N15CS Misfire

Need help troubleshooting a new misfire on my 68 N15CS. Runs smooth up to about 3500 rpm in any gear, then starts to miss and skip. Smooths out if I accelerate hard past 3500 or if I back off the throttle. I have replaced the carbs with brand new Amals which makes the bike run and idle much better but hasn't cured this problem. Replaced the plugs, condensers, and points. Ignition cables and wires look good. Plug color looks pretty good so I don't think they're fouling. Any ideas would be appreciated.

This is happening on BOTH cylinders, or just one ?

Try lifting the carb needles a notch each.
Should only take about 2 hours to find it doesn't make any difference...

Odd problem otherwise.
Your kill switch isn't suffering vibration problems on the handlebars ?

If you had a spare new good magneto sitting on the shelf, be a good time to try it out.
Eliminate a few possibilities, although a long shot.

Thanks for the reply.
It's difficult to tell if it is both cylinders or not. I was thinking about lifting the needles up a notch like you suggest but it doesn't really feel like it's starving for fuel. Almost feels like the ignition is shorting out or something. Besides, it did it on the old carbs and on the new ones. I may give it a try anyway and see if it helps.
The bike has capacitor ignition with an ignition switch, not a kill switch. The odd thing is that it occurs right at 3500 rpms. I was thinking maybe the mechanical advance unit is sticking? If all else fails, I was thinking of installing an electronic ignition unit.

A lot of things needed going from a 3 to a 31/2 slide ( cuttaway ) which makes it happen earlier .
Actually , gets the suction going earlier , so draws fuel better .
Were a few wierd ideas around , notches & shapes & bevels .
Thry needles in all positions briefly to I.D. affect .

Make Notes , in a notebook . Labled CARBS ( or that part of it ) .

The dreaded cupped or notched Main Jets , as per Mk II are anotther trick .

Is a artical or two on Concentrics on the internet , might see if I can find one with mods / blueprinting Concentrics .

This may sound silly, but are you running a choke on the carbs? If the choke has slipped, the engine won't rev past about 3500RPM, as you can't get enough air in. Make sure that when the choke is pulled off, the air slides retract all the way into the throttle valves.

Oh, yeah. To engage the choke, you slacken the cable. This lowers the air slides into the airway, reducing air flow and enriching the air/fuel mixture . Pulling on the choke lever pulls the choke off.

What number is your N15? My G15 is G15CS/123594, with a July build date. It's titled as a '68, but is actually 1967 production.

BillT:
Yes, I have checked the chokes and they are good. The carbs are synched and operating freely. The weird thing about this problem is that it started after I dismantled and cleaned the old carbs. After I installed the brand new carbs, it continued the same way. That's why I'm not convinced that it is a carb issue.

My N15CS is a '68 but it's titled as a '69 for some reason. I believe they stopped production in '68. I don't have the vin numbers with me right now but I think it is 1239?? and they are matching numbers on the engine and frame. I just bought this bike last year and it really runs well except for the current issue. I actually had one of the first 200 "Norton Scramblers" produced when I was in high school back in 1968. It was a 1963 G15CS and I paid $400 for it (a lot of money back then for a 5 year old bike!). It's very much the same bike except that it had a magneto and monoblocs. As was the style in the 60's, I chopped it and had extended chrome forks. Very cool!!!

Double-check your ignition timing.

grandpaul said:

Double-check your ignition timing.

Click to expand...

Preferably with a strobe, and to ensure it is advancing, and retarding, when it should be.
And if you give the throttle a quick blip, that the spark keeps sparking around that 3500 rpm zone.

jgmotto said:

BillT:

My N15CS is a '68 but it's titled as a '69 for some reason. I believe they stopped production in '68.

Click to expand...

Yes,they stopped production around 124500, but 5 more were built a little over a year later for one of the major European distributors, in the 131xxx range.

I knew next to nothing about these bikes until a couple years ago, and they made quite a few variants using this frame.
There was the original G15/45, which had issues
There was the '63 G15 'N', that Norton Scrambler with the funky forks (part Roadholder, part Teledraulic)
There was the G15 MkII, which looked like the G15/45, except with the Atlas motor
There was the G15 CS and Norton N15 CS, which ran pretty much from '64 to '68
There was the AJS 33, which was a G15 MkII, with AJS badging
There was the G15 CSR and 33 CSR, which had rearsets, reverse cam plate in the transmission, swept back pipes, and a few Chrome bits. Some late ones even came with 'apehanger' handlebars.

Well, I spent just a half hour this morning to open the carbs up and lift the needles one notch higher. Problem solved! Must have been starving for fuel at mid-throttle range only.

Now it runs great again, smooth & idles good. Thanks, Rohan, you hit it right on the head!

Wow glad that worked out so fast for you.. I have a soft spot for these bikes as the 1st bike I owned was a 66 Norton version of one but put togeather from many parts so not origianal at all but still I wish I had it now. BillT could you post some photoes of a CSR version? I would love to see a factory bike with rearsets from that era, and Mr Moto I personaly will forgive you for chopping & chroming a Norton way back when, hell I spent most of that era in the mountains of North California running around naked partying with 200 other people & the Hells Angels who as a ten year old seemed like "nice guys"... I had a rather unorthodox childhood. Thank Dog!

Here's a pic of a G15MKII and a G15CSR:



Specs on the CSRs changed from year to year, but usually had chrome fenders, chrome tank, low bars, swept back exhaust pipes, and rearsets with a unique trans cover and cam plate, so the rear-facing lever would still be standard AMC pattern of 1-up, 3-down