Carburetors

British Motorcycle Carburetor Advice

How do I sync twin carbs on a British bike?

There is no magic to this and it is not an impossible task. I did a huge mileage in the 60′s and 70′s on an AJS 33 (twin monoblocs) and an early Commando (twin concentrics). Maybe I checked the carbs over twice a year max. Now I hear of people trying to track down single carb set ups/heads because of frustration with their twin carbs.

1. Make sure the cables are in good condition and operating smoothy.

2. Remove the air filters – or the hoses – so you can get to the backs of the slides.

3. Open the throttle to the point that the slides are disappearing at the top of the carb bores – it is most important to make sure that the slides disappear at exactly the same time.

Adjusting the cables to achieve this, will ensure that the air intake through both carbs will be the same at all throttle openings.

4. Adjust the throttle stops so both slides touch them at the same time. From this point onwards always adjust both stops by exactly the same amount.

5. Start the bike up and set the idle by adjusting both throttle stops.

6. Adjust the pilot mixture one carb at a time – I prefer to turn them in so the smooth idle just starts to get “lumpy”. A little on the rich side is better for take off.

7. If you want/need to do one carb at a time, it is OK to remove one spark plug lead but you should fit a spare plug to it, held against the motor (make sure there are no gas leaks) otherwise you can damage the coil and other igition components. Having set the carb separately the idle with both plugs connectied will be a bit fast – so turn both stops down the same amount.

Notes –
Be sure after all this that both slides “clunk” down on the tops when closed. A quick check to see the slides move together is to hang (NOT WITH THE ENGINE RUNNING) a pair of very small screwdriver or nails in the back of the carbs and watch them move when you first open the throttle. If you find adjustment to the pilot jet has no affect on one or both carbs, the pilot passage is for sure blocked – very common problem with Mk1 concentrics.

-Mike

I’ve owned a 78 Bonneville for many years and it now won’t idle on one cylinder.Once it gets up to around 2,000 rpm it runs OK on both cylinders. I think it is electrical, it went OK with original ignition system but now has a Boyer and I think this may be the problem.

Always difficult to answer these kinds of questions by email but there’s some things which should be looked at. Any fault with a Boyer will will always show up equally on both cylinders. This type of problem comes up regularly and the most common cause is a blocked pilot jet circuit in one of the carbs. The first thing to do, if running with a Boyer, is swap the plug leads over – just take the caps off and put the left on the right plug and the right one the left plug. If the problem moves to the other side, the cause must be a fault with the coil, HT lead or the plug cap. If the problem stays on the same side, it must be something wrong in the carb or a mechanical problem connected with that cyclinder.

-Mike

I am thinking of upgrading my Mk3 Commando by fitting a single Mikuni kit.

While there is no question that Mikuni make a quality product, my own opinion is that nothing works better on a Commando than a pair of mk1 Concentrics. However a lot of people fit and like the Mikuni kits which we carry in stock. Once installed and properly tuned the Mikuni conversion will provide reliable service and rarely need attention. I did a lot of miles on a 1969 Commando Fastback with original carbs and possibly twice a year I would re-set the carbs.
This is a simple procedure. First open the throttle wide and adjust the cables to make sure that the bottoms of slides disappear at the top at exactly the same time. This means you will have exactly the same amount of throttle openings at any twistgrip position. Next close the throttle and hang a couple of thin nails (or old carb needles) under the backs of the slides and adjust the throttle stops so the needles start to move at the same time when the throttle is opened. You can re-adjust the free play in the cables if you need to but make sure you make the same adjustment to both sides. Start the bikes up (after removing the needles!) and adjust the slides up or down for a satisfactory idle. For setting the pilot screw adjustment is is OK to run the bike of one cylinder at a time but fasten the lead you take off to a spare grounded plug which can spark in the open air (away from gas) so the coil and/or Boyer, if fitted, are not damaged. I always try to get a “lumpy” idle by setting the screw inwards slightly because it helps avoid a flat spot when the throttle is opened.

-Mike

Concentric Carb Needle Clips

We are frequently asked to supply new needle clips 622/067 for concentric carbs because customers think theirs’ are broken. The clips are manufactured with a split and would only need replacing if they had lost their tension.

-Mike

What jetting changes are required to fit a T100 Daytona (dual carb) carburetor on a T100 (single carb)?

Assuming it is not one of the very first concentrics with the screwed in pilot jets (see Walridge catalogue for more details) just change the slide to a No 4 and the main jet to 180. Needle jet (.106) and needle remain the same although it is a good idea to replace the jet frequently – very high wear part.

-Mike