top of page



Over time being involved with classic Brit bikes, mainly twins and triples I have worked out a couple of things to help with carb setup.

One trick to get a starting point with your needle suitability is to remove the idle screw, the one usually at an angle . This done remove the top cap, large return spring, slide and needle clip. Leave the bowl and lower internals in place. Now, drop the slide back in the carb body and then drop the needle back down the slide centre hole. This check will determine whether you have anything near a suitable needle length. Peer down to the slide and look at the top of the needle position, if it is below the surface of the slide where the needle clip normally resides then it is unsuitable, being either too short or  tapered  too thin  over its length, it will be difficult to get rid of rich/fat mixture with this needle. If the needle protrudes out of the slide and the 2nd or 1st clip groove is visible then it is probably too long and it will be difficult to have anything other than a weak/ lean mixture. Somewhere between the top clip groove and the actual top of the needle visible is a good ballpark starting point. You may have a chance to get good running at low throttle openings. Most of your riding is probably done on the needle and to some degree at ¼ throttle , the pilot mixture and the slide cutaway air also play their part in obtaining a smooth ride. Regardless of the fact that the part number is correct or “this is the right one mate” I always give it the go or no go check.

Contrary to popular belief changing main jets to try and get rid of richness is a waste of time generally, unless of course you ride everywhere with the throttle cable stretched to the max.   I will normally go with the factory recommended main jet size and often go up one size for safety.  If you have a stretched throttle cable then the main size may require further scrutiny.Typically when I set up a carb for multi cylinder use it will be as per factory main jet or 1 size up. If the carb is a single device  on a twin then I will check the jet size as used on a twin carb application, let’s say a Bonnie 650. They are 190 but for ease let’s go 200. So I halve one and add it to one and I end up with a 300 main for a Tiger/ Thunderbird/Trophy/ Thunderbolt ,single carb 750 Nortons etc. For an 850 I might go 320. This I have on Judes ES 850 Norton with a single 30mm MK2 Amal. Great low down grunt with that small carb.  Check and adjust for engine  safety.   The factory fitted a 230 main on Triumph 650 single carb twins when the main on twin carb was 190-200. Go figure. Great for green lanes in UK but a bit weak for near or  full chat runs in OZ or western US.

So back to the area that really matters. After checking the needle , check the slide cutaway number, I learnt this when a 28 yr old after I made a single carb manifold for my Norton Commando. I just fitted one carb and thought maybe a bigger main jet would be all I’d have to do. After much fiddling about I started to file away the cutaway angle , taking very small amounts to see what may occur. After all I had another slide. Well after a few goes at it I succeeded in getting rid of the 8 stroke at cruise speed. ¼ throttle at 60mph. It was a bear up till then, dah-  dahdah –dah dah .If you know what I mean. This is most exaggerated when holding a steady throttle with no load (on flat road) or coasting down hill. Just a steady throttle when bike and engine are coasting/cruising. That is when richness rears its ugly head.  It turns out I had achieved about a 4 cutaway, so I learnt yrs later. So the cutaway on a single carb twin  would need to be about 3.5 to 4.5.  This is similar to what a single cylinder 350-500  would require , start at 3.5.   On a twin carb twin  the cutaway will be something like a 2.5 to 3 maybe a tad more but not usually. Pilot jets in 930 series concentric are set except the latest offerings. You can bore them out and add a removeable  jet in the bowl area of the body, boring out not required if going leaner. The smaller 926 series isn’t tapped so can’t be done.  Typically a twin with twin carbs may need a 20-25 pilot jet and a twin with single carb may need a 25-30 pilot jet. Likewise 350-500 single. These are only base line figures but close.

You can tell if the pilot needs to be larger or smaller by setting an idle around 1000 and turning the idle mixture. Start with air screw at 1.5 turns out from stop. Turn in and the engine will get richer( if the pilot jet is clear)and should slow down and possibly stall. Turn out and you add more air to the set fuel available through the jet and it will speed up as the mixture becomes weaker. If you are out 2 or more turns out and it is still rich ( black smoke) then you have  too big a pilot jet. Similarly if turning in makes little or no difference then the jet is too small or blocked.

When tuning twin carbs get a close pull (Synchronise) to start with and similar lift off base with idle screw, when satisfied then set the cable lift last. You can get close by hearing the slides hit bottom when engine stopped. Check cable pull and make sure there is some slack in the outer cable or throttle may hang. Then check synchronising by sight or feel. Each time you play with cables pull throttle tight to seat cables. Keep cable route slack and turns wide. Don’t let idle for long periods as carbs get hot and that makes it harder to get anything  ,if you must then put a fan on the motor as it is AIR cooled.

If you have a Mikuni, the twin or single carb needs to start with a 159P3 and 6DH3 needle and  jet. Cutaway , main and pilot about same as Amal .You may need to flick the low speed air jet , when looking at the front of the carb, it is the  middle hole deep down  and this air jet is more suited to rich 2 stroke. Flick it or bore it out some to get a nice low speed tune.

The whole trick to this is getting the transition from idle jet to needle and these ideas may help. Oh yes if your carb is one with an enrichener lever or cable ,eg Amal MK2 ,Mikuni. The  quick check is to fire up cold, flick the enrichener off straight away and give it a rev. Does it take throttle instantly or is it a bit hesitant. Instant response on a cold engine shows richness and it may well 8 stroke at the critical cruise point, drop the needle a notch but a short period of small throttle openings with the engine not wanting to pick up cleanly until engine warms and  then responds quickly is about right and should see good plug colour. If it takes forever to respond  ,raise the needle 1 notch.  If your twin Amals are just plumb wore out , don’t throw them away you might have need to use one of them in single carb application. They usually work ok when worn, by itself. If you are to try a set of resleeved carbs which generally are good you will find yourself needing to lean off a bit more than when it was a worn carb. Less air pulled around slide instead of under it. Less air ,less fuel required. So drop everything one size to start with. The story goes that the new concentric ,introduced in 1968, would stick/jam with close tolerances so the factory made them looser to counter this. Worn out from the start. The biggest problem is of course expecting any life from an instrument where both metal wearing surfaces are the same,in this case pot metal. Cast iron is OK and different  hardnesses of steel OK   with oil, as in Tri and BSA gearbox. Cams and followers etc.

Hope this is of help to someone , I remember being at Les McKittericks place one day as he was explaining something to me about maggies and I thanked him , his reply , son knowledge is no burden. Shame the same can’t be said for meat pies.

Cheers.   Brian .  For more info I have dozen tech articles on my website and may do this as a youtube video where I have other articles and mongrel swap meet bikes.   Yootoob  channel,   british bikes by brian 

bottom of page