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.
Title: Re: Pilot air screw
Post by: Minto on 29.09. 2019 22:26
Just to add to this, a couple of weeks ago while out on a run, the old girl started missing and stuttering when under load, up hills in particular. I’ve not had chance to look at her since til today. So took the carb off and stripped it down to its component parts, pushed the jet block out (easy job with no use of hammers!!!) and blew it through with some compressed air.
I’ve dropped the throttle needle a notch which on a colourtune is now showing a nice blue while idling. but when on the road is still missing under load at about mid throttle. (In 4th gear between 45 and 50 mph)!again particularly up hills.
I’m going to check the pick up brushes and slip ring next, but if anyone has any other suggestions I’d be proper grateful.
Title: Re: Pilot air screw
Post by: Gerry on 30.09. 2019 02:00
Hi Jase, Although my A10 swinging arm has 376 monoblock I had exactly the same problem, air screw right in and right out with no spring pressure and made no difference. Amal said 25 pilot jet which is what was in there. Had a 20 pilot jet, fitted it and found I could at last adjust the air screw and make a difference to the idling. Adjusted it until I got the highest revs without missing or stalling and then lowered the slide screw to get the tick over I wanted. Had to go back and forth a few times before success. This would be the easiest solution to try before dismantling. Good luck and hope you get it sorted. Cheers Gerry
Title: Re: Pilot air screw
Post by: Slymo on 30.09. 2019 02:05
What can happen with the pre monobloc  carbs is that the nut at the bottom of the body gets over tightened. This bells out the body out above the bottom nut so the the passages in the jet block lose contact with the corresponding drillings in the body. The jet body still seems to fit tightly in the body but its because its crimped at the bottom. This stops the air screw from having any effect and tends to make the carb run rich. If you can run a straight edge up the inside of the body you can spot the distortion. The only way to fix this other than boring and re sleeving  the body is to gently scrap out the bottom of the body to remove the compressed bit and then shim the jet block over so that the drillings are tight together. You'll know its worked when the air screw works again and the carb is tune-able.
Title: Re: Pilot air screw
Post by: Minto on 30.09. 2019 20:53
Thanks Gerry and Slymo
I’m hoping it’s not that the carb body is distorted, that sounds expensive!!
Not going to get to look at it for the next three weeks. I’ll report back as and when I make things worse.