Amal carbs and spark plugs issues and concerns

Hi newbie Mk3 owner here. Purchased mostly reconditioned bike and have only put 50-60 miles on it. Starts and idles magnificently. Ran beautifully at 1st, strong hard pull, but weak on the top end. Then started missing on mid to upper range 2nd gear on and gurgle almost popping on rapid decel. So I cleaned and gapped the plugs. Which for some strange reason are Champion RN9YC. Not sure if PO did that because of the colder climes where he lives (Indiana). I'm in Socal.

Back when this bike was built In had my 1st real bike (aside from a Rupp minibike when I was a kid). I built out a Yamaha xs 650 to some pretty cool specs and had a lot of fun with it. Did all the basic tuning on twin 34 mm Mikunis myself with a few screwdrivers.

So my questions are this. Can I similarly adjust these rebuilt original Amals to solve my issues or do I need to take them off and go batsh*t crazy to get and keep them performing well? Secondly, since I have resistor plugs that run hot with wide gaps, what would be an optimum swap out for me here in sunny CA?

As much as the wife will be on my case of I keep spending on this girl (gotta make her my own you know) should I just throw in the towel and throw down on a couple of Keihin fllatsides so I can have some peace of mind and better top end?

Please talk to me as if a 5th grader as the NASA science equations can tend to hurt my head sometimes.

The original plug for a Commando would be NGK BP7ES which is one level colder than your Champion. That might cause the issue, but more likely you have an air leak at the intake manifold. Check the crossover tube between the intake manifolds for leaks. The other typical is the nuts holding the carbs to the intake manifolds have been over tightened causing the carb flange to warp. Loosen those up just a bit and see if that cures the problem. Any throttle sticking problem? The other thought is valve clearance set to tight and getting some air past….particularly the exhaust. Valve clearance for all Commandos except combat engines is .006” intake and .008” exhaust.
If it is the Amals worn out I would go with new Amal Premiers. When I rebuild my ‘72 twenty six years ago there were specialists that would bore the carb body and put a stainless sleeve on the slide. My guy pasted away years ago, Mike Gaylord. Not sure I trust anyone for this anymore. Mine have another 42,000 miles and still have almost no wear.

What kind of ignition is fitted? The reason I ask is most aftermarket electronic ignition units eg Pazon need either a resistor cap or resister plug to function. This may not be your current issue but if you swap out for the standard BP7ES plug and have after market ignition then you may introduce in a new problem.

I have Pazon ignition, NGK BP7ES plugs and 5 k resister NGK plug caps. Also using old fashioned spark plug wire type leads. The pencil lead fill spark plug leads are only trouble on our bikes.

Popping on the overrun can also come from leaks at the header pipe. Often if you have a balance pipe. Check the clamping rings are tight in the head and look for signs of exhaust leaks at the joints. I use high temp red RTV to seal my headers.

Lastly on the carbs. I agree with the earlier poster. Look for intake leaks etc and if the carbs are worn out I'd go for new Premiers from someone like Greg Marsh since your in the USA. You have a Mk111 which had a large plastic airbox filter as standard and different jetting to the earlier 850 models. After all these years who knows what filter you have and therefore what jets you should use. Looking at the manuals suggested below to see what the original settings would be.

My own personal opinion is that starting with what Norton recorded is the way to go. Suggestions often go pretty exotic pretty fast online and personally I'd start with the basics by the book. My bike has Premier with 19 pilot jets and setup exactly as per the book. Then all I needed to do was raise the needle on one carb only one notch because it looked very lean on low to mid rpm. Now runs and starts perfectly. I have retained the chokes. Many people don't but I personally think if your running without a choke then you must be a little rich at low settings.

Plus if the bikes just recently been put on the road it could be something as simple as a bit of crud blocking a fuel pipe or jet.

I'd buy a manual. The Norton workshop manual. If you can find it the Clymer manual or failing that the Haynes one which is OK but not the best.

arcpoint said:

Hi newbie Mk3 owner here. Purchased mostly reconditioned bike and have only put 50-60 miles on it. Starts and idles magnificently. Ran beautifully at 1st, strong hard pull, but weak on the top end. Then started missing on mid to upper range 2nd gear on and gurgle almost popping on rapid decel. So I cleaned and gapped the plugs. Which for some strange reason are Champion RN9YC. Not sure if PO did that because of the colder climes where he lives (Indiana). I'm in Socal.

Back when this bike was built In had my 1st real bike (aside from a Rupp minibike when I was a kid). I built out a Yamaha xs 650 to some pretty cool specs and had a lot of fun with it. Did all the basic tuning on twin 34 mm Mikunis myself with a few screwdrivers.

So my questions are this. Can I similarly adjust these rebuilt original Amals to solve my issues or do I need to take them off and go batsh*t crazy to get and keep them performing well? Secondly, since I have resistor plugs that run hot with wide gaps, what would be an optimum swap out for me here in sunny CA?

As much as the wife will be on my case of I keep spending on this girl (gotta make her my own you know) should I just throw in the towel and throw down on a couple of Keihin fllatsides so I can have some peace of mind and better top end?

Please talk to me as if a 5th grader as the NASA science equations can tend to hurt my head sometimes.

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The carbs you have, if not worn out, and you don't have an air leak, will take you well past lose your license speed. You need to think of them as four carbs in one. Since it starts and idles, the first part - pilot circuit is probably close. It works up to about 1/8 throttle. From 1/8 to 1/4 throttle, it's the slide cutaway which - 3-1/2 stock and if 3-1/2, no need to think more about it. Next is 1/4-3/4 throttle. This is the needle jet, needle, and needle position. Since most of the time this is where you are riding, the needle wears and to a lesser degree, the needle jet. Finally, 3/4 throttle to wide open is the main jet - you are most likely riding here (or won't for long on the street).

If you are having trouble between 1/4 and 3/4 throttle, a simple thing to try is to put the needle clip in a different slot. The lower the clip on the needle the richer.

If it gets better - good, if it gets worse, go the other way.

Saying mid-range doesn't convey useful info. It's best to mark you throttle so you can say are what opening you are having trouble.

Of course, there are many other things other than carbs.

Do you have points of electronic ignition. If electronic ignition, which one. Is your battery good? Are your valves adjusted?

From about 3500 rpm to 6000 it should pull very strong. Explain more about what it's doing.

At the top of this forum, there is a thread "Technical Information". It's worth taking a look.

Also you can get manuals here for signing up for an account and paying a one-time 5 pounds charge: https://classic-motorbikes.net/manuals/

This is useful: https://amalcarb.co.uk/technical/rebuilding-mark-1-concentric-carburetter

The Amal tuning guide is attached.

Carbs can be electrical problems and vise a verse, put in the right plugs be the first thing, what ignition system is it running, look for air leaks, if the bike has been sitting could be a blocked pilot jet, a new bike too you I be going over everything, a fully charged battery, a lot of people have gone down the road of running different carbs, myself included but have gone back to my old Amals, my 850 runs so much better with twin Amals, coils could be faulty, bad plug leads, in other words could be anything, jets and needles do wear out, manifold gaskets leaking, you name it, just go through one thing at a time till you find/fix the problem.
The wonders of owning older British bikes and who knows what the old owner has done, could have had a problem he couldn't workout, who knows, could well be something so simple to fix, timing not set right, float levels not right, its all a guessing game really.

Ashley

Wow, first off, a huge thank you to Ashley, Greg, John, and Illf8ed for the insightful responses! The ignition is a Boyer electronic with Boyer voltage regulator. In 2021 the carbs were given new floats, jets, and needles. The resistor caps and spark plug wires seem typical new old school but wouldn't know a resistor cap or wire if it smacked me in the face. (You can tell I've been away from bikes for a while, a long while). The battery is strong. I will check for leaks at the manifolds and headers and everywhere else plus make sure of no over tightening on the carbs, etc. Thanks hugely for the links and the Amal tuning guide. I will study that pronto. I want to ditch these plugs for more standard fare but am nervous about getting a non resistor plug and causing problems. The bike also got new Barnett clutch discs and steels in 21. But have had a few times when putting into 1st it wouldn't engage, yet wasn't in neutral. Had to kick it up into 2nd and back to 1st to engage. Any thoughts on thos?

arcpoint said:

Wow, first off, a huge thank you to Ashley, Greg, John, and Illf8ed for the insightful responses! The ignition is a Boyer electronic with Boyer voltage regulator. In 2021 the carbs were given new floats, jets, and needles. The resistor caps and spark plug wires seem typical new old school but wouldn't know a resistor cap or wire if it smacked me in the face. (You can tell I've been away from bikes for a while, a long while). The battery is strong. I will check for leaks at the manifolds and headers and everywhere else plus make sure of no over tightening on the carbs, etc. Thanks hugely for the links and the Amal tuning guide. I will study that pronto. I want to ditch these plugs for more standard fare but am nervous about getting a non resistor plug and causing problems. The bike also got new Barnett clutch discs and steels in 21. But have had a few times when putting into 1st it wouldn't engage, yet wasn't in neutral. Had to kick it up into 2nd and back to 1st to engage. Any thoughts on thos?

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Ya, live with it for now - you're not ready for adjusting the spring or linkage that is most likely causing the problem It's not hard, but it's inside the outer gearbox cover and drives many mad. On the other hand, if you have the bike running on the center stand and the rear wheel is turning fast, then it's probably a simple clutch adjustment or possibily needing the primary oil changed (dragging clutch).

arcpoint said:

Wow, first off, a huge thank you to Ashley, Greg, John, and Illf8ed for the insightful responses! The ignition is a Boyer electronic with Boyer voltage regulator. In 2021 the carbs were given new floats, jets, and needles. The resistor caps and spark plug wires seem typical new old school but wouldn't know a resistor cap or wire if it smacked me in the face. (You can tell I've been away from bikes for a while, a long while). The battery is strong. I will check for leaks at the manifolds and headers and everywhere else plus make sure of no over tightening on the carbs, etc. Thanks hugely for the links and the Amal tuning guide. I will study that pronto. I want to ditch these plugs for more standard fare but am nervous about getting a non resistor plug and causing problems. The bike also got new Barnett clutch discs and steels in 21. But have had a few times when putting into 1st it wouldn't engage, yet wasn't in neutral. Had to kick it up into 2nd and back to 1st to engage. Any thoughts on thos?

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A resister cap will usually have 5 k or similar written on it in tiny letters. Have a hard look. The plug lead you want is the one with real wire in the core. Most modern plug lead is just carbon core made for cars fitted with radios etc.

Sorry last owned a Boyer around 30 years ago so don't remember if it needs a resister plug or cap.

If the bike ever stops totally after 15 mins of running and then restarts after about an hour cooling down and does this again and again come back and ask about broken wires in the Boyer pickup down at the end of the cam.

ILLF8ED said:

The original plug for a Commando would be NGK BP7ES which is one level colder than your Champion. That might cause the issue, but more likely you have an air leak at the intake manifold. Check the crossover tube between the intake manifolds for leaks. The other typical is the nuts holding the carbs to the intake manifolds have been over tightened causing the carb flange to warp. Loosen those up just a bit and see if that cures the problem. Any throttle sticking problem? The other thought is valve clearance set to tight and getting some air past….particularly the exhaust. Valve clearance for all Commandos except combat engines is .006” intake and .008” exhaust.
If it is the Amals worn out I would go with new Amal Premiers. When I rebuild my ‘72 twenty six years ago there were specialists that would bore the carb body and put a stainless sleeve on the slide. My guy pasted away years ago, Mike Gaylord. Not sure I trust anyone for this anymore. Mine have another 42,000 miles and still have almost no wear.

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Having bored out Amil carb bodies with stainless sleeves sounds like a great way to run. Seems to me like someone could make a nice little cottage business out of doing this for the Norty population.

I do not know much about ignition systems. I use a Boyer which has fixed advance, with a double-ended 12 volt coil from a CB750 Honda. It is very reliable. When tuning the carbs, I always use a number 3 slide, and adjust the idle with the idle screw until I get really good even tick-over. The needles and needle jets are critical. With Commandos two needle jets are specified for use with petrol - a 0.106 inch ID and a 0.107 inch ID - I think. I don't know what needles Amal use, but all needles are the same diameter at the parallel part. I prefer slow taper needles - (lean). To get the needles right, I usually start with them set high, then progressively lower them until I get the motor to miss when I ride the bike - then I stop and raise the needles one notch. I do not know what main jets are recommended for Commandos, but I would use the biggest recommended. For plugs, I usually use NGK 7 or NGK 8.
Pilot jets usually work in conjunction with pilot jet, slide cutaway and the idle screw. I use the idle screw to adjust low speed running. I have never needed to change pilot jets or air jets. Inlet port size affects the size of the needle jet which is required, and the taper on the needle. The taper compensates for loss of vacuum when the throttle is opened quickly. Bigger inlet ports lose vacuum quicker and need richer needles to get enough fuel into the motor. The depression on the needle jet affects fuel flow. For best performance, slow taper needles and smaller ports are usually better. I use Amal carbs with Mikuni needles. I do not place my trust in the quality of Amal needles.
My brother successfully races two-stroke and four stroke sidecars on speedway. We have almost come to blows over tuning. He has a friend who is a jeweller and can actually measure the internal diameter of needle jets. The guy also has very thin reamers and can adjust the size of the jets. A better way might be to use a flow meter.

acotrel said:

I do not know much about ignition systems. I use a Boyer which has fixed advance, with a double-ended 12 volt coil from a CB750 Honda. It is very reliable. When tuning the carbs, I always use a number 3 slide, and adjust the idle with the idle screw until I get really good even tick-over. The needles and needle jets are critical. With Commandos two needle jets are specified for use with petrol - a 0.106 inch ID and a 0.107 inch ID - I think. I don't know what needles Amal use, but all needles are the same diameter at the parallel part. I prefer slow taper needles - (lean). To get the needles right, I usually start with them set high, then progressively lower them until I get the motor to miss when I ride the bike - then I stop and raise the needles one notch. I do not know what main jets are recommended for Commandos, but I would use the biggest recommended. For plugs, I usually use NGK 7 or NGK 8.
Pilot jets usually work in conjunction with pilot jet, slide cutaway and the idle screw. I use the idle screw to adjust low speed running. I have never needed to change pilot jets or air jets. Inlet port size affects the size of the needle jet which is required, and the taper on the needle. The taper compensates for loss of vacuum when the throttle is opened quickly. Bigger inlet ports lose vacuum quicker and need richer needles to get enough fuel into the motor. The depression on the needle jet affects fuel flow. For best performance, slow taper needles and smaller ports are usually better. I use Amal carbs with Mikuni needles. I do not place my trust in the quality of Amal needles.
My brother successfully races two-stroke and four stroke sidecars on speedway. We have almost come to blows over tuning. He has a friend who is a jeweller and can actually measure the internal diameter of needle jets. The guy also has very thin reamers and can adjust the size of the jets. A better way might be to use a flow meter.

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I hear you on the Mikuni needles. Gonna give them a try. I'm not seeing the reasoning for the hot running resister plugs either. I know the PO wasn't using the choke and probably is set rich for quick start and idle, but I think hurt the top end and blackens the plugs.

Personally, I'd stick with Amal. Al's experience is from racing, not street riding, quite some time ago and he used methanol, not petrol. I've recently bought repair stuff from Amal and thee quality has been fine. They still support the old carbs. Actually, what I've had from them for my Norton is much better than the Keihin aftermarket stuff floating around for their carbs (which ive needed on another old bike I have).

Again, personally, I'd take the advice higher up, from people who regularly ride their Commandos on the street and/or (in Greg's case) make a business out of preserving them. I'd start from a clean pair of carbs in stock settings, with no leaks. If it is a carbs problem, that will get you close. If you're running with a different air filter, that may change things, but plenty of advice here.

Another thing occasionally noticed here is a carb problem often turns out to be an ignition problem! But get the carbs set up and if the problem persists, move on to that.

I had early problems when I first brought my Commando home. Advice here guided me. Mine has been reliable, fast and fun since. Just regular maintenance. So much so, I had to buy another old bike for something to do in the winter!

Mart UK said:

Personally, I'd stick with Amal. Al's experience is from racing, not street riding, quite some time ago and he used methanol, not petrol. I've recently bought repair stuff from Amal and thee quality has been fine. They still support the old carbs. Actually, what I've had from them for my Norton is much better than the Keihin aftermarket stuff floating around for their carbs (which ive needed on another old bike I have).

Again, personally, I'd take the advice higher up, from people who regularly ride their Commandos on the street and/or (in Greg's case) make a business out of preserving them. I'd start from a clean pair of carbs in stock settings, with no leaks. If it is a carbs problem, that will get you close. If you're running with a different air filter, that may change things, but plenty of advice here.

Another thing occasionally noticed here is a carb problem often turns out to be an ignition problem! But get the carbs set up and if the problem persists, move on to that.

I had early problems when I first brought my Commando home. Advice here guided me. Mine has been reliable, fast and fun since. Just regular maintenance. So much so, I had to buy another old bike for something to do in the winter!

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Thanks for the input. I agree with this platform. Getting tons of great insight here.

I was running a Boyar EI in my Norton for over 30+ years running non resistor caps and wire core leads without any problems at all and the standard Champion N7YC plugs (my Norton never did like running NGK plugs), but these days am running a Joe Hunt magneto for even bigger spark, I still run the Champion N7YC plugs and wire core leads, but with the JH the plugs are set with less gap at 18thu.
Don't worry about what Al writes as he has never ridden a road going Commando and it's been 50 years since he has ridden a bike on the road.
The stock Amal needles work fine, but depends on the mileage I always replace needles and jet every 3 or 4 years as vibrations through the manifolds wear the jets oval over time, Amal rebuilt kits are cheap maintenance.
As mentioned the pick-up wires that come out of the points housing can break inside the insulation and cause problems, I never had this problem, but others have.
Well set up Amals the Norton runs so well with them, they are a simple carb to work on and maintain.

Ashley

ashman said:

I was running a Boyar EI in my Norton for over 30+ years running non resistor caps and wire core leads without any problems at all ....

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But, a 30+ year old Boyer is not the same as a modern Boyer and you switched to a magneto a long time ago, so you must have had an analog Boyer. The digital Boyers are different, from the Boyer website:

"Why Do I Need To Use Suppressed Plug Caps?
MKIII and MKIV do not require suppressed plug caps for operation although we recommend using supressed 5000 ohm plug caps. Micro Digital and Micro Power units must be fitted with supressed plug caps. Plug caps fitted with suppression resistors are usually fitted to prevent radio interference. Radio interference (noise) can cause more complex electronic circuits like radios and computers to malfunction."

arcpoint said:

Having bored out Amil carb bodies with stainless sleeves sounds like a great way to run. Seems to me like someone could make a nice little cottage business out of doing this for the Norty population.

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There are/were folks offering amal rebore and sleeving services. Had a pair done only a few yrs ago by @bchessel in Ontario Canada...ran me about $75 each. Used a bronze sleeved slide. Unsure if he still does them.

What Greg said in the post above!

Without radio-suppression-resistors (wires or plug caps) digital electronic ignitions will create their own radio interference confusing the little computer brain. Also abide the recommended plug gap because greater gap will produce higher voltage from the coils and contribute to their own self-interference. I have run pre-digital Boyers on various bikes with and without resistance with no problems. That said I've had all sorts of problems with digital electronic ignitions of various makes on customer's bikes that weren't set up correctly.

Since you have an E-start Norton and a Boyer of unknown age DO NOT attempt to start the bike with a low battery. It may/will kick back against the starter and flip the sprags over with disastrous results if the engine catches and runs. It will spin the starter up until it explodes.

There exists a needle-jet size 0.1065 between 106 and 107. I believe LAB can come up with a part number. I found that new 106 jets are too lean (I'm at 2400 ft. altitude) and the bike must run on the chokes way too long. And the pipes are blue. These are carbs that Mike Gaylord bored and sleeved which may have an effect on mixture. It ran fine on these carbs with worn 106 needle-jets but I gained about 10MPG with the jet change. Someday when it gets to the top of my priority list I'll put the worn jets back in or order some 1065 jets.

Check the banjo screens for crud and check any other fuel filters that may be fitted. Also stale petrol or petrol with ethanol that may have separated over time can contribute to bad running. Once it separates, I'm told you can't remix it.

Start with the spark plug wires. Use solid core wire only..usually copper or steel. If you have a meter measure the wire...should be zero ohms. The spark plugs should be NGK BP7es. You will probably have to settle for BPR7es which are resistor plugs. If you can find them, the original Champion N7YC plugs are better. Beware of counterfeit Champion plugs.

Thomasa said:

I believe LAB can come up with a part number.

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