Tuning/altitude

Just got my 72 commando on the road after buying it and shipping from Texas.

I have tried to sift through the info on the forum about tuning (trust me there's lots) but I'm still having trouble understanding exactly where I should be looking.

My first thoughts are this bike may need to be rejetted due to the altitude difference. That said I'd like to get other's input before going to that trouble. After a rainy spring, I'd rather ride then tinker.

Here's what I have found thus far;

1) the bike starts up first kick
2) after about 30 sec. with the choke on I can start to turn it off
3) Idles pretty good at about 800rpm
4) I get some popping during deceleration
5) the mixture screws were set about 3/4 turn from seated.
6) opening the mixture screws seems to help reduce the popping but makes the idle less consistent
7) the bike feels very well powered and smooth during acceleration and at freeway speeds
8) best I can tell, everything is stock and the carbs are amal 930's (number stamped on the side)
9) bike came from Keller Texas (709') (not sure how it ran there) I am in Calgary (3736')

I have only a rudimentry understanding of the mixtre screws and their function.
I would love to understand exactly how these function during idle and during running and would love to know the exact process I should use for adjusting.

I have no doubt that this is covered somewhere on this forum so perhaps it might be as simple as pointing me to that link

Thanks
Jeff

Welcome to the forum, Jeff,

These should keep you busy for a while: :wink:

http://www.oldbritts.com/amal_tun.html

http://www.british-only.com/pdf/more/am ... d900-3.pdf

http://www.hitchcocksmotorcycles.com/am ... nc_ht.html

EFFECT OF ALTITUDE ON CARBURETTER. Increased altitude tends to produce a rich mixture. The greater the altitude, the smaller the main jet required. Carburetters ex-works are suitable for altitudes upto 3,000 feet approximately. Carburetters used constantly at altitudes 3,000 to 6,000 feet should have a reduction in the main jet size of 5%, and thereafter for every 3,000 feet in excess of 6,000 feet altitude, further reductions of 4% should be made.

Click to expand...

http://www.nortonclub.com/docs/TheNortonCarburetor.pdf

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

http://www.amalcarb.co.uk/Technical.aspx

http://www.gabma.us/page2.htm - Fuel Sys.

Best idle should be with the pilot-air-idle screw turned out ~1.5 turns.
Your report suggest a plugged pilot jet &/or too low a float level.
Implies not as much gas getting sucked up so less air needed to get
decent idle mixture.
Get that base line set then finer tune to elevation.
Keep in mind newer gasolines and those sold in colder-higher areas
have more oxidants in them so must fudge the older tuning references
toward richness.

hobot

Thanks for the help.

I feel a little embarrassed asking some of these questions. It sounds like most people on this forum could rebuild one of these amal's blindfolded...but I guess I have to start somewhere.

If I were to dig a little further into these carbs, possibly changing the main jets (for altitude) and possibly looking at the float and pilot jets would there be any reccomendations or tips?

I'm pretty mechanically inclined and am not necessarily afraid of tackling this but at the same time I'd like to keep this as simple as possible and cut into riding time as little as possible.

Once again, thanks for any help/advice.

Jeff

jlit said:

If I were to dig a little further into these carbs, possibly changing the main jets (for altitude) and possibly looking at the float and pilot jets would there be any reccomendations or tips?

Click to expand...

If you haven't done so already, then I suggest you click on: Advanced search (top right) and try doing a search using the word "altitude", as I think you will find some useful information in previous posts where this topic has been discussed.

Normal mechanical skill is not good enough level required in dealing
with ole Brit Iron, Nortons and especially Commando's.
Factory sabbotage to shysters supplying spares to miss-assembled
and missing components, some not available no more.

Amal carb are perfect example, hardly two ever the same right
out the box let alone 40 years used.

You may have worn slides and warped bodies that no tuning
skill can work until the throttle air leaks inside corrected.
Worn jets and needle to plugged up crude in pilot jet.

Go to the Technical advise post at top of forum messages
and go though those references to see scope of what you
need to know and avoid in following the standard manuals.
Also its way worth while to start at begining of Capt.Norton Note
and take a season or two to read the history of those
taking on Commandos before us.
http://home.clara.net/captain.norton/

hobot

I ripped the carbs apart and did a little clean up to get the slides moving freely again, reassembled everything and fired it up.

Still started first kick with the choke on, but with the air mixture screws at 1 1/4 turns out I could not ease the choke off without it starting to putter.

At 1 turn it was better but back at 3/4 turns it ran like a charm.

I have not re-jetted yet but my reasoning is telling me that at 3/4 turn I am allowing less air so a smaller jet would only hurt my cause.

Is it at all possible that (due to wear or an imperfect fit from the begining) the main slides are allowing some air by thus making the ideal mixture position 3/4 turns?

Both carbs seem to be functioning exactly the same and the exhaust pulse feels identical.

I have/had plans to re-jet to 5% smaller jets but don't know if this is the right way to proceed.

Does the air mixture have any effect while running or is this strictly an idle adjustment?
Is this something I should worry about it or just dial it in to the best setting?

My next step was to run it a little harder and check the plugs. I'm just not sure what this will tell me and how to differentiate the results between idle and running conditions.

Also anyone know a good North American source for parts? Prefferably Canada?

Thanks
Jeff

The idle screw setting gives a good idea of the float fuel level
that all the Amal circuits depend on as baseline mixture.
Aim for 1.5 turns out, or play with it for your attitude.
Your's implies too lean fuel level to begin with so less air screw
turn open for burnable mix.

I shop here and I'm not even in the Great White North.
http://www.walridge.com/section/view/

hobot