Tuning amals

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Jul 18, 2004
I have 32mm Amals on my bike - the carbs are in great shape, with choke slides - (never need them though?!) Fresh gaskets, viton needles, new o-rings etc. The manifolds are tapered down to the original 28mm head. The bike is strong but running a little rich judging by the black plugs.

My questions are:
What is a "reasonable" idle speed? I try for just under 1000 and it seems to work - nothing like dying at stoplights in rushhour traffic.

Idle is fine, (closed throttle on run down ok, smooth, no backfire or luminess), power under load is great, but holding the bike at a constant speed, just a little throttle with minimal load and the bike farts and burbles and feels generally grumpy - open it up to accelerate and it is fine. Is this a needle jet issue (drop a notch?), lean/rich idle issue or?

Any thoughts appreciated - I am new to tuning carbs. I am amazed at how much fiddling Amals seem to need, not exactly "set and forget".

It does sound like classic mid-range richness. Is everything standard ? Air filter clean ? Has a standard exhaust or filter been fitted in place of a less restrictive one ?

Presumably you have the "full circle" spray tubes and correct needles ?

I'm with you on tickover. I think tachos vary a bit. If it's not stalling and not racing to the extent that you can't select first gear then it's up to you.

The physical position of the slide governs what happens in the carb. At what sort of throttle opening is the richness occurring?

Lowering the needle (ie raising the clip one notch) is the easiest thing to change and it doesn't cost anything.

If the problems are at rather smaller throttle openings then a change of slide might help. What number slide are you running at the moment?
Thanks for the quick response,

The airfilter is new/stock, the pipes are new/stock. I beleive the carbs are stock, I was told they are and seem to be.
I'll give the needles a drop and see what happens. I was thinking that I might lean out the idle some more - I did notice that the slide moves up before the needle does so perhaps idle is still too rich.

Does fast idle when cold seem normal? The idle lowers as the bike warms up. Seems counter intuitive to me, maybe it is just over heating as I do a lot of inner city idling :?
Just my thoughts again.

I think that you might be mixing up the functions of the two adjuster screws. The one that is angled upwards is simply a tickover adjustment. It lifts the slide in the same way as opening the twistgrip slightly. It does not adjust the mixture in any way.

The other, horizontally mounted screw is the pilot air screw which affects the mix at tickover and just on the transition to where the slide cutaway takes effect. Because it screws into an air pasage, screwing it in (reducing the air supply) richens the mix.

Actually, you don't need to worry about the technicalities because it is set simply by turning about 1 1/4 turns out at tickover and then gently screwing in or out to find the position where tickover is smoothest and fastest. You can easily hear if you have gone too far.

The slide moving up before the needle does puzzles me. The needle should be held in position by the return spring and I do not see how it can move independently.

I think looking at the needles should be your first move. If that doesn't help, look for wear in the needle jet which you find screwed into the top of the jet holder.

I don't know about the tickover. It should be a bit higher as it warms up but overheating can make it erratic and lumpy. My personal feeling is that modern fuels are more prone to this in hot weather but I have no scientific basis for thinking this :)

By the way, do you have a copy of the old Amal Hints and Tips leaflet ? If not, pm me with your e-mail address and I'll scan one through to you. Lots of nice diagrams and step by step instructions.
Be aware that there were needle and needle jet changes thruout the Commando range and one kind won't work with another. Also there are jets out there that never got reemed only drilled. You can check them with a pin gage set at any good tool shop. .105 are .105 and so on but a quick look with a magnafiing glass will insure that they are smooth inside. You say that your slide moves before your needles? Do you mean to say they have found a rest point somewhere in the carb? That don't sound good. But could lead you to the problem. As rich as most Amals are I would be looking for leaner sets. You should also give your real MPG as it helps others help you. Maintaining 55 MPH in third gear for known miles would be good fill up to fill up. You should see 50 MPG or so. If you get it lean enough. If your gas is like ours latley seeing black isn't uncommon give us milage. Also what plug?
I cannot quite follow what you mean when you say that the slide moves up before the needle does. The slide and needle are held together by the return spring and hence should move together.
I have had identical symptoms of slight jerkiness on my 850. I am still trying different combinations of idle mixture screw setting, needle position and slide cutaway.
I have had it running absolutely spot on with different combinations and have also had it misbehave on these same combinations. I strongly suspect that jitter-free running on a whisper of throttle is crucially dependant on synchronisation of both slides more than the size and position of the jets. It is mighty tricky to be sure that both carbs are contributing equal amounts of fuel and that both slides move exactly together.
First you must insert two thin identical drill bits (or similar) into the bottom opening of both carb intakes such that the drill bits hold both slides open by the same ammount. Then turn each idle speed screw until the drill bits just move. Remove the bits. Close down both idle srews by exactly the same number of turns so that, when you eventually fire the engine, it will not race too much.
Now very carefully turn the twistgrip a tiny ammount. You will hear, feel or see (using a mirror) the click(s) as the slides rise and fall to their stops, as you release the twistgrip. Adjust the throttle cables until both slides rise and fall together. Keep thinking about it and take your time.
Getting the slides synchronised has has the greatest effect on my 850's ability to poodle along on very light throttles.
Just for the record I use 106 pilots, 3.5 cutaways, needles in middle groove (3 groove needles), 280 main jets. I am currently running with number 3 cutaways with needles in top grooves, which I feel may suit my particular engine slightly better.
I still need to re-sychronise occasionally - Amals don't stay the same indefinitely and need regular tweeking, particularly the throttle cables.
As an extra check on synchronisation, I remove float bowls and jet holders and put a straight edge across the bottom of the jet holder boss. It is then possible to check that both slides / needles are clearing the carb bore together.

I would like to pretend that this is because I am only concerned about carburation at full throttle but to be honest, it just seems the least fiddly way to check.
Thanks guys.

I guess I get lumpy midrange from running larger carbs. They are synched, and as I said idle and acceleration are fine, just partial throttle settings when the engine isn't under load is lumpy. I will give the needle a drop, and when they wear out, resleeve them with the proper cutaway - until then I will live with the huka-huka burble crack. :wink:

Now the one thing I can't get used to is how ficlke the bike/carbs seem to be. The carbs seem to need tweaking depending on (in no particular order):
  • Gas octane
    temperature of gas
    temperature of air
    engine temperature
    amount of gas
    alingment of planets
    month of the year
    what I ate for breakfast
    barometric pressure
    colour of my underwear
For example yesterday I had the bike at a nice smooth idle, just under 1000 tickover. I made three trips with the bike from morning to evening. Several hours to cool down.
This morning the bike was just as cooperative as yesterday - until I bought gas - I used the top octane and now the idle has dropped to almost stalling and he ride is bumpy and lumpy.... what gives?

Tweaked Carby
Welcome to the world of two carbs. Are you useing a vac gage set to set these? If your using your ear your missing on the first set up enough that you have no slack for how hot or cold and all the others talked about. The smallest adjustments make the gages go nuts. Yes you still have to synch the cables and recheck them when the stop screws are set even. The bench mark provided by using gages just helps so much gives you the needed slack.

If the idle speed decreases as the engine warms up, chances are high that the idle circuit is too rich. A quick look at the insulators at the business end of your spark plugs will verify this.

I just went through this same thing and though I believe it was/is my inability to fine tune dual carbs precisely, I changed the needles and needle jets to new stock, retuned carbs and measured every screw turn I made this time and I rode yesterday for the first time since I had the bike (6 years) without any problems. It was a lovely day in norton land.
Thanks again for the info - I will endeavour to ride lots to wear out the carbs... then I can fit new bits & try a new cutaway profile. :lol:

I gave my battery a good trickle charge over the weend and noticed a marked improvement this morning - fewer pops, snorts etc. My plugs are still pretty dirty so I will drop the needle sometime soon to see how that effects things.

Ride on!
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