June 20, 20242 yr A friend of mine has just purchased a nice C15 for Classic Pre 65 Trials. It ticks over evenly, pulls well at the bottom end, but runs out of puff when being pushed up a hill climb . I’m sure it needs a larger main jet, but I’d like to get the whole settings right , from Amal carb type to all jets , needle and slide. Thanks everyone.
June 21, 20242 yr If it revs out alright but only falters on an extended full open throttle it may just be a case that the float height is too low. Raise the float height and get a bit more fuel into the carby and you could solve the problem.
June 21, 20242 yr Chappo - Original carb. fitment would have been a 7/8" bore Monobloc type 375/34 or 375/51 ; this specn. number would be stamped on the mounting flange. As it's extremely unlikely that a Trials bike of maybe 60 years old would have retained its original carb., I wonder what the bike in question has ? Edited June 22, 20242 yr by lorenzo
June 21, 20242 yr My Amal 622 has No.4 slide 140 main 105 needle and 25 pilot. Best thing i did was a rebore to 280cc with a triumph piston. Revs v well and almost eliminated the stutter off idle unless gave it too big a handful. Slides/body can get worn as i believe they were made of same metal and 2 metals the same will wear out quicker. Amal make hardened slides now Think jets are same for the Amal monobloc carb Edited June 21, 20242 yr by timdog
June 22, 20242 yr On 6/20/2024 at 8:47 PM, chappo said: A friend of mine has just purchased a nice C15 for Classic Pre 65 Trials. It ticks over evenly, pulls well at the bottom end, but runs out of puff when being pushed up a hill climb . I’m sure it needs a larger main jet, but I’d like to get the whole settings right , from Amal carb type to all jets , needle and slide. Thanks everyone. You don't say which Amal, but assuming it's a concentric, for a C15 it should be a 22mm which is the diameter of the standard intake. Different bikes won't necessarily run the same on the same jetting so it always a bit of trial and error. There is only one needle for the four stroke concentric of that size which is U1. Problems at high revs could be either too weak or too rich, If it's too rich it can cause the bike to choke on full throttle, too weak and it will gasp/die. Main jet size is normally around 100 or 110. There should be no pilot jet, needle jet 105 / 106 and it must be the 4 stroke type with spray holes. The jet holder is longer in the 4 stroke so the main jet sits lower in the bowl. Slide is usually a 3 If it is a premier type carb the same settings apply apart from the pilot jet which is a different design to the normal concentric, screws into the outside of the carb body opposite the pilot air screw and this is usually the three ring jet There could be other reasons for it not pulling up a climb, timing, cam type, different head with big valves etc
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