BSA Owners' Club Forum

The BSA Workshop => Twins => Topic started by: CROFTY1984 on 05 March, 2018, 20:07:52

Title: '71 A65 lightning running rough
Post by: CROFTY1984 on 05 March, 2018, 20:07:52
Hello all,
Following my previous posts, the A65 is starting pretty well, 3-4 kicks regardless of the weather :)
However it's running pretty badly on light throttle openings. I ultrasonically cleaned the carbs, took it out for a test run and thought I'd cured it, then after a mile or two the stutter came back. :(

Before the carb rebuild the right plug was a bit sooty so I'm going to try leaning out the idle mixture on that carb. It starts and runs well when accelerating but at a cruise at less than 1/4 throttle it just feels like a cylinder is missing a beat every few seconds.
Any other ideas about what to look at?
Thanks, Dan
Title: Re: '71 A65 lightning running rough
Post by: JulianS on 05 March, 2018, 22:35:34
Have you checked the Amal tuning guide or the workshop manual carb section, both give info on tuning and synchronising twin carbs.

http://amalcarb.co.uk/downloadfiles/amal/Mk1_Hints_and_Tips.pdf
Title: Re: '71 A65 lightning running rough
Post by: Editor on 06 March, 2018, 11:41:16
Sounds like a weak mixture on one of the carbs on Pilot stage (first 1/8) of throttle). As you say it runs well whilst accelerating which singles out the pilot stage being the problem.
Despite ultrasonic cleaning, I would remove the offending carb and check the pilot bypass (the two small holes in the bottom of the choke bore, either side of the front of the slide, by squirting carb cleaner down one side and seeing if it comes out of the other. if it does, poke some copper stranded cable down them. I had to do this on my Thunderbolt a couple of years ago and it worked a treat.