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Re-assembling & How to trace faults


When replacing the throttle see that the jet needle goes into the hole in the choke tube and needle jet and that both the throttle and air valve spring locate correctly in the mixing chamber top.

When reassembling the float, engage the float needle recess in the horseshoe section of the float and fit in float chamber. Check that the needle jet (24), jet holder (28) and the main jet (29) are fully tightened together before screwing the assembly into the body.


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 the banjo gauze is clear and allows 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.
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.
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. Now at throttle position shown on sheet 7, fig 5, test to see if the mixture is rich or weak. This is done by partially closing the air valve, and if the engine runs better weakness is indicated, but if the engine runs worse richness is indicated.

  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 cutaway
  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.
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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