[BRADEY]

Vince the only brass screw I could find was in "the well"
and I believe it sets the idle on the motor. 

Arizoni, I have found a NGK dealer in my Region and hope to meet him
tomorrow, is it ok if I pick BPR5ES instead of 6ES or their Iridium version ??

NGK India lists BPR5ES/CPR7EA-9 or CPR7EAGP-9 and CPR7EAIX-9 as the probable choices for the UCE 350 engines. The first being their conventional plugs, the second being their G Power plugs and the third being their Iridium series plugs.

[Arizoni]

IMO, these Royal Enfield engines are low tech designs including the UCE's.
They are designed to use the old fashioned, low cost spark plugs so spending 3 times the money for a Iridium plug makes no sense to me.

If the engine was a high reving short stroke engine that is expected to run 40,000 km between spark plug changes (like modern automobiles)  it would be different.

As for the difference in heat ranges (the numbers you gave are not a 6), I can't say how well they will work.
I guess you can try them and then, after riding 300 km or so, check the color of the electrode insulator to see how its doing.

[Vince]

     That is the screw. Just mark it's position so you can return to it. Try 1/8 turn. I have not had to go further than that. It affects idle by affecting mixture. The last customer I had with a lean condition turned it counter clockwise. It helped throughout the RPM range. His plug looked better and he had better throttle response. It will either help or make it worse. If worse, go the other direction. You can always go back if you marked your starting point. Put a scrape mark on the inside of the well at the screw's slot.

[BRADEY]

thank you vince and arizoni. the current setting of this screw on my bike is just about high enough, such that the auto decompressor is not activated, even during the cold winters of North India. And I remember quite well that when the bike came from the factory it always did actuate the auto decomp 

So what do you suggest should I turn is counter clockwise (to make it still richer) or clockwise (so that the auto decomp activates on cold starts). Remember the motor can return a mileage of upto 80mpg on freeways at speeds of 50 to 60 MPH....on the current settings. I'll try to post the pictures of the Plug for your reference.

[BRADEY]

Here are the pics. On the left/lower side is my C5 plug
and the other one is my Electra CI 350 plug (which is too rich)
From whatever little I know of plug reading the C5 plug cannot be termed as running lean. In fact it is the perfect tan colour
we would want it to be.

[barenekd]

The auto decomp has absolutely nothing to do with the EFI. It is a purely mechanical device mounted on the exhaust cam. Oil weight and temp could affect it. They definitely get a bit sticky on cold mornings.
Bare

[Vince]

     Both are too rich. I've had success in running improvements by turning clockwise. Just mark the starting point so you can return to it, then try one way or the the other. If it's better, great!. If not, go the other way.
     The auto decompress is completely separate. It has no mechanical or electrical connection to fuel/ignition or anything else.

[BRADEY]

Vince do you have an example of a spark plug
that you would think is running on optimum mixture
so that I may compare it with mine.

I believe that if you lower the idle RPMs below 900 by
adjusting this brass screw, the auto decomp comes into play.
However, if you raise the idle RPMs, it may not. I understand it is a mechanical device, but is it not governed by the engines RPMs at idle. Correct me if I am wrong please !

[barenekd]

It is only affected by RPM. I've forgoten the exact rpm,but it's supposed to be somewhere around 800 RPM when the auto decomp stops decomping. It is operated by a flyweight on the exhaust cam, but as I said, the flyweight can get a bit sticky on cool mornings.
Bare

[Arizoni]

Just for the record, the Service Manual says the large brass screw down in the well on the top of the throttle body is a air adjustment screw that controls the airflow at the idle.
It does not change the fuel mixture by itself but the change in the idle speed is noted by the ECU and it (the ECU) modifies the fuel rate to accommodate the new idle speed.

[JVS]

The C5 plug does not appear to be too rich, the electrode is the tannish colour you speak of. And that air bypass screw does not have anything to do with how the mixture is set. If you've been making short trips on your RE without it getting properly warmed up, it can explain some of that sooting. Just try swapping the plug with what Arizoni has recommended, set the gap at 0.7mm and see how it goes 

[JVS]

BRADEY

Also, I just realised that you have the Classic 350, correct? 81mpg is very decent and comparable with other UCE 350s. Your bike is running fine! Keep riding!

[BRADEY]

Hi JVS its a C500 and not 350.
I did some research on the NGK plug guide,
and find that their BPR6ES and BR6ES are essentially
the same plugs. The later is available in my city and
I am going to give it a try and report on my findings.

The "P" seems to be one slightly longer. I suppose it
could have better heat transfer properties !!

[JVS]

No worries, and thanks for clearing that up. Let us see how your new spark plug goes. Hopefully, all will be well.

[heloego]

I jumped on the forums today to ask about equivalents for the BPR6ES, since everyone here wants to sell me the iridium one. Good thing I checked this thread first! 
Thanks for the post, arizoni!