B33 carburation and timing with E5 fuel

[Mr P]

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

[Derek996]

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.

[Jim S]

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

[Mr P]

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?

[Jim S]

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

[Alan wray]

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.