The BSA A7-A10 Forum

Technical => Amal, Carburation => Topic started by: Minto on 01.05. 2019 00:55

Title: Pilot air screw
Post by: Minto on 01.05. 2019 00:55
Having got my plunger A10 running reliably now, I checked the colour of the plugs which are quite sooty. I have tried adjusting the air screw from fully in to fully out and this seems to make no discernible difference to the revs as I thought it should do. Is something not right here, what would cause this? I've recently had the carb off and cleaned it out. It's ca pre monobloc 276, air screw currently set at 1 1/2 turns out.
Muchos Gracias

Title: Re: Pilot air screw
Post by: Swarfcut on 01.05. 2019 09:11
The problem you have of sooty plugs is either a little bit of oil passing the rings  or valve guides and burning with blue smoke, or too rich a mixture. Providing it runs OK and the exhaust is reasonably clear, run as it is for a while, it may settle down. There is plenty of information on AMAL carb tuning, so you should have no problem sorting it out. The pilot jet adjustment setting has no real effect on the idle speed on this design of carb.

 Richness is indicated by black exhaust smoke, high fuel consumption, smell of unburnt fuel in the exhaust, plus other fine detail regarding performance with different air valve (choke) positions.

 In an old carb, most likely cause is a worn throttle needle jet, assuming correct assembly and fuel level. Also you do not know if the carb is standard set up or has already been messed with. The air filter or lack of one affect the size of main jet required.

AMAL suggest screwing out the pilot air screw,  lowering the throttle needle, fitting a smaller main jet, and a larger cutaway on the throttle slide to cure richness. Scrounging a ColourTune will help you set the idle mixture.


 Additional.   Standard Spec. 1953 A10

         AMAL Type 276ER/1DB     1 1/16" Choke at engine end.

  Main Jet     170

 Throttle Slide  6/4

 Needle Jet   .108

 Needle position 2

Title: Re: Pilot air screw
Post by: duTch on 01.05. 2019 09:22
Jase, bearing in mind my engine specs may be different and 389 monobloc, I had that with mine and stuffed around with just about everything- float / level pilot jet and probably other stuff.... but ended up using a smaller needle jet (105)- been fine now for ~15K miles....
Title: Re: Pilot air screw
Post by: BSA_54A10 on 01.05. 2019 15:00
If you can turn the pilot air screw more than 1/4 turn in either direction and not make a difference to the engine, you are idling on the cut away and not on the pilot jet.

Usual reason is the jet passage is blocked.
Title: Re: Pilot air screw
Post by: Minto on 01.05. 2019 16:00
Thanks all, I’m very new to this so all this info is great and really helpful.
How do I remove the jet block from the carb body? It’s the one part that didn’t get removed when I stripped it a couple of weeks ago.
Thanks again
Title: Re: Pilot air screw
Post by: Swarfcut on 01.05. 2019 18:28
    This is easy in theory, awkward in practice.  Once the float chamber and the big bottom nut have been removed, you can see the base of the jet block extending below the carb body. Mark the body and block to ensure it goes back as before.

  With everything stripped off, the jet block can be pushed down and out of the body. Sounds easy, but they usually need a good smack from the top, with all the dangers of distortion and mayhem associated with big hammers and soft castings.

 You need a nice piece of wood as a drift, to get the force to the outside edges of the jet block, as in effect it is a brass cylinder in an alloy sleeve. Heating the body with a hot air gun may help, but the brass will also expand.

 Before replacing, make sure to align the block and body with your marks. That's the basics, do your research first as these parts are fragile and easily damaged.

 Altering the mixture screw should make the engine falter (too much air) or choke itself  (too much fuel) on idle, and is set in conjunction with the throttle stop screw.

Title: Re: Pilot air screw
Post by: Minto on 01.05. 2019 19:44
Thanks Swarfy,
I’ve done a little digging and found some advice like soaking the body in hot water to get the carb body expand, then tapping it out with, as you suggested a wooden drift, but taking a hammer or mallet anywhere near a carb scares me. I’ll need a few glasses of single malt courage before I go at it.