that long or steep, however I had held the bike at very high speed for several miles prior to the mountain grade,so the heat was already in it.
That one also has b7es plugs and has never since given so much as a hiccup, even climbing White Bird hill in southern Idaho, again full bore, but two up with luggage.
Maybe I came close to a blowup with both of these bikes?

comnoz said:

It melted the end of the spark plug side electrode and went into pre-ignition.
The plug shown is from the side that was still intact -except for the two cracks in the piston top. The plug from the other side was smashed.

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Its amazing a spark pug can do that much damage. :evil:

So it's the steel plug inserts here that caused the problem?

From searches on this site, it seems that quite a few of us, including myself, run BPR6ES plugs. According to NGK, these plugs make the engine run 80-100 deg C hotter than the BPR7ES plug.

My 750 Commando has a single Mikuni VM34 with 27.5 pilot jet with air screw turned 2 full turns out. The only way I can get decent plug colour is with the BPR6ES plugs.

The bike still runs a little rich with these plugs fouling ocassionally due to the variable quality of the fuel in Malta, but I still prefer to be on the rich rather than leaner side of the equation for just the reasons your post illustrate Jim.

Another question from the less informed. In your original post you are talking about B8ES and B7ES plugs. As I recall, the equivalent plug to the Champion N7Y that was original spec is a BP7ES. I also remember seeing the NGK cross reference table many years ago that said a B7ES was equivalent to an N4Y Champion. I had a discussion with an old brit bike wrench many years ago who refused to believe that the "P", protruded tip, could make that much difference, until he pulled out the NGK catalog and read it personally. I've always run N7YC's and BP7ES, depending on availability. Are you saying B7/8 should be used instead of BP7/8, quite a difference in heat range there, isn't it?

Thanks, B

Personally I would never run a NGK 6 grade plug in an air cooled motor -particularly one that runs as hot as a Norton. The plugs heat grade does not change the engine temperature- It just changes the operating temperature of the spark plug electrodes.

A plug becomes a hotter plug by lengthening the path the heat must take before it is dissipated by the head. Adding a steel insert simply lengthens the path so the plug electrodes run hotter.

If a plugs electrode is running so cool that it does not burn away the soot -then there is little danger of the plug glowing and causing pre-ignition. A mixture that is very rich does tend to keep the electrodes cool.

A mixture that is rich enough to keep a hot plugs electrode cool is also rich enough to reduce power output by a considerable amount and do slow damage to the engine by washing the lubricant from the cylinder walls and building deposits on the piston and cylinder head.

A much better and safer thing to do if the plug is becoming sooted up is to correct the fuel mixture - not go to a hotter plug. A Norton motor is fairly forgiving of a lean mixture-as long as the spark plug is able to dissipate enough heat on it's own without relying on a rich mixture to cool the electrode.

A Norton motor [or any motor] is not forgiving of a spark plug that can not dissipate heat fast enough to keep the electrodes from glowing. A glowing electrode "lights the fire" long before the spark comes along. This results in a ignition point that may be 50 to 90 degrees before it should be -which causes immediate engine damage when it happens at full throttle.

Another thing that will influence the spark plugs electrode temperature is timing. Timing that is too far advanced puts more heat into the spark plug and piston top.

Timing that is too slow puts more heat into the exhaust valve, the exhaust port and pipes.

Timing that is too slow can cause the exhaust valve to become hot enough to glow -which has the same effect as a glowing spark plug electrode. Jim

Interbak said:

Another question from the less informed. In your original post you are talking about B8ES and B7ES plugs. As I recall, the equivalent plug to the Champion N7Y that was original spec is a BP7ES. I also remember seeing the NGK cross reference table many years ago that said a B7ES was equivalent to an N4Y Champion. I had a discussion with an old brit bike wrench many years ago who refused to believe that the "P", protruded tip, could make that much difference, until he pulled out the NGK catalog and read it personally. I've always run N7YC's and BP7ES, depending on availability. Are you saying B7/8 should be used instead of BP7/8, quite a difference in heat range there, isn't it?

Thanks, B

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A projected tip spark plug is more influenced by the combustion temperature -because more of the electrode is exposed. So if the combustion temp is low the plug will be cooled more but if the combustion temp is hot then the electrode temp will be hotter than an equivalent plug with a recessed electrode.

Generally a recessed gap plug is capable of operating over a wider range of combustion temperatures without suffering from a glowing electrode. Jim

worntorn said:

Jim, my near stock 850 (Boyer MK3, open peashooters, .020"headgasket, b7es plugs) went onto one cylinder near the top of a seven mile full bore run up the Salmo Creston. After backing off the throttle for a few seconds it went back onto 2 cylinders and normal running. I assumed that the one cylinder cutout due to an overheated plug. This is the only time that it has ever done this, but it was also probably the longest and hardest pull it has done. Should I be changing to colder plug? It did occur at the time, then I forgot all about it until reading your post here.

My Vincent did the identical thing being thrashed on the mountain grade at the IOM course. Five Vincents that I had passed earlier on the Sulby straight, all of us full bore and laid flat on the tank, went by me in a flash on the mountain grade when the bike dropped onto one cylinder near the top. The mountain grade at the IOM is not that long or steep, however I had held the bike at very high speed for several miles prior to the mountain grade,so the heat was already in it.
That one also has b7es plugs and has never since given so much as a hiccup, even climbing White Bird hill in southern Idaho, again full bore, but two up with luggage.
Maybe I came close to a blowup with both of these bikes?

Click to expand...

Chances are that when one cylinder stopped running it was because a valve got hot enough that it could no longer seal on it's seat.

A hot carb can do something similar but it would take longer to clear up.

A spark plug that got too hot would simply begin to glow -and you would see the results very quickly.

I don't know what spark plug is called for in a Vincent.

A spark plugs heat range is determined by the cylinder heads ability to dissipate the spark plugs heat.
That is why you see hot plugs in a water cooled engine and cold plugs in an air cooled engine.

Very informative post. Thanks Jim.

In reading this, I come away that you are advising against hotter plugs in general.

This seems to support the idea that investing in an O2 sensor or "sniffer" and verifying fuel mixture is the best approach. They are a pretty cheap tool to know for sure what's going on. A old school "plug chop" might work but has some variables that are difficult to control.

dennisgb said:

Very informative post. Thanks Jim.

In reading this, I come away that you are advising against hotter plugs in general.

This seems to support the idea that investing in an O2 sensor or "sniffer" and verifying fuel mixture is the best approach. They are a pretty cheap tool to know for sure what's going on. A old school "plug chop" might work but has some variables that are difficult to control.

Click to expand...

I would always use the coldest plug I can get away with. Generally the higher the tune of the engine or the harder it is used -the colder the plug needs to be. Anything that makes the cylinder head run hotter means the plug needs to be colder.

A plug that gets hot enough to glow causes immediate damage. A plug that is too cold just soots up and starts running poorly.

O2 sensors are good when used as a comparison and not necessarily an absolute.
Probably of more value for an initial tune is a set of fast EGT probes and a cylinder head temp guage.

The problem normally seen with O2 sensors on a Norton is the fact that about half the length of the exhaust system is going to be contaminated by ambient air.
So the O2 sensor must be mounted in the front 1/2 of the pipe or it must use a tube that reaches halfway to the head from the outlet. A tube that long means a pump is needed to draw the sample.

Thanks Jim.

I keep forgetting that the Norton doesn't put out consistent exhaust from the pipe.

Makes it more difficult. Might have to add a bung to mine and figure out a place to mount the gauge.

I guess looking at the plugs will still be in play too.

dennisgb said:

Thanks Jim.



I guess looking at the plugs will still be in play too.

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Absolutely. Reading the plug is important. Just keep in mind that non-leaded fuel does not color the insulator like it did in years past. With a lot of miles the insulator should slowly darken with additive buildup but any darkness on a fresh insulator is just soot from incomplete combustion. That can happen because of many things- even a mixture that is too lean to burn properly.
The insulators should stay white for at least a few hundred miles.
The best things to look for anymore is the color of the carbon that builds up around the base of the threads and the position of the ring where the carbon no longer exists on the side electrode. That should happen somewhere around halfway up the side electrode. Jim

comnoz said:

Adding a steel insert simply lengthens the path so the plug electrodes run hotter.

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Steel insert - lessons learned?

Would a copper-breyllium (or comparable alloy) insert be a better option since it
1.) more closely matches the thermal coefficiant of expansion of the cylinder head and
2.) has as good if not superior thermal conductivity properties?

Dances with Shrapnel said:

comnoz said:

Adding a steel insert simply lengthens the path so the plug electrodes run hotter.

Click to expand...

Steel insert - lessons learned?

Would a copper-breyllium (or comparable alloy) insert be a better option since it
1.) more closely matches the thermal coefficiant of expansion of the cylinder head and
2.) has as good if not superior thermal conductivity properties?

Click to expand...

Sure, bronze, beryllium and aluminum inserts all work fine as far as temp is concerned. That is why I do aluminum bronze plug inserts. Jim

Interbak said:

Another question from the less informed. In your original post you are talking about B8ES and B7ES plugs. As I recall, the equivalent plug to the Champion N7Y that was original spec is a BP7ES. I also remember seeing the NGK cross reference table many years ago that said a B7ES was equivalent to an N4Y Champion. I had a discussion with an old brit bike wrench many years ago who refused to believe that the "P", protruded tip, could make that much difference, until he pulled out the NGK catalog and read it personally. I've always run N7YC's and BP7ES, depending on availability. Are you saying B7/8 should be used instead of BP7/8, quite a difference in heat range there, isn't it?

Thanks, B

Click to expand...

I had to use a P plug on one of my Bultacos. Without it I'd get fouling in no time.