Amal Mk I (Specific application: 932/ Norton Commando)

 

Stuart SS

History of symptoms and causes ~ my experiences!

 

I feel I have just overcome several issues regarding the Amal Mk I Carburettors fitted as the standard unit to most 1960 and 70s British bikes, the venturi dimension being the main difference between machine.

 

My Commando was initially totally restored in 1974. This project was the end product of a two and half year project.  Little to no expense was spared.

Features included:

·         The retention of the original Mk1 carbs, with stainless steel inserts being fitted to both the body and slides.

·          Additionally Bransden Boyer Ignition was fitted and,

·         At that time, the new Australian Auto cam chain tensioner.

 

Over the first several years of running the bike ran just fine.  While no great mileage was added to the speedometer reading before the bike spent some time in the back of the shed as it were, all appeared fine.

 

In 2000, the machine became increasingly difficult to start before it stopped and refused to start.  After a Harley- dog ride home, and much time spent head scratching two issues arose. 

  • The mounting nuts on the right hand carbie were slack. The O-ring on the RH carbie was NOT fitted. 
  • After fitting this O-ring and refitting the carbie, tensioning the mounting nuts, the body distorted jamming the slide.

 

This was eventually resolved after much tedious “bluing’ and polishing of the slide. The slide then worked just fine. The Left carbie appeared to be fine.

 

The bike resumed its classic performance and starting which I had expected initially.

 

In 2001 I moved to Queensland and after a period of hibernation, I put the bike back on the road yet again. 

Initially during our Summer- autumn period all seemed fine, until seemingly over a relatively short period of 2-300 hundred miles. A sensation of ‘snatching’ began to become more and more obvious.

The problem was indicative of a faulty ignition. But ~ under acceleration and/ or under load the machine appeared fine.

 

But under ¼ to 1/3 throttle on cruising mode, the snatching sensation immediately set in.

  • It gave all the indications of being in too high a gear or final gearing being too “tall”!
  • Running/ idling became erratic.
  • Idling gave an uneven and flat tone on exhaust notes.
  • Initial starting responded with heavy fuel vapour smoke emitting from the left pipe. (No choke usage.)

 

On inspection of the carburettors and spark plugs:

 

  • Spark was healthy.
  • Left hand slide was occasionally sticking.
  • Left plug was always severely rich.
  • Filters were all clean, as the tank was sealed with inner coating on restoration.
  • Slide adjustment and synchronising/ tune was poor.
  • Air filter only 400 miles old and clean.
  • Timing OK.

 

Servicing included:

  • Checking the float settings and needle settings – (leanest setting: 3rd notch.)
  • Resurface all abutment joints,
  • Polish and free slide/s in their respective bodies: check for excessive clearances.

 (The disadvantage here is that now the respective slides are suited only to the specific body and not interchangeable.)

  • Replaced all cables with Nylon inner style cables.

 (This in itself was a revelation, with the throttle action instantly becoming a sheer pleasure to operate rather than a constant exercise in stretching the Brachioradialis  (forearm) muscles, which was the case previously!)

  • Check all passages and jets.
  • Replace all wear components: Needles, needle jets and gaskets etc.
  • Replace NGK plugs with NGK Iridium plugs.
  • Additional considerations included the change of altitude (2000 feet above) now experienced as opposed to originally residing BELOW sea level ~ and up 1000 above sea level.

 

Final overhaul analysis: The new needle jets (106) against the original jets, also 106: showing   obvious wear.

 

Initial response:

  • Initial start was even more instantaneous with only half crank required.
  • Throttle ‘flicking/ blapping’, at too early a point, not warmed up, caused a carby spit back, no choke.  (While this may appear as normal to most ~ it had Never been experienced previously. Weather at this initial test was cool ~ over the week: 12- 15 deg C., being our winter.) 

Proper warm up/ idle eliminated this reaction.

  • Exhaust tones even and noticeably ‘crisper’.

 

Road test:

  • Excellent throttle response.
  • Good acceleration.
  • NO ~NUNNA ~ZIP~  “snatching” on ALL road conditions experienced over a two hour ride.
  • Road conditions varied from suburban, highway and country/ Hilly/ Mountain winding roads.
  • Plugs BOTH show CLASSIC/ textbook (ideal) colouring.

 

Additional experiences symptoms/ causes: (I am probably stating the very obvious to many but I BET ~ not all ~!)

  • Needle jets in Amals ARE prone to excess wear after 9,000- 11,000 miles!
  • Spark plugs electrode insulators can do break and crack, even from new: typified by erratic or cylinder failures. As do plug leads!
  • High level/s of cleanliness MUST be maintained when over hauling Amals or any carburettor as the idle jets in particular, or any jet can easily become fouled by foreign materials, creating additional problems which are bound to complicate any problems!
  • When fitting fuel bowl gaskets, try nipping off the inner square edge of the gasket that sits over the float-seating spindle/ rod. Just enough to clear the chamber recess.   This gasket edge can foul the float operation.
  • I always specifically like to surface the Inlet nipple (the ol’ emery on the flat glass panel.) This will minimise the risk of seepage.

 

Final comment

 While appreciating all the positive feedback I received, I suggest ~ use the K.I.S.S. method. (Don’t go complicating the issue, it may well NOT be too complicated.)

 I was ready to start ditching the Amals, fit up CV off a jap-a-heinous machine, and started hunting 6V high output sports coils and more.

 

 

·         Before ‘jumping the gun’ look for the simpliest cause of the problem/s.

·         IF opting for ‘the carburettor insert’ option, I believe it is essential to quiz the contractor as to the boring and alignment method, i.e.; will the carburettor maintain it’s shape completely, once installed and the mounting nuts tensioned up?

In short ~ get a guarantee of the final product will maintain its ‘smooth operational   function”, i.e.; jamming the slide.  Don’t believe this can’t or doesn’t happen.

After all if one of the slides jam open while riding, particularly in the event of a single carburettor application, the consequences doesn’t warrant contemplation.

 

 

As has been suggested by others in my research ~that if ‘you’ have extended your repairs to extraordinary lengths eg: the fitting of expensive and or ‘way out’ components to get the machine to run anywhere near correctly~ then you are over riding the real problem!

 

After all, for example the Norton or Triumph factories went to great lengths to compromise between effective performance and economic machinery.

I believe there are ~ many ~ effective aftermarket up grades, but then ~ there is effective aftermarket and then~ there is the purely extravagant!  And then of course there’s the hobbiest and the enthusiast!

 And in this article I am endeavouring to offer remedies for a specific range of problem.

 

One classic example I found was the fitting of three Amal MKI to a Hilda CB 750 four! I guess if you have the tools and the enthusiasm ~ Why not!?

Stuart SS

 

 

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