Strange Norton missfire/backfire

So Ive got a 1974 850 which has always had a strange missfire. It just got worse when I took it on a long trip I noticed when it started backfiring out of one cylinder. When I am cruising at 60 mph it is very smooth with no missfire but gets worse as the rpms rise. It is most noticeable when cruising through town going about 30mph in third, it will jerk forwards and back and cough like the engine is fighting itself. But to make any of the missing go away all I have to do is accelerate and it will not miss at all. I have replaced the points a year ago along with the condensers. The point gap is always set to .015in and I check the ignition timing often and always set it at 28 degrees btdc. The carburetors have new standard jets and the mixture is correct because the plugs always have the right tanish color to them. Even with all of this set correctly it has always missed at 30mph or any low mph but if it is lugged in 4th going about 40 it doesn't miss. I was thinking about getting the power arc ignition to just replace the system completely, what do people think about it?

Sure sounds like electrical or air leak or float level or slide cut out but not points which i live with too, just fine in the low down almost lugging states. A stuck advanced might do this but my bad AAU's just didn't return to idle but didn't missfire.

30 mph in third gear? you are lugging the motor, way too much, you need to keep it spinning over 3000 rpm

why would anyone want to be on a big bore parallel twin at such low rpm?

under some 3000 or so rpm the isolastics are not in match frequency with the frame, hence vibration

just a guess, but when this amount of shaking directly to the frame is going on under 3000 rpm, could be that there is an insecure electrical wire connection problem, somewhere

could be a loose ground, maybe the wires to or from a coil are not securely push on, maybe the wires running down the front frame tube and into the back of the timing cover are time and fatigue worn and fractured enough to lose their connection when you allow the shaking to take over by riding lugging the motor, such as 3rd gear at 30?

I think your problem IS an electrical connection somewhere and you just gotta start getting intimate with all your wiring to find it, a labor of time and frustration, hopefully ending with an Eureka moment of disovery

just some thoughts from one who has been there

Since an ignition problem would likely be worse under load- because that is when the voltage requirement is highest- I would guess you must have mixture or mechanical problems.
I doubt an electronic ignition would help from the description.

It could be related to vibration or grounding issues. Have you checked the ground wire connection to the head. They notoriously break. Jim

Ignore Dances novice advice as 30 mph in 3rd is over 2000 rpm and is not lugging condition if just in steady cruise not trying to accelerate hard or pulling up a good steep. 3000 rpm held long in 3rd gear will dry out the sleeve bushes on shatts so wearing out the clutch wobble stability before its time. Over 2000 rpm is cam break in rpm and enough to surf the lifters on cam lobes and pressurize rod shells fine on low throttle loads. I do this routinely and have checked closely the nil wear effects evidence on direct exam of issue areas and sump magnet barely visable fuzz. There is a motto in Brit Iron when it acts like fuel smells like fuel and sounds like fuel look to electrics first. I had similar issues that turned out to be a shorting ground strap in the tail lens area. Sometimes plugs get fouled so only takes new set to clear up the bad zone of sparking regularly. I am now spoiled by pure factory Combat Trixie for one ^step on* starts cold or hot, steady idle no throttle by time sump pumped out 30 sec or so and low throttle in congested pedestrian clogged Eureka Springs European like caravel town and creeping on loose steeps on high-ish throttle to climb and squeeze pass tree falls no problemo then roll on throttle w/o stumble to rooster tail the rest of the way up in what ever gear allowed 2000 rpm at slowest speed. Choke cable hole might be air leak, cross over balance tube or even the exhaust nuts sucking back in. God I love a plain old Combat ease of wide scope use even if it can't beat modern cycle in race like conditons that ain't how I live routinely on mine. Always worth a look with tugging and jerking loom after dark lights out.

I had a weird missfire that I finally tracked to old spark plug wires.

Balance tube between carbs gets my vote.

My guess is it needs new needles and needle jets. The jets get beat on by the needles and it soon wallows them out causing richness ---> stuttering around town at low throttle openings.

I just replaced the plugs as they where old and it did nothing to help however the spark looks pretty pathetic, it is barely blue and I had a hard time even seeing it on the missfiring cylinder. The needle jet and needle was new a year ago. I also tried playing with the idle mixture screw on the missfiring cylinder and it didn't seem to be affecting much. The bike starts very easily cold but when it gets hot it takes a few kicks. I have the needles set in the middle groove so should it be lower?

Take the tank off and clean up every electrical connector or replace with modern ones. Change condensers too. Check battery strength/age.

hmm now sounds like might be a condensor breaking down. Takes a real mechanic to figure out a Cdo, even if ya don't want to feel like one just to ride off into sunset. I've had glass fuse go bad inside terminals for intermitent mis fires at some rpm resonance zone, so can't have two things at once each doing the same thing.
I've a word that covers this limbo state - ugh.

hobot said:

Ignore Dances novice advice as 30 mph in 3rd is over 2000 rpm and is not lugging condition if just in steady cruise not trying to accelerate hard or pulling up a good steep. .

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Steve
I don't think "Dances" if you are refering to Dances with shrapnel is even on this thread. Unless I am missing something.
Got a bone to pick with him?
CNN

I changed the condensers about a year ago but even after I did that the points would still occasionally spark and the book said something about the condensers controlling this? The points have the full 6v so it would seem to me like its either the coil or condenser, which one would restrict a full spark?

Ugh, alls I know is I've had new condensors bad out the box so had to dig old ones out of trash to relieve my mind I wasn't the worse mechanic ever. Coil I've had go bad usually just quit when hot or shorted to case but of course could vibrate or heat for half fast failure mode. Be a bit careful if leaving key on to check this stuff as ballast can get paint burning hot and so can the coils. Bypass kill switch occurs to me just now as shot in the dark to feel useful long distance.

A cough or a backfire, is usually a sign of a lean spot in carburation. I'd be checking the needle jet sizes and the needle shapes. I don't use petrol in my racer, but methanol is just as critical if you want real speed out of the motor. I use Mikuni 6DP6 petrol needles, and I made my own needle jets. Your needle jets will be 10 thou of an inch smaller (probably 0.107). The trick is to ride the bike around twisty roads with a lot of throttle changes. Lower your needles until the bike coughs when you use 3/4 throttle in corners , then raise the needles one notch. The motor should clear, and accelerate cleanly without a cough or backfire, right throughout all changes of throttle position. I'm assuming that you set the float levels to makers spec. and have adjusted the idle settings. and have checked to see that the slides are correct to the manual, and have the same cut-away. It is possible you have an electrical problem, but from your post it seems unlikely. Check the tank breather.

1up3down said:

30 mph in third gear? you are lugging the motor, way too much, you need to keep it spinning over 3000 rpm

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Nonsense. I cruise at 35 mph in 4th gear. The 850 handles it fine, it just buzzes a bit.

why would anyone want to be on a big bore parallel twin at such low rpm?

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Fuel economy.

The OP's problem sounds like clogged pilot jet to me. Surging and backfire just off idle that goes away when you twist the throttle.

Postby acotrel » Mon Sep 17, 2012 3:19 am
A cough or a backfire, is usually a sign of a lean spot in carburation. I'd be checking the needle jet sizes and the needle shapes. I don't use petrol in my racer, but methanol is just as critical if you want real speed out of the motor. I use Mikuni 6DP6 petrol needles, and I made my own needle jets. Your needle jets will be 10 thou of an inch smaller (probably 0.107). The trick is to ride the bike around twisty roads with a lot of throttle changes. Lower your needles until the bike coughs when you use 3/4 throttle in corners , then raise the needles one notch. The motor should clear, and accelerate cleanly without a cough or backfire, right throughout all changes of throttle position. I'm assuming that you set the float levels to makers spec. and have adjusted the idle settings. and have checked to see that the slides are correct to the manual, and have the same cut-away. It is possible you have an electrical problem, but from your post it seems unlikely. Check the tank breather.

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Useful info for me acotrel, as Trixie has intervals of rough sounding on some lower speed throttle, especially cold leaving drive way twisties and swoops. Dances and I go round and round but meant 1up/3dn who also tangles with me here. Lugging ain't happening above 2000 rpm unless nailing in 4th from there, but if feathery throttle up w/o rough pulling then within normal Commando health zone or shift down before reving up harder. Pilot jet is always a suspect on low throttle upsets.

Also coil clamps get overtightened shorting out a coil ? 2 -4 lb. tightening is like next to nothing. Betcha the HT leads to the plugs are internally oxidized too. Just wondering when was the last time the points plate was off to grease all the moving/sliding/pivoting parts of the neglected AAU unit ? Pilot jets clog from disuse for sure.

Hobot,
If you read Phil Irving's 'tuning for speed', it says that if you run alcohol fuelled motors rich, they still make good power. Our guys read that as being -'run alcohol rich'. Alcohol hides up tuning errors, but to get the most out of it, it is as critical in response as petrol. I use a petrol needle and run it as lean on the needle as I can get it without the motor coughing at 3/4 throttle. I don't use petrol, the motor runs too hot, and you don't get the bike performing so well. But there is one thing which should be noted. In the fifties and sixties in Australia we ran everything on methanol, so our guys were used to fast racing on bad circuits. They did very well in the UK, but the Poms could get their Nortons going just as fast on petrol, as we could on alcohol. The best fuel that I have used was BP JA. It was about 60% methanol 40% benzene. The benzene is of much higher calorific value, and the methanol freezes the charge with its high latent heat of vaporisation. They used it in Maseratis in the early days. ( It is poisonous )

Well it looked like the miss firing cylinder's point set retarded itself in the middle of my ride. I advanced it to where the bike seemed to like it (as I don't have a timing light at the moment) and the miss went away. It will still do its weird jerking at 30mph but it is much less (doesn't affect speed at all) so it leads me thinking that it is still something with the ignition. The carbs should be fine because they have all the same parts just new jets of the same size. It starts great, idles fine, accelerates cleanly at any rpm and cruises fine at all speeds other then 30.