Mk 3 Main Jet Confusion

According to my manual for the MK-III the standard main jet is 230. I put 260's in mine after fitting a more free flowing K&N filter and it runs very well. My limited knowledge of the Amal concentric suggests you should be looking at the slide cutaway, needle groove and possibly even the pilot adjustment. The main jet only really comes into play above 1/2 to 3/4 throttle openings.

Similar that Martin.

See if this pilote jet is not mouth, easy on Amal Premier, it is him who manages the circuit of gasoline up to half, at least, of the levying of throttle. Sees so so ALL THE CIRCUIT OF CARBURETORS ARE VERY CLEAN.

And of course put the needle in the good position, often in the middle.

Previously mike_steer wrote:

Just put a brand new set of Amal Prem's on my Mk 3 ES 850

Hi Mike,

After reading your ps (glowing red pipes), I would start with looking for air leaks check the carb orings and face up your manifolds then check your valve clearances. Confirm float heights/fuel level throw in a timing check for good measure, before you start buying any more jets lift the needles one notch , fit the 220's from your old carbs .

Settings for premiers on my MK3, I run K&N air filter and RGM straight through Black Cap replicas.

No 19 pilots

3 1/2 slides

So called "Special needles" (centre groove)

260 mains

All good now, however, "it's been emotional"

Regards

Katherine Scott

Run my Mk 3standard Amalswith 230 main jets. Needs full choke to start (first kick usually with Boyer ignition)and continues to needfull choke for a fair old time before engine up to running temp - set off withfull choke thentake it off completely once out on the road andthrottle opened beyond athird. If I pull up ata junction / lights in the first few minutes of journeyneed to apply full choke again to prevent stalling or blipping throttlebut after those first few minutes / milescan put away choke and forget about itwhen engine temp up to normal and ticking over reliably when stopped.

I had this bike out in Kenya for many years and living in Nairobi which is at 4,500 foot altitude used to run it with 180 main jets to prevent over-rich mixture , as prescribed in the workshop manual. When I took it down to Mombasa on the coast never bothered to change the main jets - bike used to still run OK albeit on thelean side.

On return to UK had bike rebuilt and found left and right cylinders ran at different temps with the left downpipebadly blued while the right stayed a cooler light yellow (no balance pipe). Needle position was middle for both carbs so dropped the left needle to bottom groove to richen mixture on left cylinder. After fitting a new set of downpipes (with balance pipe)left pipe now runs just a pale yellowfor a couple of inches from manifold , as with the right.

On glowing downpipes ,my 961 Sportdoes this for the first few inches of downpipeas observed when starting upin a dark garage - never been a problem. Downpipe material for original Commandos and the 961 are different however. Suggest checking timing onyour Mk 3.

... treat twin carb tuning on a twin cylinder engine like tuning two separate single cylinder engines - carb settings may not necessarily be the same for each cylinder ...

As Francis indicates this is not normally caused by the main jet.

It's usually the result of tooweak a mixture on the primary circuit.

Try adjusting the mixture screws to 1/2 turn out rather than the recommended 1.1/2 turns out, or fit larger pilot jets which is possible on the premier carbs.

The other possibility is a smallair leak on the primary circuit,areas known to leak are poorly fitted small blanking plug on the carb air intake side or the blanking plug next to the main jet holder (which was my problem).

You can test this by removing the mixture screw and spraying brake cleaner into the pilot jet whilst blanking off all the primary circuit exit holes with your fingers. You may see brake cleaner bubbling out from around the offending blanking plug.

Good Luck hope you manage to fix it.

Just because the carbs are new does not mean they are made right. I would strip and clean them checking for bad manufacturing, swafe left in the drillings, badly fitted welch plugs, mating surfaces flate, all flashing is removed, floats move freely. Then check float height and float valves working correctly.

Have you got standard exhausts and the black airbox and filter?

Have a look at Bushmans Amal site.

I bought a new concentric last year and that had swarf in the pilot circuit

Hi Mike,

it may just be one carb that's giving you the problem and from what you say it may be the right hand side carb.

One way of establishing which carb has the problem is to adjust the mixture screws with the engine running, the offending carb may not respond to adjustment as well as the good carb.

This was the case with mycarb which had a leaking primary circuit blanking plug.

Hope this helps.