Spark plug colour

Good day all,

I have been struggling with the carb jetting set up for a 1970 NC I acquired from the USA and imported to Tripoli, Libya where the fuel here is not refined so well and very rich.

The bike came with a single Mikuni VM34. I changed it out for a new identical Mikuni VM34 as the previous one was about ten years old. I was concerned that there may have been issues with the old carb, and for about $100, I thought the money was well spent.





The new Mikuni was set up specifically for a Norton Commando by the reputable UK seller.

Notwithstanding the plugs sooted up really quickly within minutes of starting. I ended up dropping the pilot jet size in steps from 35 down to 25. The helped a lot. On taking the bike for a 10 mile run on the weekend, I pulled the plugs to find this:





Better, but still not there with the right plug lighter than the left.

Maybe I should have gone for a longer run to allow the heat to fully burn off the built up carbon. Other than that, I'm not sure what the bike is doing to give an uneven colour. It ran pretty well over the distance and pulled smoothly across the rev range. Idling was also good.

Any comments welcome.

Cheers,

joe

Change the plugs for another "new" pair.......... or swap the same over to assess which cylinder is leaner or richer?? ( this will ensure the plugs are not dysfunctional)

What brand and type/heat range sparkplugs are you using?

Plugs are NGK IRIDIUM BR7EIX

With a single carb there are only 4 possibilities for uneven burning/mixtures between the 2 cylinders:
1) Uneven cooling: Not likely on a Commando without a fairing. Airflow with fairings is a whole different subject.
2) Uneven mixture distribution: A function of carb, manifold & intake design & not easily addressed by an owner.
3) Uneven ignition timing: A basic maintenance task. Make sure the ignition timing is exactly the same between the two cylinders.
4) Uneven combustion chamber pressures: The most easily addressed task to address a basic cause is to make sure that your tappet clearances are correct & the same between cylinders. Uneven tappet clearances result in a difference in valve timing & opening resulting in significant differences in pressures, therefor temperatures.

you might review the classic article by Gordon Jennings on reading spark plugs for heat range, mixture & ignition timing: "How you can read spark plugs and select them - by Gordon Jennings" to be found at http://www.strappe.com/plugs.html

In order to accurately get a plug read you'll need to do some 'plug runs & chops': Find a stretch of road on which you can stretch the bike out & have your plug wrench with you. The 1st task is to get the main jets right. This is done by accelerating through the gears at full throttle, probably a few times to clear the plugs. On the last run, simultaneously close the throttle, pull in the clutch & kill the engine from full throttle & brake to a stop. Pull the plugs & read them. Gloves to handle the hot plugs & a 2X or 3X magnifying glass are helpful (read the Jennings article, you'll understand why).

The 2d task after the main jets are correct is to set the needle. This is done using the same procedure but at 1/2 throttle. It's helpful to mark your throttle with a Sharpie, or some such, for repeatability. A higher needle (lower clip) is richer, lower needle ( higher clip) is leaner.

The last step is to check the throttle cutaway in the same manner but at 1/4 throttle. A larger cutaway is leaner, smaller is richer.

You may want to check that your fuel lines arent touching your motor... you don't want them to melt.

Toss in leaking intake valve guides darken too but these don't look oil fouled just fuel over dose. There is ~15% less spark power current in the reversed polarity spark of wasted spark ignition systems. I put 1000's of miles on two Combats with 34 Miki + boyah analog ign. but never saw that much variation. Get a V8 pack for plug chopping trail and error. Best spots I found for WOT plug chops was a steep climb I'd shut off at top then coast back down to truck brake check pull over to examine. I wonder if the aim direction aka plug indexing of electrode makes a detectable change in the plug carbon? May be the Commando is cooling so well may need a higher heat range.

A faulty ignition system can give exactly the same symptoms as incorrect mixture. On my bike I use a double ended Honda coil with wasted spark. If you have two separate coils, I suggest you should have them checked, also look at the wiring to each of them . When you do a plug chop, it should after a long hard burst using the main jet (full throttle) preferably on a long slightly up-hill road. What you are looking for is the 2mm wide black ring on the porcelain right down inside the plug where it meets the metal. If the plug heat range and mixture are correct you will see it. A sooty looking plug on a twin carb bike can mean that the petrol in one of the float bowls is frothing due to vibration and poor isolation of the carb. You obviously don't have that problem. I don't believe you can successfully set the low and mid-range jetting by doing plug chops - only the main jets. I don't think it is possible to get what you've got through bias of the single carburetor, however it might be due to a leaky inlet valve guide making one side lean.

clearly it is time for some brake cleaner and your wife's toothbrush

that one plug looks badly fouled

clean em both and take her out for a good ride then cool down and pull em to look again

Now a days with the lean burn fuels, [over all a good thing], the traditional plug looks do not apply but for very crude mixture indicators, such as above photo's obviously showing wasting of unburn fuel and likely washing bore oil off too. A propper plug chop starts with new plugs put in right before the test run then out to reuse some time later but not much good to do another chop on them. Not my opinion to recoil from - look it up your selves like I did a decade+ ago. A well used plug does give idea of average over all ball park of mixture as does a look at the exhaust coating, which in water cooled 'puter controlled craft should be light layer of mostly dark carbon but better be fairly thick dark black on our type engines on new gasoline or risking lean over heating.

Here's a good looking Enfield vintage air cooled engine plug, essentially white porcelain, rim dry dark sooty layer and thermal electrode stain just before the bend indicating spot on ign time. Too hot stain by rim, too cold stain past bend. If ya look till a professional race shop shows up they open up plugs to look at very base of porcelain to know for sure. I always like to ride out when on a mission.

http://www.youtube.com/watch?v=VLehT1m0Ztc
http://www.youtube.com/watch?v=VLehT1m0Ztc

Why happens that one spark plug is very very clean (like new) and the other has a correct colour white/brown?
The cylinder of the clean spark works yet.
Piero

You'll rarely get perfectly equal mixture distribution with one carb feeding 2 cylinders.
Mismatches at the manifold joints can affect it,but the biggest factor is usually the angle of the carb.If the carb faces more toward one cylinder,that cylinder will run richer than the other.

Air leaks on one side from intake, exhaust or internal head gasket could cause this too.

Update - I've made a number of changes to the pilot/main jet/spark plug set up that has improved the performance of the bike, to the point that I'm happy with the left side spark plug colour. The right side remains fouled with black sooty dust indicating a too rich condition on that side. The bike runs well and pulls strongly across the rev range to 5k rmp and beyond. It's still not a 1-2 kick start, more like 10 kicks.

I removed the tank to check and reset the tappet clearances. All were within spec except the right side inlet which had no clearance. I reset that tappet, and the right plug showed some improvement after a ride but still relatively dark and sooty compared to the left plug which has a nice tan porcelain centre and black ring outside around the porcelain centre.

It is possible that I have a damaged right inlet side valve seal? If the carb settings are ok, and tappet clearances are set to factory specs, is the valve seal the remaining factor that could be causing the right plug to be too dark and sooty?

Next question is, is it a relatively straightforward job to pull the head and replace the valve seals or should I send the head to a Norton mechanic for this work? Adjusting the tappets and carb settings is about my level of mechanical (in)expertise.

cheers,

Joe

One cylinder correct and one slightly rich may be as good as you'll ever get, with a single carburettor.

Does the bike run ok?

The shape of the manifold internal bridge can effect the flow, slighly off centre seperation will feed more cylinder mix..ask Jim to test them on his flow meter.

FROM THE LOOK OF THE PLUGS IN THE FIRST PHOTOS, THEY ARE NOT REALLY A PROBLEM. I SUGGEST YOU JUST RIDE THE BIKE AND ENJOY IT. I f you suspect oil fouling, maybe do something. Oil fouling will cause a slight powdery look on the porcelains if a small amount of oil is present. If it is just the colour that is different, I would suspect you are not having an oil issue.
As far as ignition is concerned, if you are running a boyer, then both plugs always fire at the same time every rev of the engine. [ not possible to have the timing out on one cylinder ]
Dereck

OldForge said:

With a single carb there are only 4 possibilities for uneven burning/mixtures between the 2 cylinders:
1) Uneven cooling: Not likely on a Commando without a fairing. Airflow with fairings is a whole different subject.
2) Uneven mixture distribution: A function of carb, manifold & intake design & not easily addressed by an owner.
3) Uneven ignition timing: A basic maintenance task. Make sure the ignition timing is exactly the same between the two cylinders.
4) Uneven combustion chamber pressures: The most easily addressed task to address a basic cause is to make sure that your tappet clearances are correct & the same between cylinders. Uneven tappet clearances result in a difference in valve timing & opening resulting in significant differences in pressures, therefor temperatures.

you might review the classic article by Gordon Jennings on reading spark plugs for heat range, mixture & ignition timing: "How you can read spark plugs and select them - by Gordon Jennings" to be found at http://www.strappe.com/plugs.html

In order to accurately get a plug read you'll need to do some 'plug runs & chops': Find a stretch of road on which you can stretch the bike out & have your plug wrench with you. The 1st task is to get the main jets right. This is done by accelerating through the gears at full throttle, probably a few times to clear the plugs. On the last run, simultaneously close the throttle, pull in the clutch & kill the engine from full throttle & brake to a stop. Pull the plugs & read them. Gloves to handle the hot plugs & a 2X or 3X magnifying glass are helpful (read the Jennings article, you'll understand why).

The 2d task after the main jets are correct is to set the needle. This is done using the same procedure but at 1/2 throttle. It's helpful to mark your throttle with a Sharpie, or some such, for repeatability. A higher needle (lower clip) is richer, lower needle ( higher clip) is leaner.

The last step is to check the throttle cutaway in the same manner but at 1/4 throttle. A larger cutaway is leaner, smaller is richer.

Click to expand...

I do it systematically in the reverse order. I start by fitting mains which I know are slightly too rich, I set the idle and low running, then I adjust the needles and needle jets. After that I do the plug chops to find the right main jets. If I then fit a main jet which is richer than the optimum found by plug chops, if the plugs don't darken I know the thing is metering off the tip of the needles and needs a different taper needle, or recessed needle jets.
You need to look at the ring on the porcelain right down where it meets the metal down inside the plug. What the electrodes etc look like is pretty irrelevant. If everything is correct a 2mm wide ring should stay on the porcelain, and if it disappears you have a problem unless you are brave and looking for maximum performance. The bike will always go quickest just before it destroys itself, through lean mixture or timing adjustment. It is worth noting that a bad ignition system can give the same symptoms as poor jetting, so get that right first.
Please don't get confused, - when the needle is in the needle jet from 1/4 throttle up to 3/4 throttle, that is where the metering is happening, the metering is done by the mains a bit after that. The mixture at 3/4 throttle is the hardest to get right, and it is critical otherwise you get coughing or a flat spot as you open the throttle coming up through the gears. You usually only use the main jets when you are really getting stuck into it or the bike is greatly over-geared and you are making it pull hard. On most race circuits, it is usually a long hard controlled squirt out of corners, and wide open throttle for the last half to third of the straight.

I know this all sounds like silly stuff however my brother and I have almost come to blows over it. While jetting his H2 and H1 Kawasaki speedway sidecars, we've had an expensive nightmare getting it right. You don't get much warning with a two stroke when you tune it for methanol. This the reason I discuss this stuff on this forum - if you are racing and get it really wrong, it can put you out of the sport financially.
The only time I've ever seen my brother lose a race with the 750cc H2 sidecar was once when it came on song and jumped 9 feet into the back of another outfit and did a loop. - You don't need that.

john robert bould said:

The shape of the manifold internal bridge can effect the flow, slighly off centre seperation will feed more cylinder mix..ask Jim to test them on his flow meter.

Click to expand...

Nice thought, but there is no way a single manifold can be balanced to provide the same mixture on both cylinders by doing flow tests. I don't think I have ever seen a single carb setup that ran the same mixture on both sides when measured with a gas analyzer. If it were to happen it would just be by chance as there are too many variables to affect the mixture and no good way to adjust one side only.

If your jetting with leaded fuel then jet so the lean side is light tan and live with the rich side.

If your jetting with unleaded -both sides should be white as there is no lead to provide color.

If they are dark then it is dark because of soot from incomplete combustion and that can be caused by a lot of things like ignition, compression, cam timing or mixture that is way off -rich or lean. Jim