Carburation.

[matchbox500]

I would appreciate some advice re setting up the carb on my 51 rigid G80. The bike starts fine and pulls well, but sometimes it backfires on shut off. Also it wont tick over. It is the right carb for the bike.

[clive]

air leak between carb and spacer, or exhaust pipe and head or exhaust pipe and silencer will all cause popping/backfire on over-run. To tell if it is the right carb for the bike details such as carb number, slide number, main jet and pilot jet number. from memory a 51 G80 should have a pre monobloc 89 but it should run fine with a nonobloc 389 provided its the right bore and jet sizes are correct. I would start by checking that there is nothing in the pilot jet and that the very fine hole into the bore is clear. Poor tickover could be worn slide, also poor magneto, tickover will need to be set with mag advance backed off a bit.
This is serious tried to find my copy of AJS and Matchless the post war years but it is not on the shelf. The correct carb numbers are in there .

[clive]

Found my copy. 1946-53 G80 type 89 pre monobloc, bore 1 3/32, main jet 180, slide 29/4, needle position 3, needle jet std. 54 bore of type 89 increased to 1 5/32, with 180 main jet and needle position 2 (measured from top of needle i think). 55-59 it was a type 389 monobloc, bore 1 5/32 with either a 250 or 260 main jet, 30 needle jet, 3 1/2 slide, needle position 3 and 106 needle jet. still think you have an air leak probably around the pipe into the head. Cannot cure the pop on mine.

[Janet]

Some people who were at at the Kimbolton Jampot may remember my G2 popping when I shut off the throttle quickly, complete with flames from the silencer. I gobbed loads of gasket goo round the pipe where it entered the exhaust port. It was fun, especially with tailgaters as I slowed for bends, but common sense prevails.