Amal 376/67 carb

bogthick · Post by **bogthick** » Mon Mar 22, 2010 11:21 pm

Hi folks, I have a 1955 AJS Model 20 500cc that's fouling the plugs (running rich). The number on the Amal carb is 376/67. The carb is correct according to data contained in "Service and overhaul manual for the AJS & Matchless twin motorcycles by F. Neill".
If all the jet numbers are correct would I be best to order a complete set of jets and needle and rebuild the carb as I am unsure whether the jets have been tampered with.
Any other suggestions regarding this excess fuelling?

To clear up a few possible answers.
Yes the choke lever opens the slide fully.
Yes the float is set correctly and the needle valve shuts of the fuel supply.
Spark plugs, unsuppressed plug caps, Copper core HT leads and terminations at the magneto end are all new.
Not using open exhaust pipes.

The jet sizes are as follows;

Needle jet, 106
Pilot jet, 25
Needle position, Centre
Main Jet, 240
No air filter fitted.
Carb outlet ID 1 1/16"
Manifold inlet ID 1"

When I use the bike I get about 8 miles before the plugs are fouled enough to cause spluttering and poor running.

Any suggestions from the learned many would be appreciated.

wilko · Post by **wilko** » Tue Mar 23, 2010 2:11 am

Check you have the right length needle. There's a C and a D size. I forget which goes with which.

GOLDSTAR · Post by **GOLDSTAR** » Tue Mar 23, 2010 7:50 am

I would obtain a full set of new jets and needle from the real AMAL company, wo'nt be very expensive and start again. kind regards

GSAX1 · Post by **GSAX1** » Tue Mar 23, 2010 7:54 am

The '55 model 20 should have the 376/6 carburettor. This is however only interesting if you seek originality, since the number only denotes the jetting.

You didn't say what slide cutaway you have? I thinks it should be 4. I would check/change slide, needle and needle jet first.

Also be aware of that these symptoms could be an indication of a weak/faulty magneto.

Michael

Flash · Post by **Flash** » Tue Mar 23, 2010 8:58 am

Brendon, I think the pilot jet on that model should be a 30, not 25.

Regards John

Eric · Post by **Eric** » Tue Mar 23, 2010 9:00 am

I agree with GSAX1 its just as likely to be a magneto fault that brings the bike to a halt but of course you do need the carb to be reasonable.

bogthick · Post by **bogthick** » Tue Mar 23, 2010 12:43 pm

quote:
The '55 model 20 should have the 376/6 carburettor. This is however only interesting if you seek originality, since the number only denotes the jetting.

You didn't say what slide cutaway you have? I thinks it should be 4. I would check/change slide, needle and needle jet first.

Also be aware of that these symptoms could be an indication of a weak/faulty magneto.

Michael

Thanks for the info.

How do I identify the cutaway slide?
Is it by size or should it be numbered?

Any suggestions how I might check the magneto for proper performance?

Does the difference in ID of the carb and manifold mean anything?

Ah! old bikes I just love them. There's nothing like getting down and dirty!!

dave16mct · Post by **dave16mct** » Tue Mar 23, 2010 12:56 pm

The slide size is stamped on the top. I think your carb should be a 1'' , not 1 1/16. If the throttle slide is very worn (can you feel a lot of wear with your finger in the intake--ooh maitron!) then it will run rich. A new carb works wonders!

condor · Post by **condor** » Tue Mar 23, 2010 5:10 pm

Hi
I had a problem with the mixture being to rich on my model 20.

I lowered the needle a click at a time and that sorted the problem, you will need to reajust the slow running jet after lowering the needle.

Hope this helps.

Mike

bogthick · Post by **bogthick** » Tue Mar 23, 2010 7:07 pm

Ok, so I bit the bullet and ordered a new carb. After carefully examining the one I have, it needs a new slide as it's 3 1/2 and should be 4. The needle looks poor, jet block was beaten with a hammer at some time, wear between throttle valve and air valve. List is endless.

Anyway, how do I check a magneto?? Help appreciated.