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Choke Control Diaphragm

NOTE: Three different types of vacuum systems are used to provide vacuum to diaphragm. Internally ported has an internal passage to diaphragm. Externally ported without vacuum trap has vacuum tube direct from manifold vacuum source. Externally ported with vacuum trap has a vacuum trap in the vacuum tube from manifold source. Visually inspect vacuum routing to determine vacuum system used.
  1. Start engine and watch that choke diaphragm rod retracts within 2-5 seconds. To perform this test on externally ported diaphragm with vacuum trap, by-pass vacuum trap. If rod retracts within specified time, diaphragm check is complete. Reconnect vacuum trap (if equipped). If rod does not retract within specified time, proceed to step 2  ) for internally ported diaphragms or step  6) for all others.
  2. Remove carburetor. Remove diaphragm and cover as described under OVERHAUL in this article. Using compressed air or vacuum, check and remove any restriction from internal vacuum passage. (Choke air passage connects to diaphragm passage in casting).
  3. Inspect diaphragm for cuts or tears and replace if necessary. Reassemble diaphragm and cover. Tighten to ensure no vacuum leaks are present. Connect vacuum line between casting bottom port and external vacuum source. Seal junction at casting to prevent vacuum leak.
  4. Block choke air inlet by capping casting connector tube beneath diaphragm cover. Apply 18 in. Hg vacuum to diaphragm. If diaphragm rod retracts within 2-5 seconds, control diaphragm is operating properly. Remove test equipment and reconnect all vacuum hoses.
  5. If the diaphragm holds vacuum but diaphragm rod does not retract, replace cover and repeat test. If diaphragm does not hold vacuum, remove diaphragm cover and block upper diaphragm vacuum port at lower side of diaphragm seating surface. Apply 18 in. Hg vacuum again. If vacuum is held, replace cover and repeat test. If vacuum is not held, locate and service vacuum leak at choke cap gasket, choke shaft bearing or other similar areas. Repeat test.
  6. Remove diaphragm and cover as described in OVERHAUL procedures. Inspect diaphragm and replace if cut or torn. Install diaphragm and reassemble diaphragm with original cover. Tighten assembly to ensure no vacuum leaks occur.
  7. Connect external vacuum source to connector tube on diaphragm cover. Apply 18 in. Hg vacuum. If choke diaphragm rod retracts within 2-5 seconds, test is complete. If rod does not retract within 2-5 seconds, replace diaphragm cover and repeat test.
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