Plugs and Pipes - What color should they be

I have a MK III with a single Mikuni carb. I finally figured out it had the wrong intake mainfold and jetting. I have since put on a proper one to two intake manifold and rejetted the carb. After sitting for several months ( I have been injured) it started first kick. That makes me think I am on the right track.

The ceramic on plug I was fuel fouling is now a light tan, just like the other side. I noticed the new pipes are turing a bit golden color at the first bend from the head. Are these the proper color for plugs and pipes?

By the way, thanks for all the advice I have been given during this year long project. About all I have left to do on this bike is have the fenders polished and get the back end of the frame, where it holds up the rear fender straightened a bit. I think a prior owner used a gorilla to tighten up some tie-down straps.

calbigbird said:

I have a MK III with a single Mikuni carb. I finally figured out it had the wrong intake mainfold and jetting. I have since put on a proper one to two intake manifold ...

Click to expand...

I am curious: What manifold did you have before (the one that was wrong) and what did you replace it with? I would have thought that any manifold would have flowed to both cylinders, although some will do it with less restriction than others. How did the manifold make one cylinder run at a different mixture than the other?

As for pipes, if they're chrome then I think they are going to go blue eventually unless you have them coated internally to insulate the chrome from the exhaust gas heat.

The manifold on the bike when I got it was a Sunny Angel. It was such that the card sat canted to the right side of the frame backbone to clear the throttle cable. Bill Bibiani told me that most of the Sunny Angel manifolds were made for the Atlas which has cylinders that sit more straight up than the Commando. I bought a new manifold from Old Britts. It let's the carb sit straight up and level. It now starts and idles better than ever.

I believe the carb sitting in the wrong position coupled with the wrong jets caused the left cylinder to run abnormally rich at idle and lower RPM.

The head pipes are ceramic coated inside and have about 150 miles since installed. As I said, the first bend is starting to turn a bit gold. I had a guy tell me gold equals lean. Another guy told me blue equals lean. Now I'm confused.

the colour of the pipes has alot to do with their thickness ,mine are gun metal blue on the first bend,straw on the bottom bend, doesn't do the chrome any good though, the settings on this forum for the mikuni carbs are pretty close ,I do have mine set lean at light load highway cruising,60/70mph,its still coming off the slide there as I see no point in wasting fuel. Ifeel it is safer to start rich and lean down in small steps ,

Steel heated to straw colour is around 500 deg c
Steel heated to blue temper is around 700 deg c
So blue indicates more heat than straw.
My pipes are L.H. straw for round the first bend. R.h. Blue to straw round the first bend.
So my R.H. cyl runs a bit hotter.
Most Commandos will be blue round the first bend.
Some are blue to right down to the second bend.
In my opinion with your ceramic coating you should not see more than blue for the first 4" of pipe. Then straw for 2".
(Max)

Your plugs should be the colour of a digestive biscuit brown (do you have them in USA?), or darker, but not sooty black.
Remember, slightly rich is MUCH better than slightly lean. My plugs are brown at mid and full bore and blackish on idle.
Also remember that you need to check the plug colour in three different positions:-
At idle : adjust the pilot air screw.
Full bore : adjust the size of the main jet.
Mid bore : adjust the height of the tapered needle on the 'c' clip, if your carb has one. Amals do.

When you check at half and full bore run the bike at that position for at least 25 seconds and hit the kill switch at those revs....dont pull up and then hit the kill switch, the plugs change colour very quickly once you change revs.

All or most of this you probably already know.....but if you didn't then here it is

That definately answers my questions, thanks. I think I'll leave it as it is for a while. Apparently I can live with straw or golden pipes and light tan plugs.

bigstu said:

Your plugs should be the colour of a digestive biscuit brown (do you have them in USA?),

Click to expand...

I would aim for a light tan colour like a Rich Tea biscuit as the very leanest you want to be

The nose of the ceramic is the wrong part to look at. The deposits on the nose of the insulator have more to do with the heat range of the plug. A well set up engine can have a completely clean insulator by burning off any deposits.

Have a look here- http://www.dansmc.com/Spark_Plugs/Spark ... talog.html

You'll see a huge range of insulator colours all in the "good" category.

There seem to be a few different methods of reading plugs, some look deep into the plug at the bottom of the insulator and look for where the carbon ring forms,
others look at the outer metal base ring and ground strap that's welded to it, but all agree that the top of the insulator does not really indicate much about mixture.