Amal concentric mixture screw

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baz
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Amal concentric mixture screw

Post by baz »

My b50 will only run with the mixture screw half a turn out ,does this indicate a partially blocked pilot jet bush? I have cleaned it many times with a 16 thou jet cleaner ,am I missing something?
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Re: Amal concentric mixture screw

Post by mlb50 »

Have you examined the tip of the screw? I've had some dodgy ones from new!
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Re: Amal concentric mixture screw

Post by eebtr7 »

You can always give it a healthy squirt with aerosol brake cleaner, then a blast of 120PSI compressed air. Then a length of suitable wire down the main passage way with yet another squirt and air blast. Have you tried sanding off any crusty deposits from the metering needle and/or the mixture screw?

The AMAL Mk 1 Concentric is not too far removed technologically from a Neanderthals stone axe.
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Re: Amal concentric mixture screw

Post by baz »

mlb50 wrote: Fri Apr 29, 2022 3:28 am Have you examined the tip of the screw? I've had some dodgy ones from new!
The screw looks ok but I will try changing it
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Re: Amal concentric mixture screw

Post by baz »

eebtr7 wrote: Fri Apr 29, 2022 3:32 am You can always give it a healthy squirt with aerosol brake cleaner, then a blast of 120PSI compressed air. Then a length of suitable wire down the main passage way with yet another squirt and air blast. Have you tried sanding off any crusty deposits from the metering needle and/or the mixture screw?

The AMAL Mk 1 Concentric is not too far removed technologically from a Neanderthals stone axe.
Yep I've tried all that
I'm starting to wonder if there's some crap in the mixing chamber
I may have to dig the Welch plug out to take a look
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Re: Amal concentric mixture screw

Post by Jim S »

I had problems trying to clear a blocked pilot jet on my recently acquired (and difficult to start) B50. I finally got it cleared by using a straw normally supplied on WD40 cans as a guide for the thin wire, in combination with carb cleaner and compressed air.

You can confirm that the jet is clear when you get carb cleaner or compressed air flowing out the fuel intake hole from the bowl. See the attached.
Attachments
Bushmans Carb Tuning Secrets.pdf
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baz
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Re: Amal concentric mixture screw

Post by baz »

Jim S wrote: Fri Apr 29, 2022 7:03 pm I had problems trying to clear a blocked pilot jet on my recently acquired (and difficult to start) B50. I finally got it cleared by using a straw normally supplied on WD40 cans as a guide for the thin wire, in combination with carb cleaner and compressed air.

You can confirm that the jet is clear when you get carb cleaner or compressed air flowing out the fuel intake hole from the bowl. See the attached.
Yes I have done that
The bike is actually running quite well now with the pilot air screw 3/4 turns out
Just seems wrong to me but I'll leave it set there now
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Re: Amal concentric mixture screw

Post by Mark Cook »

A nice document Jim. Thanks
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Re: Amal concentric mixture screw

Post by Mr Mike »

I have encountered your exact situation before. I tried the solvents and compressed air and eventually ended up taking a #78 drill bit (from McMaster Carr) and gluing it into a straw from a WD-40 can. I was able then to clean the pilot out thoroughly and the mixture screw would then idle best at 1 1/4 turns out...where it should be. A clean pilot jet is essential to proper idling and easy starting. You might use a guitar string, but the drill works good as the flutes on the drill do a good job of getting the varnish and buildup off the pilot jet. Earlier carbs had a removable jet, but later ones have it pressed in.

Hope this helps,
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Re: Amal concentric mixture screw

Post by Jeff K »

Here in the North East where people park their bikes for the winter, come spring time, varnish was a real problem. Every spring that seemed like all I did for a month was pull and clean carburetors, No matter how many times that I told them to drain, run until dry or add gas Sta-bil. There was always a bunch who just parked it at the end of summer. The new Alcohol gas made it worst because it is corrosive. I talked to a Large farm owner in the Mid USA and he grows 1000's of acres of corn just for the new gas. He told me that they do not have any problems with it because there is some additive that they use to stop the corrosion, but that additive costs money and most refineries in this part of the USA do not use it.
So here is what I used to do every spring, large coffee can, small tin pan, can of Gum-out carburetor cleaner. I would disassemble the carburetor and using the spray can with the red straw, I would put the plastic straw in every orifice and give it a spray and watch for the spray to shoot out. If it did not then I had a plug. (Wear safety glasses while using the spray, it is nasty if you get it in your eyes). If it was plug, I would let it set and try again in a few minutes, this worked 95% of the time. If it did not work, then I would pour carburetor cleaner into the coffee can and soak the carburetor over night and use a rubber tipped air nozzle and 90 psi to blow it out. I never poked anything into the jets, too easy to ruin the brass.
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Re: Amal concentric mixture screw

Post by Mr Mike »

Jeff,
Never had a problem with poking a #78 drill bit thru the jet. I believe it is about .016" in diameter and it gets it cleaned out real fast. A shot of carb cleaner kinda softens thing up. Ssomeone on the Brit bike site put me on to it years ago and it is a regular drill for me. I don't even have to take the carb off the bike.
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Re: Amal concentric mixture screw

Post by Jeff K »

Mr Mike
You sound like you know what you are doing!. problem is when someone who does not tries using a drill bit, they can ruin the jet. Drill bits are sharpened on the sides also and if you are not careful can easily cut on the sides. You are smart enough to let the spray soften it up and then a light and easy push with the drill and out goes the plug. But if it is hard and some one keeps twisting, then they will enlarge the holes. Many years back some one posted about using welding torch tip cleaners --- :uhu :uhu :uhu . If you look at one of the "wires" from these under a magnifying glass, you will see that they are a minature round file and they are designed to "open" up the hole. A fast way to ruin a brass jet. I have found that using a good Carburetor cleaner like Gum-out and some patience, that I had no problem removing even the even the difficult varnish build ups.
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Re: Amal concentric mixture screw

Post by Mr Mike »

Jeff,
More than one way to skin a cat. Never thought about torch tip cleaners. They would work great if you had the right size. We had a set in our shop at home when I was a kid but I no longer have a set of torches. What is important is that little jet behind the mixture screw must be clean however you get it done.!

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Re: Amal concentric mixture screw

Post by eebtr7 »

(The grumpy old man speaks.)

Aaahh! You young whippersnappers with your Neanderthalic AMAL Mk.1's!!! If you don't mind, I'll just stick with my gas soaked sponge dangling in front of the intake manifold. It works just has good!

(The grumpy old man has now spoken.)
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Re: Amal concentric mixture screw

Post by Mr Mike »

Neanderthal Amal Mk 1's....hardly. Two or three sizes of these carbs have fit thousands of bikes. And with slide changes, needle changes, main and needle jet changes, pilot air adjustment you can tune it to any application. My brother had a pair on this old air-cooled VW bug. Wish more things were as universal as the lowly Amal.

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Re: Amal concentric mixture screw

Post by eebtr7 »

My dearest Mr Mike of the Great State of North Carolina,

I fear you may be missing out on the 'Grumpy Old Man's' sarcasm regarding the Neanderthalic AMAL Mk1 Concentric. Should you ever venture far & wide to ol' San Antonio, you will kindly observe a serious lack of Oriental carburetion devices chugging away on a certain 1972 BSA B50SS Gold Star.

Just this afternoon the Dragon fired up on its AMAL Mk1 30mm for a bit of exercise up & down the local hills to test a different heat range Champion spark plug. I suspect the N-3 is too cold so a N-6 was installed. Starting was easier and no detonation was evident in the short run with temperatures in the mid 90's. I'll keep you posted as testing continues, I've got to figure a way to make starting less of an exhausting chore and this modern swill gasoline is not making life easier.
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Re: Amal concentric mixture screw

Post by Mr Mike »

I am getting pretty old and sometimes well-intentioned sarcasm goes right over my head. Two things I might recommend. (1) Change that 230 AMAL to a 232. Mild porting at the inlet is helpful. It will run like a scalded cat. I was able to go up to a 250 main jet with this set up and at WOT it runs fantastic and is absolutely smooth throughout the range. (2) read the B50 starting procedure that I wrote and had posted in the tech section of this site. When I first put my B50 together it was an absolute bear to start. I decided I was going to learn how to start it or someone else was going to own it.

For those that don't visit the tech section, here is the B50 starting procedure here:
Starting a BSA B50 (cold)
1. Make sure timing is reasonably close to spot on.
2. The carb must be clean. Especially important is the idle mixture jet which is cleaned with a #78 drill wedged in a red WD-40 spray tip. It must be clean!
3. The throttle slide stop screw needs to be screwed in about 1/2 to 3/4 turn. This would be a very fast idle when the bike is warmed up. I welded a knurled post onto the stop screw so I can adjust it with a gloved hand and don’t have to get out a little screwdriver.
4. Liberally flood the carb with the tickler
5. With ignition off, prime the cylinder by kicking over the motor using compression release a couple of times with the throttle open. Re-flood the carb with the tickler again.
6. With ignition still off, ease the motor over TDC using the compression relief. Continue tuning the motor over with the kicker until you feel the springiness of the exhaust valve opening. This is about 70 to 80 degrees past TDC. (This is very important!) It helps give the engine more speed as it goes thru intake and then compression and the extra speed helps it got over TDC.
7. Reset the kicker to the top, turn on the ignition and give it a real hard kick. Don’t use the decompressor as you are over TDC. Mine starts like this even when battery less. Kick it all the way through and keep your foot firmly on the kickstart lever so if it backfires your leg can come up with the lever and you don’t get hurt.
It should start easily. After it warms turn the idle adjustment screw to achieve the desired idle speed.
On a hot start (like after a stall), you really only need follow steps 6 and 7. If the bike sits for about 20 minutes follow steps 3, 6 and 7.
Works for me!!! Unfortunately, it took me many years of BSA single ownership to figure this out. My B50 is generally a one kick affair. Sometimes when the engine is hot after a long ride, and you let it sit for five minutes or so it can be difficult, but it always starts.

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Re: Amal concentric mixture screw

Post by KiwiB44 »

Ref good starting, it’s essential to have the correct retard on the ignition, so either the correct (and properly working) original mechanical one, or a good electronic system. Apart from making the bike easier to start, and idle, the correctly retarded ignition timing during starting almost eliminates the risk of kickback.

Until I “adjusted” the springs on the mechanical Advance/retard mechanism on my bike the kickbacks were downright dangerous, let alone damaging to the kickstart mechanism.
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Re: Amal concentric mixture screw

Post by eebtr7 »

Yes! Our friends Mr Mike & KiwiB44 both make very good points. The very essence of the starting of an internal combustion engine lies in compression AND correctly adjusted ignition AND correctly adjusted carburetion. All well and good...until we mix in the vagaries of a BSA B50 with its slow cranking speed thru the kick starter, very light weight flywheels, and somewhat primitive ignition advance unit. Combined with an ability to smell fear from the soul about to have his right knee, ankle, and bottom of his foot mangled, these delightful machines can and do defy efforts to start depending on its own mood.

For the past 50 years for me, it is a simple matter of superior will power pounding down on the kick starter to drive the crankshaft around to initiate a running engine, whether cold or hot. None of my three B50s thru the years have paid any attention to the published starting routines. If your machine is somewhat more compliant, I envy you. But, once running, the ride is like nothing else out there. No other motorcycle conveys such a sense of at-oneness with the bike and the surroundings we are propelled thru. I sensed this in 1971 and I still do today.

The only thing is...an electric starter would sure be nice on these 70 1/2 year old bones!
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Re: Amal concentric mixture screw

Post by Mr Mike »

A B50 is a "Real Motorcycle". Decent handling for it's day, lightweight, powerful. It is the only BSA I have kept. I have no use for 600-800 pound machine that everywone rides today. I want a bike that tours the backroads, can cut through a field, can be seviced by it's owner and 50 years later I can get parts to rebuild it. Mayby I'm good or mayby I am just lucky, but my B50 is an easiy starter...easier than my long gone B25 that I rode in college.

Mr Mike
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