Misfire Troubleshooting

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Tornado said:
No facility to perform that unfortunately. No air compressor. Would need to take it to a shop.
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Just read this thread. Sounds like you have not tried bypassing the ignition switch which is easy to do, although from your swapping components it doesnt sound like an ignition problem. You have good compression so unlikely to be rings. If you swapped back to a single carb you could definitely rule out carburation - which is usually not the problem anyway since it's relatively easy to fix. My money is on a loose, cracked or worn guide. Might try a slightly hotter plug to see if that makes the misfire go away. If it does you have achieved a successful symptomatic treatment without solving the root problem. However you could then ride the bike for the season and perform major surgery next winter.
 
Fast Eddie said:
I’m gonna go out on a limb here and say... I think you have more than one problem.

Yes, the plugs are a bit darker than ideal. But not to such an extent that oil burning would cause a misfire. The smoking exhausts also point to oil being burned, but again, not to the extent it would cause a misfire.

A sticking valve might cause a misfire / very poor running though. You could take the exhaust pipes off and have a look up in the ports with a torch to see if the ex valves look particularly carboned up.

It would be a heck of a shame to spend a load of time and money rebuilding the top end only to find it still has a misfire when you’ve done... that would NOT be good for the soul! So I’d definitely test out both carbs one at a time on your single carb manifold, before stripping the top end, if I were you.

It‘s kinda difficult to understand what someone means by ‘misfire’ or ‘herky jerky’ etc via text, but generally speaking, such dramatic misfire type scenarios are down to either carb or ign 99% of the time IMHO (we’re not looking at a race engine and megaphoneitis here).

Its interesting that your misfire is at certain revs. That could link to certain vibration frequencies. That was the case when my Boyer back plate terminals broke and I was convinced my issue was carb related! I know your chirpy chirp was the left hand cylinder, but that’s gone now and the misfire remains, you can‘t tell for sure when riding which side is misfiring. In fact, if it’s causing a pronounced ‘herky jerky’ I’d say it sounds like both cylinders are misfiring. So you COULD still have an ign issue.

And although your carbs have been sleeved etc, you COULD still have a carb issue. I don’t know how you ascertained the pilots are clear? If it was by inserting wire or similar then they should be clear provided you used the correct gauge wire and blew them out fully afterwards. However, if all you’ve done is squirt carb cleaner through, all you’ve ascertained is that they are not FULLY blocked. You have not ascertained absence of a partial blockage that could effect flow rates. FWIW I chased carb issues for a whole season once only to discover it was machining swarf in the pilot jet, swarf from having them bored and sleeved! As already mentioned by a previous poster, the best way to diagnose carb issues is to mark your twist grip, you can then ascertain whether or not the misfire is related to a particular throttle position, and therefore a particular carb circuit. Revs don’t tell you that, at 3,000 rpm you could be at 1/8 throttle, full throttle, or anything in between.

Sorry for waffling on so long (lockdown symptoms) but I’m just trying to say that I think you need to do more problem solving BEFORE stripping the top end.

All only IMHO of course.
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Misfire isn't just at certain rpms...I said most noteable at lower revs riding along at steady street speeds...b/c I can feel the effect much more without all the wind noise/road vibes and overall bike inertia when at high speed.

As for carb work, was last into them a week ago. Needles were/are correctly seated (viewed them from above the open carb tops after fitting on bike) and in middle grooves. Floats were at 2mm as per spec, but tried moving them up to 1mm below bowl edge as per the Bushman Guide rec. I also did his drill out of the blank side on idle jet to gain full access to both sides of the jet. I could see light straight through the brass jet. And could poke a straw all the way from fuel pick up point underside of carb body to the 90 deg bend just at the jet. Spraying from the fuel pic up opening give two solid sprays out the two tiny throat holes. Not sure what else can be done more thoroughly than that?

The smoke is worrying me more right now than the misfire. I agree these may not be related issues....one a carb/ign issue and the other something wrong inside the head/cylinders. Fact bike was not smoking until this week, 6k fresh miles since getting her two years ago, tells me something has recently happened...and smoke is noticeably worsening since testing earlier this week, now clearly visible at idle and even appearing on RH a little. I'm thinking rings are failing or headgasket letting oil into combustion chamber. Cannot a HG leak, causing loss of compression, give a misfire? Oil burn?

The head has not be removed since I got the bike. Likely has a lot of coke built up. Would be good to get in there to more thoroughly investigate and clean it up and see if there has been a re-bore previously. There's a good chance HG is 45+ years old.
 
OK, a potential fault....I pulled tank, intake rocker cover. Some pooled oil at bottom. Added a 100ml or so of fresh oil. Didn't disappear for over 15 min, though perhaps it lowered a bit. Mopped out remaining and had a close look around....is this the infamous drain hole sitting between the valves? Looks/feels more like a dark smudge. Could not poke it with anything (WD40 straw, tie wrap, piano wire). Wrong place to be looking? What's that dark bit towards upper right corner of intake box in first pic?

Misfire Troubleshooting


Misfire Troubleshooting
 
what level do you see on oil dipstick , has the level changed suddenly , overfull will make smoke as you know ... still think miss is one of two functions mentioned several times , too easy to come up with worst case scenarios , make damn sure on simple obvious things first , run both carbs as singles to prove they good , then back to electrical .... sorry starting to get so repetitive , just that I have gone in circles many times, when in your shoes, thinking worst case and yet manage to have eureka moment without tearing into engine yet .....
 
Tornado the drain hole is about 4 o'clock on the right intake when viewed from the rear of the head. Not easily visible with spring in place. Have a loose head handy if you need a pic.
 
The drain hole on the intake side is on the RH side all the way to the back of the head. Here's the picture of the barrels with the hole on the RH rear of the barrels it's the small hole in the picture

Misfire Troubleshooting


and here it is in the head.

Misfire Troubleshooting


I would go with a fat bronze wound low E string... if you can fit it to get the best possible clearing of that hole as you can get. IF you can't get the fat guitar string to make whatever bend, then try a narrower one. You should remember that the engine facilitates the drainage with the sucking and blowing of the pistons moving air around inside the crankcase, but with the valve covers off there is no reason the oil should not drain...
 
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OK I managed to poke that hole. Used a #78 (or whatever is recommended as carb jet poker) wire, go it down around bottom of cylinder block before could no longer push further due to bendiness of wire. Tried a thicker safety-wire bit, but it had more trouble going more than 3 or 4 inches...too many pre-exisiting bends in the wire. Will rummage for something better.
I did try watching the hole as I poured in more oil. Could see level above hole, then over a minute or two, I could see the oil surface change shape as top of hole exposed. Just doesn't seem like much flow...why is this drain so small? Is there a suction effect helping to draw oil down when operating?
 
Since everyone else who is responding has gone down the "It's the carburator rabbit hole", I'd like to add something to my advice that has been part of those other people's advise.... Essentially, do the investigations that allow you to check a possible causes before you just tear the engine apart. You never know if you find something stupid like this possible clogged drain hole and can simply clear it with a guitar string, pop the valve cover back on, and be fixed again... Give the less invasive diagnosis first priority...

I would poke the head's drain hole, put in NEW plugs, then eventually run the bike for a while. (maybe take a ride going uphill first in case it dies so you can coast home) It will take a while for the oil to burn out of the exhaust pipes, so you won't see the smoke clear up for at least 50 miles, but the misfire may disappear if that drain hole was the problem.

If I still had the miss after the hole poking manuver, I would change the oil, and strain it into the pan so I could look for some sort of broken metalic parts. (All of this kind of stuff is investigative without tearing the engine down as I said)

Again, if I still had the miss, I would pull the exhaust pipes and the intake manifold, then roll the engine over looking at the valves through the ports looking for some clue to oil infiltration down the stem or around the guides.

If nothing in the head shows any sign of oiling, and poking the hole has no effect, I think NOW you are in disassembly mode. I would hope it's just a leaky head gasket, so you pull the head and all you need to do is some clean up and a new gasket, but it could be the dreaded circlip issue too, and you'll be able to see the telltale sign of cylinder wall scoring with the head off and the pistons all the way down.

Other than that it could be a broken ring, but that wouldn't explain why both sides smoking really... and I would think the broken ring would smoke like crazy too...

As I said previously, you'll know when you've exhausted all the simple explanations that it's time to pull the head off... The silver lining in these things is that once you pull an engine apart, it's no long a mechanical mystery you bought from some guy claiming it's worth $25,000. dollars. It's a bike you've had apart and inspected things like the cam, the followers, the amount of rod play. My own commando makes an incredible sound from about 50 to 85mph. It just winds up and sounds really perfect about right there. I often marvel at the sound, and the fact that I built it and it didn't blow up,... yet! If I could do it way back when I first rebuilt it without any help, nor any internet, you can do it easily... except for the drain on the wallet of course...
 
Craig said:
what level do you see on oil dipstick , has the level changed suddenly , overfull will make smoke as you know ... still think miss is one of two functions mentioned several times , too easy to come up with worst case scenarios , make damn sure on simple obvious things first , run both carbs as singles to prove they good , then back to electrical .... sorry starting to get so repetitive , just that I have gone in circles many times, when in your shoes, thinking worst case and yet manage to have eureka moment without tearing into engine yet .....
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Dipstick typically a wee bitover low level mark. Doesn't seem to change much in recent past. This is fresh oil a month ago. 20w50 BelRay EXP synth, one of the types found to a good'on in Comnoz's scar testing.
 
Tornado said:
OK I managed to poke that hole. Used a #78 (or whatever is recommended as carb jet poker) wire, go it down around bottom of cylinder block before could no longer push further due to bendiness of wire. Tried a thicker safety-wire bit, but it had more trouble going more than 3 or 4 inches...too many pre-exisiting bends in the wire. Will rummage for something better.
I did try watching the hole as I poured in more oil. Could see level above hole, then over a minute or two, I could see the oil surface change shape as top of hole exposed. Just doesn't seem like much flow...why is this drain so small? Is there a suction effect helping to draw oil down when operating?
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Tornado, I'm with Craig. Put the single manifold back on a test each carb as a single. You don't need to connect the air filter etc. as I reckon you'll quickly know if there is a difference, even at tickover.

Regarding the over oiling, I found this thread from last year which may help. https://www.accessnorton.com/NortonCommando/help-with-blowing-smoke.27835/page-2#post-430457 ( and I realize you have a pressure gauge fitted) but you never know.

You also mentioned somewhere that when you changed the inlet valve seals in situ that the old ones crumbled & fell apart. I wonder if a piece of that has found it's way into the drain passage? Personally I would be more inclined to use something to suck on that hole rather than potentially ram something further into it. (which I think turns 90º further down anyway) So maybe get a couple of feet small diameter hose which will fit into it tightly and give it a suck? You may not get the obstruction out, but you'll know if it's clear or not.




Rooting for you.

cliffa.
 
cliffa said:
Tornado, I'm with Craig. Put the single manifold back on a test each carb as a single. You don't need to connect the air filter etc. as I reckon you'll quickly know if there is a difference, even at tickover.

Regarding the over oiling, I found this thread from last year which may help. https://www.accessnorton.com/NortonCommando/help-with-blowing-smoke.27835/page-2#post-430457 ( and I realize you have a pressure gauge fitted) but you never know.

You also mentioned somewhere that when you changed the inlet valve seals in situ that the old ones crumbled & fell apart. I wonder if a piece of that has found it's way into the drain passage? Personally I would be more inclined to use something to suck on that hole rather than potentially ram something further into it. (which I think turns 90º further down anyway) So maybe get a couple of feet small diameter hose which will fit into it tightly and give it a suck? You may not get the obstruction out, but you'll know if it's clear or not.




Rooting for you.

cliffa.
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No possible to get a tube to seal in hole with the valve spring seat partially obscuring it. I guess it could be blown out from inside the timing chest if that is where the lower end appears. I did manage to stuff a very long straw (from the SeaFoam can I bought yesterday) quite a long way down...at least to bottom of cylinder block before it would go no further (perhaps a tight bend in the hole?). Also fired some WD40 while that straw was all the way down. Drained the sump and re-poured fresh oil into rocker tub...it seems to flow down at a rate of about 1 spring wrung per 1.5-2 mins. so it is flowing but can be sure this is normal flow for cold oil.

Will button up the rocker and have a go at fitting back my single carb setup...to appease the masses ;-)
 
Pull the head. Put it into a machine shop familiar with Nortons . $pend. Pull the barrels. Inspect rings , pistons and bores for damage or blowby. If so , put into that machine shop too. The camshaft is now visible. Inspect for any left side lobe breakup. Pull all oil filters first too and after. Inspect.
 
Before you go tearing the head you may want to consider renting a camera scope from Princess Auto
 
acadian said:
Before you go tearing the head you may want to consider renting a camera scope from Princess Auto
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You can get very inexpensive "endoscopes" or "boroscopes" that plug into your phone and work with an app that you download. They are good enough to show you if your bores are scored. But if you have equal and correct compression I doubt that is the cause of your problem. Wonder if one of the valve seals you installed has destructed. As many posters have said, retracing your steps is often diagnostic.
 
Put an amazon order in beginning of this week for a $15 CDN borescope from China.....we'll get some inside views of all sorts of things....
 
Maybe its just as simple as a valve not seating properly it would cause a misfire when running.

Ashley
 
You indicated in your first post you had done 6K miles with the bike. Has it had this misfire issue the whole time? In other words, do you have an existence proof they CAN work? I had a set of Amals on a new to me '73, and while I had no idea what I was doing, nothing I did made them run right. I rebuilt them, had them sleeved, replaced needles, jets, floats, etc. Nothing seemed to make em work right. I finally got feed up and purchased a new set. Put the new ones on, and all the issues disappeared. I guess my point is, if they've never worked - maybe you should consider a new pair? BTW my 2 cents is the huffing/missing thing is a carb issue.
 
Tornado,
There are two further carburettor possibilities... It is possible to get fuel starvation and flooding as a consequence of the isolastics , try changing clearances or tension . The other is you can have a more or less completely blocked carburettor filter and it will still flood on tickling but be erratic in fuel supply.
Do apologise for my seemingly elementary 'tips' but have personally have had weeks wasted on them . BTW to be sure you donot have a dropped needle you have to either sight through the carb intakes with mirror and torch or by feel if you have sensitive fingers
All the best with it
 
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