SUNBEAM S7 and S8 MOTORCYCLES
  • SUNBEAM MOTORCYCLES. HEADER PAGE.
  • SUNBEAM NEWS and START PAGE
  • About the bike.
  • Distributor Internals
  • Redundant spark ignition.
  • Coil Information.
  • Beam-Spark Ignition Unit
  • FOR SALE
  • Smiths Chronometric Speedometer
  • Calibrating Smiths Chronometric Speedometer.
  • Speedometer Drives.
  • Companies and Products.
  • My other bikes.
  • The Horn. and Rear Brake Spring.
  • HOW TO PAGE
  • Modifications Page One
  • Modifications Page Two
  • Modifications Page Three
  • Dipsticks,sumps and oil weirs.
  • The Electrics
  • Sunbeam Dynamo in depth.
  • Repairs and Maintenance.
  • Carburettors
  • S8 Front forks.
  • Full width front brake on the S8.
  • Top dead centre.
  • Your Sunbeam.
  • All things wheels.
  • Data Sheets.
  • Engine breathing and oil leaks.
  • Putting the engine together.
  • Stainless Steel Exhaust
  • Tools
  • All things final drive
  • I80 Degree Crankshaft
  • Completed engine speed ignition sytem.

AMAL CARBURETTORS FOR THE SUNBEAMS

On this page :                  Original Amal carburettor.
                                          Concentric carburettor.
                                          Offsetting the carburettor.

The original Amal was a DO76/DO3A which is a ' left handed' version. That is, the very small diameter float chamber is on the opposite or right hand side  of the main body and the pilot and slide screws are on the left. The choke diameter is 15/16th inch. The cold start choke is a spring loaded plunger assembly. These old carburettors are available from a carburettor restoration specialist, Mr. Martin Bratby. The carburettor is restored to a better than new specification with rebored body and a hard brass slide along with all new internal components.

I am aware these carburettors do not have a really satisfactory float needle system and can cause problems round the base of the main body by 'sweating' petrol. Always turn off the petrol tap immediately after stopping the engine. Make sure the petrol taps are not leaking internally.

Picture of restored original carburettor model DO76.

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New Amal Carburettor.

The new Amal carburettor is available from Stewart  Engineering and is model type 624/4 concentric type. The choke diameter is 24 mm, this carburettor is very easy to set up requiring very little adjustment on the air or throttle stop screws. The choke is operated by a handlebar lever. I did away with this, blocking off the cable adjuster top to prevent air leakage, I enrich the mixture by flooding or 'tickling' the float chamber. The carburettor has a large 'tickler' button for this purpose. This is my favourite carburettor of the two types. Burlen Fuel Systems do a ' Teflon ' coated slide for this carburettor. It is a straight forward replacement for the original and should reduce any wear significantly.
Below is shown the new teflon coated slide on the left.

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The back plate of the air cleaner for the two types of carburettor are different in the diameters of the threaded hole, so be aware they are not interchangeable. The two air cleaner backs are available from Stewart Engineering.
Picture shown below, left hand type for old DO76 carburettor. Right hand type for new 624  carburettor.
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Picture of Amal 624/4 carburettor.

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Picture showing choke cable adjuster blanked off.

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Carburettor offset.
The standard spacer for the carburettor to cylinder head is 3/8th inch. I wanted to see if I could get away with a bit more offset. I machined up two new longer head studs and obtained a 3/4 inch spacer. This is about the maximum possible offset with the standard 'teardrop' air cleaner cover without the riders right knee rubbing on the paintwork.  I wanted to give the carburettor a longer inlet tract and also to get a bit more cooling air to the carburettor body by moving it out from between the hot exhaust headers.

I decided to see if I could offset the carburettor by one inch for 2014 season. I machined up two new mild steel studs on the Myford lathe which is idea for screwcutting Whitworth and BSF threads. I lengthened the support pillar with a piece of brass rod soft soldered  onto the air cleaner end. The only problem I have is the lack of one inch Amal carburettor spacers so for the time being I have used an additional 1/4 inch one to give me the extra length. Initial riding has shown the air cleaner teardrop can just be felt with the right knee if the tank knee rubbers are gripped. Temperature of the carburettor body after a long ride is now cold to the touch so the tickover remains constant. I would think that trickling along in heavy traffic is going to be much improved.
Picture of one inch offset at end of article.

Longer carburettor studs.

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New 3/4 inch spacer.

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Longer support pillar.
The brass support pillar for the air cleaner cover has to be increased in length to 3.75 inch including any plain and locking washers. This length of support pillar applies to both types of carburettor.

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Carburettor offset by one inch for year 2014.

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Petrol Feed Pipe.
To keep the incoming fuel to the carburettor cool I ran the feed pipe next to the air intake. I made up a brass adaptor for the float bowl with a short length of 6mm  copper pipe soldered into it and curved upwards. ( First picture ). The plastic petrol pipe was covered in nylon electrical insulating braid. The petrol pipe is loosely secured at the outer end of the brass spacer rod with an aluminium clip which keeps the braid covered pipe close into the air filter cover ( Second picture )

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