1. Amal Carburetor Tuning
Peter S. Ballard
Consort to “The Red Lady”
1933 JAP powered Sports

2. Original fitment General belief is that the following were fitted when leaving the factory
Matchless MX
1 inch choke (25.4 mm)
140 main jet
6/4 slide
Needle at position 3
Matchless MX2
1 1/16 inch choke ( mm)
180 main jet
Matchless MX4
29/4 slide
JAP LTOWZ
1 1/16 inch choke ( mm)
180 main jet
29/4 slide
Needle at position 3
JAP Sidevalve front start
15/16 inch choke ( mm)
130 main jet
6/4 slide

3. Only two scenarios There are only two possible faults in carburation, either richness or weakness of mixture
Richness
Black smoke in exhaust
Petrol spraying out of carburetter
Sooty spark plug
Heavy, lumpy running
Weakness
Spitting back
Erratic slow running
Overheating
Poor acceleration
Runs better if air valve is partially closed

4. Start at the beginning
Before any tuning starts check that the carburetter is suitable for the engine and fitted with jets close to the original fitment
Next if the engine is in running condition, start and once warm check for air leaks around the mounting point and also the manifold nuts.
I use an unlit propane torch, if there is a leak the engine will speed up.

5. Remember
Poor ignition timing can cause some of the previous issues also. So before blaming the carburetter please be 110% sure you have timing correct and good points etc.
Also ensure carburetter is clean and all jets and passageways clear before commencing tuning.

6. Tuning, 5 easy stages Tuning is accomplished in 5 stages
1. ¾ to Full open
2. Up to 1/8 open
3. From 1/8 to ¼ open
4. ¼ to ¾ open
5. Recheck stage 2 for idle

7. What you need Long straight deserted road, preferably slightly uphill
Spark plug wrench
Gloves, plugs get hot
Screwdriver
Wrenches to fit jets etc.
Knowledge of throttle lever relationship to slide position
Patience

8. So what does what 1. ¾ to Full open - Main Jet size
2. Up to 1/8 open - Pilot Jet
3. From 1/8 to ¼ open - Slide cutaway
4. ¼ to ¾ open - Needle position
5. Recheck stage 2 for idle - Pilot jet

9. Main Jet size
From ¾ to full open the main jet controls the fuel supply.
Fuel supply still goes through the main jet at smaller throttle settings, but the main jet if sized correctly has no effect at smaller settings.
If on a full throttle run the engine runs heavily, i.e. feels like it is choked, the main jet is too large
If on full throttle closing the air slide gives better power then the main jet is too small
Change jet as appropriate and retest
Once satisfied, run at full throttle, preferably on an uphill to load the engine, then whilst still on full throttle, declutch and immediately switch off the ignition, coast to a stop and check plug colour
If black and sooty, main jet is too large, if there are signs of intense heat then the main jet is too small. If a nice tan colour all is fine.

10. The Pilot Jet system
Up to 1/8 open the Pilot Jet is responsible metering the correct amount of fuel into the engine
This is done with a fixed jet and a regulating screw.
Fuel flow is fixed by the jet air is controlled by the regulating screw
With the throttle set between closed and 1/8 open unscrew the slide stop screw until the engine slows and begins to falter
Now adjust the pilot regulating screw so the engine runs smoothly and speeds up
Unscrew slide stop screw again and repeat with the pilot regulating screw
You may find that the settings were close and have to screw in the slide stop screw a tad to get a good idle

11. Throttle Cut-away
From 1/8 to ¼ open the amount of cut-away of the slide controls the amount of air let into the carburetter
If as you take off from idle there is spitting from the carburetter then richen the pilot mixture by screwing in the pilot screw
If there is no improvement then a slide with a smaller cutaway is needed and the pilot screw should be reset
If the engine jerks under load then either the needle is set too high or a slide with a bigger cutaway is required

12. Needle Position
From ¼ to ¾ open the needle controls the amount of fuel let into the carburetter. The needle also controls how the carburetter works during acceleration
The needle controls the greatest range of throttle openings. Start with the needle clip in the groove closest to the top of the needle
If acceleration is poor and closing the air slide improves acceleration then raise the needle by two grooves
If there is great improvement then lower the needle by one groove
Leave it in the best position.

13. One last job Go back and recheck the idle once more
Make a note of all your settings

14. Resources Amal a division of Burlen Ltd. UK Hitchcock’s Motorcycles UK
Parts and new carburetters
Hitchcock’s Motorcycles UK
Surrey Cycles