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Home >> Plymouth >> 1971 >> Cricket Automatic Trans >> Repair and Diagnosis >> Engine Performance >> Engine Control Systems >> ROOTES Group Single Carburetor Engine Modification >> Service Procedure >> Checking Time For Engine To Decelerate
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Checking Time For Engine To Decelerate

Open throttle slowly until engine speed reaches 2500 RPM. Release throttle and check time it takes for engine to return to idle. If time is 4-6 seconds, system is working correctly. If not, proceed as follows:

  1. If engine stays around 2000 RPM, throttle by-pass valve is opening, but vacuum retard diaphragm is not working. Lubricate internal baseplate of distributor if its movement is sticky, and check vacuum retard actions.
  2. If engine slows to 1500-1700 RPM, but does not return to idle, it is possible engine has become too hot. Allow to cool for at least 10 minutes (with hood up), and repeat test.
  3. If engine still stays at 1500-1700 RPM, make sure air bleed orifice in carburetor air cleaner flange is not plugged. If clear, it indicates that control valve is faulty. To adjust, see ADJUSTING CONTROL VALVE .
  4. If engine slows down in 21/2 seconds or less, throttle bypass valve is not opening. Remove plastic cover from cylindrical end of control valve, press exposed end of control valve spindle to open valve. Vacuum advance diaphragm should now move inwards and engine may speed up to 1700 RPM.
  5. If it does not speed up, hold control valve by hand until engine speed is 2500 RPM. Release throttle, speed should settle to 1700 RPM. This indicates throttle by-pass valve and vacuum retard are working correctly and control valve is not opening under deceleration conditions.
  6. If engine slows to idle speed or less, check for broken or plugged lines. If defective line is found, replace it and repeat test. If still unsatisfactory, remove line from vacuum retard diaphragm and plug end.
  7. With engine idling, press control valve spindle, engine speed should now rise to 2000 RPM. This indicates throttle bypass valve is now opening and vacuum retard diaphragm is leaking. Distributor must be replaced.
  8. If engine speed does not increase, check setting of throttle by-pass valve. Remove screws holding by-pass valve base and cover together, separate these parts. Take care not to lose spring, or adjusting screw and "O" ring. Drive sealing disc out of outer body, reassembly valve unit. Adjustment screw is now accessible. Turn adjusting screw clockwise as far as possible and then turn counterclockwise one turn. This is the normal setting.
  9. Install by-pass valve and check its action by pressing control valve spindle while engine is idling. If engine speed does not increase, replace valve. Connect vacuum feed hose to retard side of vacuum actuator. Recheck slow down time from 2500 RPM. If time is above 4-6 seconds, adjust screw up to two full turns counterclockwise from the set position.
  10. Reconnect all components and check that all lines and fittings are correct.
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