Carb Jetting

Hi gents
When i buy new 30mm Amal Premiers carbs for the 750 the jetting as supplied is being offered as:-
220 Main Jet
106 Needle
17 Pilot jet
No 3 slide cutaway
Air filtration will be by some kind of aftermarket Cone/wire mesh/ K&N type... yet to be decided
Some kind of exhaust noise reduction (minimal obviously) will be used.
Road use only & not likely to be ridden hard
UK operation mainly at under 1000ft & majority of summer only usage
Appreciate some tweaking will be necessary, but do these carb specs look to give me a good starting platform??

Thank you

What ever you get, don't just bolt them on and start riding - tune them first - particularly the needles and needle jets. I'd go a bit richer on the mains until you can do a plug chop to get them right.

I have found that the #17 pilot jet needs to be changed for a #19. Try a 107 needle jet just to see what happens

https://andover-norton.co.uk/en/shop-details/20569
https://www.carbtune.com/colortune.html
https://classiccarbs.co.uk/wp-content/uploads/2017/03/MANOMETER2d.jpg
These tools help me to get a good setting for both carbs to be in sync
Clean out all ports before hand with carb cleaner and compressed air.
idle screw should be close to 1-1/2 turns out for each. If not reset the float level. Take it for a run then do various plug chops with new plugs and carry the plug wrench with you for removal. I would try the various positions of the needle before investment of the .107 needle jet. 107's were used for the early concentric s which had the screw in idle jets. If you have the #19 available then try it all over again and see if it gets better or worse. Be methodical and only change one thing at a time and then retest for results.
Cheers,
Thomas

With the smaller needle jet, by lowering the needles you should be able to induce a cough when riding the bike changing up and down through the gears. If you cannot induce the cough, your needle jets are too big and the motor will be slightly sluggish - you might not notice it. But if you get the needles and needle jets right, your bike would perform much better. I always try to get the motor to cough, then raise the needles one notch. With experience, you can tell if you have got the mid-opening jetting wrong. Very cold weather should make your bike a pig to ride because the jetting is leaned-off. If it goes faster, then it is jetted too rich for normal use in warm weather.

I'm reading up on the change from 17 to 19 pilot jets as the new standard for Amal premiers. Is that for 30mm carbs as well as 32mm and does this lean the mixture or richen it?

Dommie Nator said:

I'm reading up on the change from 17 to 19 pilot jets as the new standard for Amal premiers. Is that for 30mm carbs as well as 32mm and does this lean the mixture or richen it?

Click to expand...

19s are richer than 17s.

My ‘68 650 Bonnie with 30mm Premiers needed the 17s replacing with 19s.

Dunno about them Commando wotsits...

The differences between a Triumph Bonneville and a Commando are the ignition timing, the amount of restriction in the exhaust and the air cleaner. Otherwise what works for a Bonneville should work for the Commando. Ignition timing probably has the greatest effect. There is a balance between comp.ratio, ignition timing and fuel mixture which determines combustion conditions in the cylinder head.

Your specs are exactly what I'm using in my 750 with good results. I'm running the needle in the bottom groove(richest). I have K+N airfilter and open peashooters. I've found the #17 pilot works in my 750 but my 850 likes the #19. I'm using NGK BPR8ES plugs.