My First Commando...

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Could be a worn needle jet.
The slide needles are very hard and wear very slowly but the needle jet or needle block is brass and it does wear out fairly quickly on all carbs, not just Concentrics. Its an inexpensive item.
Most people get a new slide needle at same time, just to be sure
 
Update:
Took it in to a British bike shop to have them set dynamic timing and go over things in general. He somehow got it idling much better...she ticks over just under 1k when cold and just over 1 k when warmed up...can barely hear/feel it idling from the riding
position.
They swapped in a 280 main jet from the 260 that was there. Left needle and 106 needle jet
untouched. He said plugs badly
fouled (werenew) and put in a fresh set. Reported headlamp not getting brighter with rev'ing, so likely not charging and that may
contribute to plug fouling /stalling.
Also reported some oil presence on front shocks...new seals needed.

I spent a few days going through
electric checks. No more than batt
volts when rev'ing. No AC detected reaching battery (good). Rectifier testing on bench with spare headlamp showed all diodes
working in one polarity only as
required. Don't yet have a 1 Ohm 50W resistor (slow boat from china) so can't do all tests as described. Did check for AC direct off stator
connections to rectifier...seems ok
at 30-50 VAC while rev'ing around 3k. Removed Zenier and found the Lucar connector cover quite melted...cleaned connectors and replaced. Checked 2 MC on bench
by applying 12v a few seconds and checking output above 9v after 5-10 minutes.. actually held to 12v for ten minutes. All good as per shop manual. Cleaned all
connectors with small brass brush. Checked at battery terminals at 3k and now getting 14-15v. Seems good to go.
Took her out for a ride...running
really well. First time out she hasn't stalled.
Did a cafe stop and got swarmed by three older guys wanting photos of it. :)
 
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Well, spoke too soon. Still getting a stumble just as throttle is moved off idle a tiny bit. This did give me a couple of stalls on the road during my ride home today (stop & go traffic).

Uploaded a video where you can observe the stumble on two throttle blips...second one nearly stalls and a black puff comes out the pipe....



Any suggestions?
 
Tornado said:
One issue it does seem to have, a dip in rpm just as you roll throttle from idle the tiniest amount. It was enough of a dip to stall once today. What should I look at, more idle jet or elsewhere?
Click to expand...

Thats either your air screw, pilot jet, or slide cutaway. Could be too rich or too weak.

Could even be both, meaning that one aspect is overly rich and another has been adjusted overly weak to compansate (or vice versa).

What carb set up are you running?
 
When the bike shop checked your carb did they replace the needle jet ? They wear out very quickly
 
Carb is a single 932 Amal running 3.5 slide. Shop only did main jet, 260 swapped out for 280. This was their own decision, not my request. When I asked about needle jet they said they last longtime and no need to change.
I will order new needle and jet...should I stick to 106 or get several and experiment a bit?
 
I'd go with stock.
And the shop is wrong about needle jets lasting a long time. They wear out pretty quickly. I would say they need replacing every 20,000 miles on average.
My 850 has 26,000 on the original carbs and they have started to do just what your bike is doing, just less pronounced.
You get a slight hesitation when lifting off idle plus gas mileage has dropped noticeably.
A lot of focus is on worn slides, but I've found that things will work ok with a fair bit of slide wear. A tiny bit of needle jet wear seems to cause more problems.
A worn slide allows more air than desired in the mixture ( lean) a worn needle jet allows more fuel ( rich)
You have the hesitation, fouled plugs and black smoke. As noted by mk11a850, that means things are on the rich side.

However, if you are feeling a bit rich then slap a pair of new Amal Premieres on there.
Then you can forget the carbs for a long while.

Glen
 
Tornado said:
Agreed but isn't that more a consequence of nearly stalling then getting flooded just before it recovers?
Click to expand...
I think your video is very clear on this (unlike many – a useful video! :)) – put it this way maybe, if it were weak there'd be no black smoke. So, rich is where I'd put my money (max £0.05). And I'm with worntorn on the needle jets. Plus, I hate to say it, but if the carbs are otherwise 'stock', then I think the problem might be the 280 main jets, which you didn't ask for, and I'd put 260s back in, with new needle jets while you're at it, and see. Then you can try other things like raising/lowering the needles or even swapping the slides – my best result so far is with no. 3 slides. But I'd try the other things first. Btw did anyone mention checking the float heights yet? Anyway persevere, sweet carbs are, well, sweet!
 
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Your problem has nothing at all to do with the main jet , you can take it out whilst tuning the low end of you like
A 280 may be fine for a bike fitted with a single carb and air filter
You will need to do a plug chop at full throttle to get the main jet right
Have you tried adjusting the pilot air screw out a little?
Also as has already been stated check the float height
And change the pilot jet for a new one
 
baz said:
Your problem has nothing at all to do with the main jet , you can take it out whilst tuning the low end of you like ...
Click to expand...
True – off idle it's simply not involved – and it seems I'd lost track of how many and what carb(s) you have – sorry, today seems to be my day for running in a new brain for an idiot! Anyway I stand by 'rich'.
 
At the risk of a mild hijack, I’d like to chime in here with a question.

Is Baz right about the main jet having zero effect on the low throttle opening circuits? I’ve heard many people claim this fact (not just you Baz).

But I doubt it to be honest, for two reasons:

Firstly, Amal themselves say that the main jet must be set correctly BEFORE anything else. If it had no effect, why would they say this?

Secondly, as a pretty inept carb fiddler myself, I know from experience that when I’ve got the main jet badly wrong, it hasn’t effected all sorts, from low speed running to poor starting (particularly when I’ve got it way too rich).

What’s the consensus here...?
 
I would make sure the float needle tip is sound and that the float chamber itself is moving on its locating rod rather than the rod turning in its recess.. gentle slight bending of rod will lock it into position Otherwise you may have variable float height Another important check is to ensure that this locating rod is properly positioned in the gasket..

Yes Baz is right , main jet size only comes into play from roughly three quarter throttle prior to that the fuel is metered by the needle position and needle jet size .
 
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Fast Eddie said:
At the risk of a mild hijack, I’d like to chime in here with a question.

Is Baz right about the main jet having zero effect on the low throttle opening circuits? I’ve heard many people claim this fact (not just you Baz).

But I doubt it to be honest, for two reasons:

Firstly, Amal themselves say that the main jet must be set correctly BEFORE anything else. If it had no effect, why would they say this?

Secondly, as a pretty inept carb fiddler myself, I know from experience that when I’ve got the main jet badly wrong, it hasn’t effected all sorts, from low speed running to poor starting (particularly when I’ve got it way too rich).

What’s the consensus here...?
Click to expand...
To be honest I was being a bit flippant here I was just trying to make the point that the main jet only really comes into play when the carburetor is 3/4 open,
I have heard of people setting up with out a main jet fitted and I have heard of getting the main jet right first
Of course if you were setting up a carburettor from nothing IE a completely different carburetor and the needle jet was too small you could change your main jet all day long and it would make no difference
 
Your point about the main jet having no impact on the lower range jets if they’re too small certainly backs up my own experience, whereby symptoms were only seemingly obvious on the rich side of things.
 
Just had a look on NOC website technical, they recommend an 850 with pea shooters and unbalanced down pipes and a k&n with a single 32 concentric fitted should have a 106 needle jet,a 3 cutaway slide and 280 main jet but to be honest this information is quite old and petrol has changed quite a lot,and bikes vary a lot ,but it's a good starting point
 
Some more details....the same stumble was happening before taking to local shop (which claims to be Norton specialist as owner has several on his show floor). Dynamic timings didn't solve stumble. Change to 280 MJ didn't solve it. Float was set to recommended height by myself
previously (2mm below edge of bowl as per Bushman guide). Float valve new. I even polished the valve brass seat using qtip on a drill chuck and Autosol.
I've recieved one suggestion to try lowering needle reduce richness...it is on middle position now...to see if that affects stumble...if worse then raise needle to increase richness.
 
Oh, one other point, the stumble only seems to appear after riding around town, stop and go, for a good 30+ minutes. Not a detectable problem earlier on. Could the carb be heating up and this plays a role? I do have the phenolic gaskets between 2into1 mani and head.
 
Did it have a new needle jet fitted?
Have you tried adjusting the the pilot air adjusting screw out a little? What is the screw set at at the moment?
 
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