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How to trace faults

There are only TWO possible faults in carburation; either RICHNESS of mixture or WEAKNESS of mixture, so in case of trouble, decide which is the cause, by;
1. Examining the petrol feed. Verify jets and passages are clear.
Verify ample flow.
Verify there is no flooding.
2. Looking for air leaks.

At the connection to the engine.
Or due to leaky inlet valve stems.

3. Defective or worn parts. Slack throttle or worn needle jet.
The mixing chamber union nut not tightened up, or loose jets.
4. TEST WITH THE AIR VALVE, to see if by richening the mixture, the results are better or worse.

Black smoke in exhaust.
Petrol spraying out of the carb.
Four strokes, eight stroking.
Heavy, lumpy running.
Heavy petrol consumption.
If the jet block F is not tightened up by washer and nut E, richness will be caused through leakage of petrol.
Air cleaner choked up.
Needle jet worn large.
Sparking plug sooty.
Spitting in carburetter.
Erratic slow running.
Acceleration poor.
Engine goes better if;
throttle not wide open or air valve is partially closed
Has air cleaner been removed
Jets partially choked up
REMOVING the silencer or running with a racing silencer requires a richer setting and a larger main jet.
Verify correctness of fuel feed, stop air leaks, check over ignition and valve operation and timing. DECIDE BY TEST WHETHER RICHNESS OR WEAKNESS IS THE TROUBLE AND AT WHAT THROTTLE POSITION. See throttle opening diagrams, sheet 7 .

If at a particular throttle opening you partially close the air valve and the engine goes better, weakness is indicated. If the running is worse, richness is indicated. THEN YOU PROCEED TO ADJUST THE APPROPRIATE PART AS INDICATED AT THE BOTTOM OF SHEET 7 FOR THAT THROTTLE POSITION.

FAULT AT THROTTLE POSITIONS (Indicated on sheet 7)

  1. Fit smaller main jet.
  2. Screw out pilot air screw.
  3. Fit a throttle with larger cut-away
  4. Lower needle one or two grooves
  1. Fit larger main jet
  2. Screw pilot air screw in.
  3. Fit a throttle with smaller cut-away.
  4. Raise needle one or two grooves.

It is not correct to cure a rich mixture at half throttle by fitting a smaller main jet because the main jet may be correct for power at full throttle, the correct way is to lower the needle.

CHANGING FROM STANDARD PETROLS TO SPECIAL FUELS, such as alcohol mixtures will, with the same setting in the carburetter, certainly cause weakness of mixture and possible damage from overheating.
Opening times: Monday - Friday 9am - 6pm | Closed: Saturday and Sunday
Units 7 & 8, Rosemary Court, Oldwich Lane West, Chadwick End, Solihull, B93 0EY, UK

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