BSA Owners' Club Forum

The BSA Workshop => Twins => Topic started by: TTJOHN on 17 March, 2017, 10:14:10

Title: Lack of adjustment on carb air screw
Post by: TTJOHN on 17 March, 2017, 10:14:10
I have had quite a problem with the A65L, now, I know it should have a monobloc carb on it as its a 63 model, however, it was fitted with a 930 concentric when I got the bike so I thought I'd leave that on there, I can run the bike on choke, when I take the choke off it cuts out, the mixture screw does not seem to have any effect at all, although I have cleaned out the pilot jet which ids the sealed type and blew it through with an air line, I have just raised the needle to the middle slot which I believe seemed to have  improved it slightly, the book states the needle should be in the first groove, now I'm thinking maybe I should try the third groove, what do the team think? :-\

TTJohn
Title: Re: Lack of adjustment on carb air screw
Post by: Mike Farmer on 17 March, 2017, 10:30:31
 :) Good morning.

Is the slide/throttle valve correct??

Mike
Title: Re: Lack of adjustment on carb air screw
Post by: JulianS on 17 March, 2017, 18:39:15
John

Sounds like a blocked or partially blocked  pilot bush.

This from Amal may be useful;

http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter

You may need to remove the crud in the bush using the N0 78 drill they mention in the text.
Title: Re: Lack of adjustment on carb air screw
Post by: 5800N19-12 on 19 March, 2017, 11:57:55
Go to your local musical outlet and buy a .016" guitar string.
File the end to remove burrs, etc.
I use a LED torch (flashlight), in order to see the small jet.
Insert the guitar string into the jet, should go in about 1 1/2" from the outer edge.
Spray carb cleaner into the jet, you should see spray from the small holes in the venturi.

The float should sit at  .080" ( 1/12") below the cusp.  Press lightly on the tangs to check.
The "Stay-up" floats make adjustment easy.
If you have brass float needles, replace them with the lighter aluminium type.
Check the small O-ring seal on the mixture (air) screw, replace if in doubt.