BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: Phil C on 28 January, 2018, 12:37:40

Title: Choke
Post by: Phil C on 28 January, 2018, 12:37:40
My recently-purchased 1965 B40F won't run without a bit of choke, even when warmed up. But the plug is black and sooty. So on the one hand the bike appears to be saying it needs choke, ie it needs a richer mixture, but the plug is telling me the mixture is too rich. Any thoughts, please?   Phil (novice.)
Title: Re: Choke
Post by: AWJDThumper on 28 January, 2018, 14:27:29
Are you able to run the bike on the road and, if so, what's the colour of the plug after a reasonable run? If you have only done a static test, the colour of the plug could be a bit misleading.

I would run through the normal list of checks. What condition is the carb in? Is the jetting and needle position correct. What condition is the throttle slide and bore in? If the slide isn't a good fit then air will leak past and you may have to richen the mixture to compensate and make it run better.

Is the ignition timing correctly set. If the ignition is too retarded you may well have to run on the choke which will richen the mixture and effectively advance the timing.

If carb and timing and correct, you may then have to look at the condition of the engine. Normally a compression test is a good measure of the condition of the piston, bore and valves. Hopefully, the problem may be a bit simpler to sort out.
Title: Re: Choke
Post by: Phil C on 28 January, 2018, 15:25:31
Thanks for the suggestions. I did about, say, three miles on it this morning and the plug was black and sooty when I got home.

Being a novice, I don't know what condition the carb is in. As I say, I know nothing about carbs other than what I've read, so don't know whether it has the right needle position and jets. How do I find out what they should be? It's an Amal 626 L301, which I think is a Concentric modal and I guess would not have been the model of carb on the bike when new. Obviously I'd be up for taking it off and having a look at it, but I'd need to know what I'm looking for.

Ignition timing is not yet checked. Would you do that first, or take the carb off?

I haven't got a compression tester - are they expensive? There's no visible smoke to speak of coming from the exhaust, if that's significant.

Thanks again.        Phil (novice.)

Title: Re: Choke
Post by: A10 JWO on 28 January, 2018, 17:20:36
Hi Phil and welcome. First of all I would buy a aerosol of carb cleaner. Those carbs have fine drillings in the body that spray primary fuel to the engine. Clean the carb as much as possible, every nut and bolt. Take photos with you phone to act as a reference. Spray shed loads down the two holes at the front face. A fine spray should come up at small drilling on the engine side of the needle. You can get a service kit from Ebay or Hitchcock M/C's. Adjust mixture screw as per book. Put Amal into Youtube search engine, lots of info and demo's on there. NO Smoking near the carb cleaner. If no joy come back here. Regards Colin
Title: Re: Choke
Post by: AWJDThumper on 28 January, 2018, 17:44:07
Phil. I would start with the carb which I would remove from the bike so that its settings can be determined. If you unscrew the float bowl at the bottom of the carb, you can then unscrew the main jet and needle jet assembly from the middle of the underside of the carb. With these removed, you can then read the sizes stamped on the two jets.

If you then unscrew the top cover of the carb and remove the throttle valve, you can then work out which needle position has been selected. The copper clip will have been inserted into one of the slots at the top end of the needle. Look to see which slot has been selected with number 1 being the uppermost. At the same time look to see what part number is stamped on the throttle slide.

Ideally, both the throttle slide and the bore in which it moves should be in reasonable condition and should not show any scratches or ridges due to wear. If you remove the throttle slide from its cable, you can then insert it into the carb and check the clearance between the slide and the body of the carb. Ideally, it should be fairly snug but free to move. If the slide is too loose a fit, it can let air leak into the engine weakening the mixture.

The last thing I would check is whether the mounting flange is straight. To do this, remove the O-ring and put a straight edge across the flange across the top of the mounting holes. If the flange is not straight but bent then it could allow air to leak into the engine, again, producing a weak mixture.

With all this information, it should be possible to work out whether the carb is a possible cause for your problems.
Title: Re: Choke
Post by: Phil C on 28 January, 2018, 18:40:35
Really helpful suggestions, thanks both of you. Something I'm wondering about though: you say it's possible that air is leaking into the engine, either via a worn throttle slide or its bore, or via a bent mounting flange, and you explain that either of those would give me a weak mixture. But if that were happening why then would my spark plug be black and sooty? I'll do as you advise and take the carb off and have a look at it (it will be good learning for me) but could you explain the above please? As I said at the beginning of the discussion, the machine is asking for choke and yet has a black sooty plug.  Phil (novice.)
Title: Re: Choke
Post by: JulianS on 28 January, 2018, 19:09:12
Is the bike fitted with a monobloc carb?

If yes then check that the float needle seat is correctly assembled.

If an original monobloc which takes a thick o ring there should be no fibre washer between carb body and needle seat. If there is a fibre washer in this carb then the float level will be too high and could flood and/or cause it to run rich.

The modern made monoblocs which takes the thin o ring needs the washer - there is a machined recess for it which is absent in the original production carbs.

Photo shows modern production carb with the washer.
Title: Re: Choke
Post by: Phil C on 28 January, 2018, 19:32:15
No, it's a Concentric.
Title: Re: Choke
Post by: AWJDThumper on 28 January, 2018, 20:12:58
If the carb is leaking air, that could be why it won't run properly. Running on choke might be trying to correct for this weak mixture but may result in being too rich at low throttle opening. It's best to check out each possible source of the problem in turn. Since you haven't got the standard carb fitted, the state of the carb needs to be checked to show a problem or to rule it out.

Also, what's the history of the bike. For example, have you recently acquired it and found it has this problem or have you owned it some time and this problem has only recently developed?
Title: Re: Choke
Post by: Phil C on 28 January, 2018, 20:40:42
I recently bought it - the problem came with it.

Do I need to know anything about jet sizes, needle positions, etc, and if so where would I find out, given that I don't think the bike would have had this type of carb when new? I think they had monoblocs.

Title: Re: Choke
Post by: AWJDThumper on 28 January, 2018, 21:02:26
Look at hoogerbooger thread which gives his settings for a concentric carb.
Title: Re: Choke
Post by: JulianS on 28 January, 2018, 21:07:54
Any numbers on your carb?
Title: Re: Choke
Post by: Phil C on 28 January, 2018, 21:39:24
Amal 626  L301
Title: Re: Choke
Post by: JulianS on 28 January, 2018, 22:30:19
That is one of the baseline spec Amals;

If it has seen some use it would be worth going through this Amal article on rebuilding the concentric;
Title: Re: Choke
Post by: Phil C on 29 January, 2018, 17:35:01
Excellent - thanks very much. I'll see what I can make of it (if anything!)  Phil.
Title: Re: Choke
Post by: hoogerbooger on 29 January, 2018, 19:47:47
Just to note that my Concentric is a 627  - so set up will differ.

just tried recheking where I got the 626 reference .The BSA 1965 service sheets indicate a monobloc 376 was fitted.

found this should it be of use:
but not sure if the set up listed is specifically suitable to the australian bike  rather than Uk B40F model

[Whilst not what was fitted, does the concentric have functional advantages over a monobloc ? Just wondering  whether I get a new concentric should mine prove not sensible to repair  - I note they are cheaper !]
Title: Re: Choke
Post by: Phil C on 29 January, 2018, 21:10:17
Thanks Hoogerbooger (interesting name!)  Phil.
Title: Re: Choke
Post by: MICKREEVES on 30 January, 2018, 10:31:56
You can check your compression fit by slowly pushing  the kickstart lever.
fI it is hard to turn past compression, its OK.
If it turns over without much effort, then you may need a rebore.
Title: Re: Choke
Post by: Phil C on 30 January, 2018, 15:09:39
Do you mean with my hand, or with my foot and therefore with my weight on it?