72 Commando spark plug colours and carb tune.

Check the fuel heights in the carb float chambers, if one is low then even with the same jets in both carbs the low level one will run weaker, also check the tappet clearances and if that throws nothing up then you need to do a compression test and a leakdown test to check both chambers are close to one another in performance, other possibility is a worn camshaft lobe on one side.

I'm having a very interesting time checking float heights myself at the moment, I'll post if I have any results worth sharing ... This is with the float bowl off the bike and using a sight tube plugged into the bottom plus the supply hoses attached, by which method I'm finding a very different story to that told by setting the float height alone.

Later ... Just to be clear, the left plug in the photo is from the left cylinder? And, you mention 'carb' singular, but then pilot screw 'on both sides' â you still have two carbs, right? So is one carb new, or both? Just to be clear!

I fitted a brand new pair of 932 Mk1 Concentrics many years ago, something wasn't right so I checked the float heights â both were way off the setting that's usually given of 0.080" below the top of the float bowl. More recently I've instead checked the fuel level, as this is really what we're interested in. The range usually given for this is 0.170" â 0.240" below the top of the float bowl, and both my carbs were way off that, so I'm now ignoring float heights ... If that's your problem, maybe it'll help.

Otherwise, a quick blast around the block doesn't really give the carbs a chance to perform properly â I'd go out with a plug tool and a screwdriver suitable for the idle (first) and pilot (second) screws, ride long enough for the bike to warm up and do plug chops. It'd probably be best to take a spare plug or two or the means to clean them, in case of fouling through richness. Otherwise I'm assuming both carbs have matching parts in reasonable condition, and wish you luck! Supposing I haven't just told you a load of stuff you already know ...!

What carb setup have you got?

Just because carbs are new does not mean they are any good

Have you checked float/ fuel level?

It twin carbs swap floats and bowls, if colour changes side problem float height.

Are the carbs clean?

By all means check the level, but if this has delevloped over 2 weeks I doubt its fuel level related as that does not change, could still be wrong anyway so do check but even if its wrong there will be another contributing factor.

I had a similar problem some time agowith two brand new concentrics (non-premier). Initially it ran beautifully but after a few weeks and around 500 miles it suddenly started to run noticeably weak on one cylinder. I cleaned the carbs (more than once) using the strip and carb cleaner then blow through with compressed airmethods, but without success. For various reasons the bikewas thenleft to stand for a while... Fast forward to more recent times and in the meantime I had bought an ultrasonic cleaner with some 'special' carb cleaner fluid. I decided to put the Commando carbs through it (having stripped them yet again) in the hope of sorting the problem.After the carb bodies had gone through the cleaning cycle there werea few tiny lumps of aluminium swarf in the bottom of the cleaner bath. Wherever they were from- the bike ran fine once it was all assembled...

Hi Alex

Where did you get your "special carb cleaner fluid" from it sounds worth trying.

Bob

There have been a lot of reported quality issues with new Amals. Floats hanging up on flashing not trimmed off of casings, pilot chamber Welch plugs leaking. The pilot chamber is a source of many problems as varnish like contamination can build up there and in the blind drilling's then bits break off and clog the pilot jet. Swaff can get trapped there waiting to cause trouble.

Hi - my cleaner and the carb cleaning solution came from BestUltrasonic.co.uk.The solutionhas a very soapy texture (alkaline characteristic?) and smells like a cross between brake cleaner and detergent. The solution is heated to circa 60degC and the individual components immersed for around 20mins. The solution gets pretty dirty and no doubt loses its effectiveness as itremoves grease etc, but I decant it off and leave it to settle for a few weeks then syphon off the clean fuid to act as the first 'dirty wash' of the next batch. Whatever, it works very well in my mind and has not only cleaned the concentrics as mentioned previously but has cleaned muck out of an apparently pristine Mikuni that Iuse as an alternative fit. It has also done a good job of the wife's jewellery... though I have to remind myself that was not why I got it!

Are the carbs handed? Can you switch then side to side?

If you do switch them and the poor running switches from side to side then you know that the problem is with the carbs. If it does not switch then I would expect that the problem is with the ignition.

Mike

Michael, you are obviously not aware that Commando carbs are "handed" with the adjustment screws being on the outside. However, as CW says, the float bowls are interchangeable and may deliver the problem to the opposite side and that would be a simple and easy check. Also check the black deposit on the plug. If it is soot then suspect mixture, if it is greasy suspect oil. Make sure you do a good run and get the engine up to cooking temperature. Whip the clutch in at a decent speed ( 60 +)hit the kill switch avoiding letting the engine run down to low revs and avoid crashing in the upcoming lay-by. check the plug colour. My money is on the plugs being quite closely matched. When you get home at normal traffic speeds, check the colour of the plugs again, if the soot is just a light layer then the problem lies in your pilot jet system.

Let us know how you get on

At least you can take the pilot jets out to clean them. What float needle valve's are you using are they Aluminium or Brass?

Previously richard_chan wrote:

I got the Aluminium ones.

I had taken both floats off and set them as close to the 2mm as I can.

Any differences in Aluminium and brass setting then?

Previously christopher_winsby wrote:

At least you can take the pilot jets out to clean them. What float needle valve's are you using are they Aluminium or Brass?

When I got stay up kits Aluminium ones were supplied I found that under the vibration spot they would leak causing the carbs to flood. I ended up getting new brass ones. My carbs are worn but I am not impressed with the build quality of new Amals. My anodized slides are showing wear through the anodizing after 1700 mile. Needles and needles jets wear quickly.

Modern petrol seems to to gum up quicker I have had to clean out the pilot systems several times, not difficult as I have an Ultrasonic cleaner but getting the carbs off is a real pain.

Guitar strings are good for cleaning pilot jets. The right size will not enlarge the hole like a drill might, just scrapes out the varnish.

Fuel level,s I check with tubes out of the drain holes along the side of the carb without and then with the engine running. Running when on the wheels not the stand.

Believe a guitar G string is the correct size, as you have premier pilots it will work but on non premier it pushes the dirt back into the pilot circuit so it can then reappear shortly afterwards, the flutes on the drill collect the dirt so its removed with the drill. The pilot hole is 17 thou, a 16 thou drill would not oversize the hole except in the hands of a gorilla.

If the carbs got blocked once (if that was part of the problem originally) they will do so again until you find the source. I have a pretty clean tank treated with POR15 but because I have a habit of running the level right down there was always something getting through. I eventually added small filters and solved the problem (for the most part)Just a thought....