I have a 928 concentric Amal carburettor fitted to my 1962 A50, which is running very rich & rough, coughing & spitting under load. Engine number - A50195 - The bike has electronic ignition.
I have just removed the plugs and they are completely sooty black.
I have not yet removed and inspected the carb yet, to check the jets & needle position.
What little research I have done leads me to think that this carb was attached to 650 Thunderbolts. So, if need be can this 928 Amal be re-jetted to suit a 500 twin or should I be looking for an original one - Amal 389 689?
Any suggestions for a starting point would be most welcome
regards
Ian
The 928 may bit a bit to large bore. You could experiment with jetting.
The 1962 A50 would have had a 376 monobloc 1 inch bore with a 220 main jet 25 pilot and 3 1/2 slide, needle in position 3 (from 1962 Instruction manual). Or you could use a 626 concentric.
Quote from: blackeyeian on 26 Oct, 2018, 15:41
I have a 928 concentric Amal carburettor fitted to my 1962 A50, which is running very rich & rough, coughing & spitting under load. Engine number - A50195 - The bike has electronic ignition.
Any suggestions for a starting point would be most welcome
regards
Ian
I had these symptoms on my twin carb Bonny fitted with Boyer and 6v coils. Drove me mad for ages until I went back to basics.
Mine would start easy but run party and fluffy before giving up with dry sooty plugs. New plugs would see the procedure repeated. I went through everything before checking the 'New' inlet rubbers carb to head. One had split causing air leaks - result easy to start on choke when enriched (mine have Mk2 Concentrics) but hunting after when warm. And the plugs fouled as if it was rich which in fact would result in wet fouling not dry.
Might be worth checking the air seal between the carb and head. Start it up and spray some GT85, WD40 or carb cleaner something volatile directly at and around the joint NOT the air intake. If the revs rise you know you have an air leak. It's a simple test and often proves fruitful. Might just need a new o ring.
Main jet on Thunderbolt 230 Needle jet .106" Position 1 on needle. If you have these in the carb it is jetted wrong.