Carburettor

[guest146]

I have an AMAL concentric on an old 2 stroke. It was running rich throughout its high rev range but weak at idle. I changed the idel jet for one size larger but now its worse and wont idle without choke. Can anyone confirm that the idle jet passes fuel  and not air as it would seem a larger jet lets less fuel through.

Regards Ken

[Steve H]

I'm not familiar with AMALs but a rich top end and weak bottom would either suggest worn needle and jet or it needs a larger air jet. The air jet has a bigger effect at higher revs and helps compensate for a rich top end.

[guest146]

What I have done is bought a special trails bike. It has all new parts but I prefer to use a modern fully synthetic oil with my mix. This it thinner and so make a less viscous mix that flows through the jets faster so making a richer mix. to counteract this i have replaced all jets  with one size smaller and replaced the needle. Even before the change it would not idle well. With the new jets it seems OK at revs but I still have an idle problem. I was just wondering if the jet passes air or fuel so i can change accordingly

Regards Ken

[themoudie]

Aye Ken,

On the Duke Amal concentric, air is drawn through the pilot air inlet and contolled by the pilot air screw. The pilot air screw controls the idle mixture. You should also look for a very small mixture bleed hole to the cylinder side of the pilot outlet in the floor of the carburettor throat. If this is blocked then getting an idle is not going to happen. Don't ask why, I don't know.  Also if the throttle slide (valve) is worn, this will make it very difficult to get a good idle as it will 'flutter' in the carburettor body, allowing more air past than required. Maybe why the old carb was thrown out!  Worn body and worn slide equates to crap carburation. Re-sleeve carb and new brass slide or else new carb for best results. 

This link will help, give info and 'downloads'. : Amal carburettor downloads

I hope this helps, Bill.

[guest564]

I have an AMAL concentric on an old 2 stroke. It was running rich throughout its high rev range but weak at idle. I changed the idel jet for one size larger but now its worse and wont idle without choke. Can anyone confirm that the idle jet passes fuel  and not air as it would seem a larger jet lets less fuel through.

Regards Ken

There are a couple of other possibilities, wrong shaped needle or incorrect throttle slide cut away.