Reading Spark Plugs

Joined
Feb 16, 2005
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16
Just thought this topic could be better served in a new post. I've been struggling with the spark plug/carburetor setting issue for some time now and would like to offer this link to anyone who hasn't already seen it.

http://www.ingfatrygg.se/Spark_plug_diagnostics.html

My 74 850 has a single 34mm Amal Mk2 (deep float, 20 pilot, 260 main, and 3.5 slide) and the Interstate fuel tank. I've tried a number of different spark plugs and have finally settled on the Champion N5C. They keep themselves clean, the engine idles at a steady 800 rpm, and there's respectable power throughout the throttle range. Other cooler temp plugs, particularly platinums and iridiums caused visible tailpipe smoking throughout the throttle range. Everything being equal, the smoking stopped with installation of the N5Cs. Idle improved as well. And the 20 pilot allowed more sensitivity setting the air screw.
 
Joined
Feb 16, 2005
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I've had an ongoing problem fouling the plugs like many of the Nortoneers who posted on this forum. I've focused my tweeking on the carburetor, and I'll be the first to admit that the carb's important. But, I didn't realize what a significant role the plugs play. The N5Cs were nearly miraculous on this bike.

BTW the plugs look like the middle photo in the link given by norbsa...full circle soot on the base thread and heat range halfway down the strap. In my experience, platinum plugs are the worst, easily sooting and resulting in a noticeable loss of performance.

I suppose my point is to not overlook plugs as a major source of engine performance problems.
 
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Feb 22, 2006
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Hi Gene,

I run Iridium plugs in my 850 with no fouling, no smoking whatsoever and no breaking down at all through the rev range, it pulls away in top gear from low revs without any kicking, coughing or spluttering, they also help to cover any timing irregularities, i love 'em :lol: but that's just my experience.

Mike
 
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May 19, 2006
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I'm a bit puzzled by all this talk of plug problems. I use boring old Champion N7YCs. I bought my last box of ten some years ago and just change them if the outsides start looking a bit corroded 8)

They are always dry and don't appreciably suffer from build up of deposits. I have used them with leaded and unleaded fuel (currently 98 if I can get it). My 850 mk111 with early airbox and silencers has Martin Bratby sleeved carbs and runs standard 850 Mk1 jetting.

I really thought that the Commando was one of the most carb and plug-tolerant motors you could find ? Mine pulls strongly all through the range and doesn't cough, splutter or fluff anywhere. I really think that's normal for a Commando.

I use NGK plug caps but with the logo filed off and polished to avoid anyone thinking that I'm turning Japanese :wink:

Twin Carbs for ever ! :D
 
Joined
Feb 16, 2005
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Hi Mike, norbsa may be right, this single MK2 carb may be running rich but it might be that the N5Cs are matched to it and burning more efficiently. I did have visible smoke at idle and during acceleration with other plugs but that's all disappeared with the N5Cs (N4s worked pretty well, too). There is a very noticeable increase in performance as well. I tried these as a result of a post here that said Mick Hemmings thought the N3s were a brilliant plug for the Commando...anybody know more about that? Your carb/s may be set leaner than mine so that the platinums work for you. I prefer platinum and iridium plugs in my autos but they just won't work in this Commando. How is/are your carb/s set-up?

Hi Norbsa, what set-up would you recommend for a single MK2?

Gene
 
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Apr 7, 2004
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I run a pair of 34mm 106 NJ,2A1 N, 240 main's and 25 pilot jets. I also use the two cycle floats and the two cycle brass slide that's chromed. Very light wt. cables hand made.This is important so as not to make the slide bind in the bore. You must add a rubber band hanger to the carb. I can send you pics. Without hangers you wear the NJ right out and get all kinds of bad running. The price of running rubber mounted carbs on a rubber mounted engine.
If I were going to run it as a single that's just the jetting I would start with. You may find the main just a tad lean on a full out fourth gear run. So you may end at 250 there.
I run an anolog Boyer,Solid copper leads,dyna dual output coil.and Autolite Platnum AP-64 plugs gaped at .035 to .040 A lot of folks run AP-63's one step cooler. Hotter plugs burn lean mixes better. On side roads doing a 250 mile trip I got 59 MPG. The main jets and the plugs need close investigation at flat out your bike may be differant , what works for me may not work for you no suprize there.
 
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Feb 16, 2005
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Norbsa, I've already figured out the rubber band hanger...I use an O-ring on a custom made hook through the frame hole above the carb. I also use a rubber flange rather than the rubber tube coupler. It needed a guide tube for the cable similar to the Delortos to keep the cable from binding. I also use the chrome plated brass slide, 3.5. The 25 pilot is too rich for mine, I moved down to a 20. 260 main colors the plugs OK at 3-4K as mentioned previously.

Tried the AP64 as a result of one of your earlier posts. Didn't work for me, sounds like it may be a good plug for you given the ignition system you're using.
Gene
 
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Gene, have you removed the alum. enrichner bushing that is located under the bellmouth? If not that is why you have to use the 20 idle jets. Also why you need 3 1/2 instead of 3's.
 
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Feb 16, 2005
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Norbsa, I have another MK2 carb body with the bushing removed. I swapped just the bodies...no other changes (20 pilot and 3.5 slide). It should have run very lean but it didn't...back to hard starting and smoking at idle. It wouldn't adjust out.

I scrapped that body originally (it was a new carb) because once the engine revved past 2-3K, the slide would stick open...it made for an exciting ride. I fabbed a cable guide for the top of the carb and installed a chrome #3 slide hoping to solve the sticking. Didn't work, so I replaced the carb body. No problems with the new one but I didn't realize the bushing was still in it.

The sticking happened again on the body with the bushing removed. Checked the cable for binding and couldn't find anything. When I was originally working on the sticking, Fred at Old Britts told me that he'd heard that some 850s had that problem using a single MK2...he thought that the slide might be getting sucked to the throttle throat and sticking. He suggested an additional spring which worked better but still not good enough. Do you suppose removing the bushing is related to the slide sticking?
Gene
 
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Aug 21, 2006
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Are you not worried about holing a piston with the hotter plugs?



Gene said:
Just thought this topic could be better served in a new post. I've been struggling with the spark plug/carburetor setting issue for some time now and would like to offer this link to anyone who hasn't already seen it.

http://www.ingfatrygg.se/Spark_plug_diagnostics.html

My 74 850 has a single 34mm Amal Mk2 (deep float, 20 pilot, 260 main, and 3.5 slide) and the Interstate fuel tank. I've tried a number of different spark plugs and have finally settled on the Champion N5C. They keep themselves clean, the engine idles at a steady 800 rpm, and there's respectable power throughout the throttle range. Other cooler temp plugs, particularly platinums and iridiums caused visible tailpipe smoking throughout the throttle range. Everything being equal, the smoking stopped with installation of the N5Cs. Idle improved as well. And the 20 pilot allowed more sensitivity setting the air screw.
 
Joined
Feb 16, 2005
Messages
16
tpeever, maybe I'm wrong to decide that the N5Cs are safe, but I'm using the fact that the plugs show no signs of overheating.
Gene
 
Joined
Feb 22, 2006
Messages
200
Hi Gene,

I said earlier that i run Iridium plugs and they work a treat, but i also run a single Mk. 1 Amal and once set up gives me no grief, i have never had a problem up till now with it sticking, the slide has been sleeved but everything else is fairly standard apart from a 260 main jet.

Mike
 
Joined
Apr 7, 2004
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1,691
Hey Gene, I found that I had to make my own cables. Useing the lightest wt. Barrnet from Flanders in Calf. The housing has a red nylon liner and the cable itself has kind of a bumpy texure. These very light wt. cables don't put any bind on the slide. You have to loose the 45 adapters and go with this new system. When a cable that's thick does that sharp bend just before entering the carb it trasfers the force onto the slide and they will bind even brand new. You may not like the hassle or my answer but that's what I learned. Once you get it right you will be riding for ten years on mark two's. I make my own cables for all the bikes now getting this right is worth the time.
 
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