Amal MK2, how to tune, start to finish

CraigT

Joined

Aug 23, 2019

Messages

22

• Aug 24, 2019

• #1

So I have this, new to me, 74 Commando Roadster with a single MK2 Amal (My other two Nortons have twin MK1 Amals) The bike runs pretty good at it's current carb settings (although my plugs are consistently black). It starts easy, idles nice and runs great (at least to me) up to about 3/4 throttle. At 3/4 to full throttle the engine cuts out. I think I read in the Amal lit. that the main jet is responsible for 3/4 to full throttle and that if the engine is cutting out it means you need a larger main jet. Does this make sense and if so how much bigger do I go? Or what should be my next step to work this out? Thanks!

 

marinatlas

Joined

Nov 4, 2007

Messages

1,413

• Aug 24, 2019

• #2

https://www.rgmnorton.co.uk/csi/1245184/f/pdf/single_carb.pdf

have a look there , you could find maybe what you need!

 

acadian

VIP MEMBER

Joined

Mar 5, 2010

Messages

1,446

• Aug 24, 2019

• #3

https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf
http://www.jba.bc.ca/Bushmans Carb Tuning.html

 

htown16

VIP MEMBER

Joined

Apr 29, 2009

Messages

1,518

• Aug 24, 2019

• #4

Yes, main jet above, 3/4 throttle. Try the next size up. Not a very expensive part to experiment with. They make a jet wrench that you don't have to drop the float bowl to change jets.

 

B

bill

Joined

Jun 1, 2003

Messages

2,151

• Aug 24, 2019

• #5

i would make a 2 size change 1 size is to small to see a real difference.

 

Triton Thrasher

Joined

Feb 10, 2009

Messages

2,971

• Aug 25, 2019

• #6

bill said:

i would make a 2 size change 1 size is to small to see a real difference.

Click to expand...

Yes it would take more than one size too small to make the engine cut out.

 

Fast Eddie

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Joined

Oct 4, 2013

Messages

21,594

• Aug 26, 2019

• #7

What main jet is fitted?

Is there one fitted?

Is it screwed in properly?

You need to have a look inside and let the experts know what yer got before you start buying random jets...

 

To surgeon

Joined

Jun 11, 2019

Messages

59

• Aug 26, 2019

• #8

I just installed two new Amal carbs (premier), idles super high. Can someone give me the ballpark starting point for the two idle/air adjustment screws, ie both 1 1/2 turns out?
Thanx

 

acotrel

Joined

Jun 30, 2012

Messages

14,004

• Aug 26, 2019

• #9

I think the idle screws on my Mk2 Amals adjust the fuel. I just lean-off until I get the miss, then back off until I get the smooth idle. It is important that the two slides hit bottom at the same time.

The comment that the plugs are black - what is important is the appearance of the porcelain, not the metal part. There should be a black ring on the porcelain, right down inside where it meets the metal. The heat range of the plug and the fuel mixture determine how far the soot burns off. I'd use 8s for tuning and 9s for running. That way you stay safe.

 

MichaelB

"Sons of Arthritus"

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Joined

Jan 14, 2004

Messages

2,448

• Aug 26, 2019

• #10

To surgeon said:

I just installed two new Amal carbs (premier), idles super high. Can someone give me the ballpark starting point for the two idle/air adjustment screws, ie both 1 1/2 turns out?
Thanx

Click to expand...

Leave the air screws at 1 1/2, adjust the idle screws for around 900 or so, then fine tune the air screws, adjust idle again.
Air screws are horizontal and control air, idle screws are upward and control the slide position.

 

htown16

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Joined

Apr 29, 2009

Messages

1,518

• Aug 26, 2019

• #11

MKII mixture screws control the air same as MKI. In for richer, out for leaner. Start at 1 1/2 and should need no more than a 1/2 turn either way. Original poster was talking about above 3/4 open performance which is main jet territory. If you want to talk idle start another thread.