Hi I am getting to the end of my renovation and wanted to pick some experienced brains on timing and carburation. My bike (a 1947 B33) now has a Amal 389 1 1/8" Carb rather than the standard 289 1 1/8". I guessed at the jetting based on 1957 B34 scrambler which uses the 389 1 1/8" carb (the engines have same piston stroke and cams according to Roy bacon book). The book suggests a B34 with 389 should240 main jet, 25 pilot jet, Slide cutaway 3 and 106 needle jet. I set the timing to standard 7/16btdc (38Deg) but it was not starting well. I understand E5 has a slower burn rate than 1947 fuel so I am guessing the timing will need to be advanced. The question is by how much?
I went to 45Deg BTDC and it started 1st kick but I think it is too advanced now.
Additionally when at standard 38Deg or 45Deg the bike pops and flames come out of exhaust when I open the throttle. It ticks over on idle very nicely which implies the pilot is good as a 106. I tried lowering the needle 1 notch but the issue still persists. I am guessing the issue is with the needle jet / needle position timing or cut away. Any thoughts. Cheers Mr P
I don't know if burns slower than a fuel from 1947 but it's reckoned that modern fuels need less advance than fuels from the '50s. Whatever, the more alcohol in a fuel the greater the oxygen content which has the effect of making the mixture more lean. I'm running my 1948 B32, also with Monobloc, with standard jetting for a 1953 B31 and it runs fine on E5 but if I switched to E10 I'd expect to maybe go up 1 size on the main jet, raise the needle a notch and some adjustment to the idle mixture.
I have a 1949 B33 with the max advance timing set at 7/16in btdc. In 2023 it was mandated to have 10% ethanol in all fuel in Canada. I haven't changed settings in the carburetor however I tend to very slightly reduce the advance when riding. The bike runs great. The spark plug is a nice clean brown colour. It works for me...
Jim
Thanks for the info Chaps. I have done loads of reading on this, it gets confusing with rate of combustion vs resistance to detonation, vs effects of increased bore. However I am more knowledgable but not much wiser for it . I suspect its a case of experimentation and what works for the individual bike and fuel being used. Jim can I ask which carb you are using and which jets and needle position?
Hello Mr. P
My carburetor is a 289g/1at pre monobloc. The main jet is 200, needle jet is marked "std" which I believe is a 106 and the slide cutaway is a 4. I'm not sure what the current needle position is.
The carburetor and float bowl housings are bronze castings rather than the common zinc alloy.
Jim
Timed my B40 SS by ear, Started with it retarded a little and advanced little by little till i got it running lovely. It runs well on E5 and E10 with a stabeliser/lead additive added.