Author Topic: Amal 375 carburetor  (Read 276 times)

0 Members and 1 Guest are viewing this topic.

Offline Paulflor

  • Full Member
  • *
  • Posts: 13
  • Country: us
  • Location: San Diego, CA
Amal 375 carburetor
« on: January 25, 2019, 04:29:10 pm »
Hi All,
I have been working on bringing a C12 with a c11g engine back to life. 
I rebuilt the carburetor (Amal 375) with all correct jet sizes and needle. To my surprise the bike came to life on the third kick and was idling very high.  I was afraid that i was going to damage something so i shut it off. 
My question is, and sorry for possibly being redundant, how do you tune this type of carb. Which is the idle control screw and air screw? 
I also think that the cutaway slide may be positioned too high as the throttle cable may be short.  Does the cutaway slide sit on the bottom of the throttle body or is there a gap?
Any advice or information would be greatly appreciated.

Thank you,

Online Vreagh

  • Full Member
  • *
  • Posts: 199
  • Country: england
  • Location: Cornwall
Re: Amal 375 carburetor
« Reply #1 on: January 25, 2019, 04:46:48 pm »
With the cable slack, slide should bottom out. Tick over speed screw is angled upwards, air screw is horizontal.

Online hampshirebiker

  • Gold Member
  • ****
  • Posts: 5586
  • Country: england
Re: Amal 375 carburetor
« Reply #2 on: January 25, 2019, 04:46:58 pm »
All C12s have a C11G engine number. If the cable's a bit short that could be your problem, The cutaway does not disappear on a closed throttle. On the offside (right hand) the near vertical screw is the throttle stop and the horizontal screw above it is the mixture; start at about 1.5 turns out. Come back if you can't make any sense of it, but they're pretty self explanatory.
P.S. Posted in parallel to the above.
Postal - Liphook Hants - But just into West Sussex.

Offline Brillandy

  • Full Member
  • *
  • Posts: 435
  • Country: gb
  • Location: Brill, Bucks
Re: Amal 375 carburetor
« Reply #3 on: January 25, 2019, 04:55:35 pm »
There's a service sheet on this site which you may find useful in understanding these carburettors, https://bsac10c11c12.co.uk/smf/carburettor/carburation-all-models/

Andy
56 C10L
60 Nuffield DM4

Offline Ginge

  • Full Member
  • *
  • Posts: 405
  • In NZ sometimes. In Oz sometimes.
Re: Amal 375 carburetor
« Reply #4 on: January 25, 2019, 11:15:26 pm »
Slide should go click as it bottoms out when you let the throttle go with engine off.

Ginge.

Offline ThomasC11

  • Full Member
  • *
  • Posts: 430
  • Country: de
Re: Amal 375 carburetor
« Reply #5 on: January 26, 2019, 06:45:10 am »
Just for a quick check: I had the very same situation at the beginning and discovered a small crack in the nozzle.   
Cheers, Thomas
1946 BSA C11
1937 Sunbeam Model 9
... and a dented Opel

Offline timsdad

  • Gold Member
  • ****
  • Posts: 7100
  • Country: england
Re: Amal 375 carburetor
« Reply #6 on: January 26, 2019, 08:03:40 am »
As has been said, Paul, the slide has to start off 'bottomed out' so you can hear a distinct clack when the throttle is abruptly shut off. Before firing up a carb that's been stripped, I then screw in the angled throttle stop screw just enough to move the slide up off its base when you watch through the rear. This sets the tick-over a bit high - say about 1200 revs -  if you can count quick enough, and will keep the engine running. With a warm engine, you can then gradually lower the slide as far as possible that still keeps the engine running.

With the mixture screw 'right in' (lightly screwed to the bottom) and then back out one and a half turns, as advised, turn this back and forth about 1/4 turn at a time, and then less and less, until the motor is running at its highest and smoothest. The throttle stop screw can then be lowered to get the slowest and smoothest tick over. I would expect this mixture screw to finish up no more than a 1/4 turn from where it started off, one and a half turns out.

If the mixture screw has no effect, there is likely a gallery blocked in the slow-running system of the carb or air sucking in somewhere.

All carbs, except CVs, are tuned much the same except on some makes the mixture screw cuts the fuel down and some cut the air down so they go the opposite way to make it richer or leaner.


Ray
Just a motorcyclist.

Offline Paulflor

  • Full Member
  • *
  • Posts: 13
  • Country: us
  • Location: San Diego, CA
Re: Amal 375 carburetor
« Reply #7 on: January 27, 2019, 04:52:53 pm »
Thank you All!
 i'm still working on it but did manage to get it running.  It sustained idle for a few minutes but then it cut off and i have not been able to get her going again for more than a few seconds.
looking through past posts i think i may have a clog in the pilot circuit.  As it starts but dies within a few seconds before cutting out.

Online hampshirebiker

  • Gold Member
  • ****
  • Posts: 5586
  • Country: england
Re: Amal 375 carburetor
« Reply #8 on: January 27, 2019, 04:58:51 pm »
Take the brass jet block out (set screw in the side) and soak in thinners for a couple of days. Blow all the drillings through with a high pressure CA nozzle.
Postal - Liphook Hants - But just into West Sussex.

Offline enfield49

  • Bronze Member
  • **
  • Posts: 1112
Re: Amal 375 carburetor
« Reply #9 on: January 27, 2019, 05:20:24 pm »
They will run with the pilot jet blocked but will not tick over . I would suspect the main jet assembly .  Graham

Online hampshirebiker

  • Gold Member
  • ****
  • Posts: 5586
  • Country: england
Re: Amal 375 carburetor
« Reply #10 on: January 27, 2019, 05:45:47 pm »
It could well be and wont involve stripping the carb. If you've got crap in the main jet after a carb. rebuild I would question where it's coming from. Have you flushed out the tank? Some people's interpretation of a carb rebuild is different to others. Unless you've done something daft it'll be an obstruction somewhere. Should be easy enough to find and you'll learn more about the carb as you proceed.
Postal - Liphook Hants - But just into West Sussex.

Online Tman

  • Silver Member
  • ***
  • Posts: 2200
  • Country: england
Re: Amal 375 carburetor
« Reply #11 on: January 27, 2019, 10:27:42 pm »
They will run with the pilot jet blocked but will not tick over . .  Graham

A bugger to start though, if it'll start at all...

Offline Paulflor

  • Full Member
  • *
  • Posts: 13
  • Country: us
  • Location: San Diego, CA
Re: Amal 375 carburetor
« Reply #12 on: January 27, 2019, 11:12:49 pm »
Once again, thank you guys for all your wisdom.  I got her running today, took it around the neighborhood.  She idles well and runs as well as i guess she should.
I cleaned the tank,  stripped the carb, used a small copper wire on the jets, and pushed some compressed air through it.  She starts first kick, Yay!
working on the wiring for the rectifier next.

Cheers from San Diego.  Its beer time now, on a perfect sunday afternoon.

Offline Paulflor

  • Full Member
  • *
  • Posts: 13
  • Country: us
  • Location: San Diego, CA
Re: Amal 375 carburetor
« Reply #13 on: January 27, 2019, 11:23:58 pm »
In the garage, showing the new generation how to make things come to life.

Offline Owen

  • Administrator
  • Gold Member
  • ****
  • Posts: 7644
  • Country: england
  • Solutions not problems
  • Location: Hampshire
Re: Amal 375 carburetor
« Reply #14 on: January 28, 2019, 07:05:08 am »
You have done a good  job rebuilding her.
1940 C12 (350cc)
1945 C10 & C11
1953 C10 & C11
Anyone want a B20 in need of a lot of tlc