Holley Throttle BodiesA propane mixer combined with a throttle body forms a propane carburetor. The Impco Model 425 mixer is designed to directly bolt up to a Holley throttle body.  Gann Products sells a 4bbl, Holley compatible, throttle body with part number TB9-8.  Impco's equivalent PN for this is AT2-42.  This throttle body has throttle bores of 1.538" and has both a full vacuum and a PCV vacuum port.

For a timed (spark advance) vacuum port, the Model 425 mixer is equipped with two ports (1 front & 1 rear).  The Model 200/225 mixers do not have this capability and a custom spacer is required between the mixer and the throttle body to add a timed vacuum nozzle.

The Gann TB9-8 was designed for LPG applications based on specifications from Don Bass (Impco's first employee and engine & fuel specialist).  The obvious differences between an LPG throttle body are that the TB9-8 has slightly smaller throttle bores and a couple fewer mounting holes.  The TB9-8 is generally more expensive that a new replacement from Holley.  Other equivalent replacement throttle bodies may also be purchased from Holley or performance parts retailers.  The difference between some throttle bodies can be with the accelerator pump cams, which are not used in propane carburetors.

Replacement Holley Throttle Bodies

Throttle Body Throttle Bore Diameter (in) Carburetor Vacuum Ports
112-116 1.688 750 CFM 4150
PN 0-4779C
1 full and 1 PCV
112-118 1.563 600 CFM 4150
PN 0-4776C
1 full and 1 PCV
112-119 1.750 850 CFM 4150
PN 0-4781C
1 full and 1 PCV
112-120 1.688 750 CFM 4150HP
PN 0-80528-1
No Ports
112-121 1.688 650 CFM 4150HP
PN 0-80541-1
No Ports
112-122 1.750 950 CFM 4150HP
PN 0-80496-1
No Ports

 

When using either a Gann TB9-8 or a Holley gasoline carburetor throttle body, be sure to use the Impco mixer to throttle body gasket (PN G1-74).  It would be wise to seal unused holes in Holley throttle bodies for vacuum leak prevention.

A propane carburetor requires a mechanical secondary (ie, a 4150 "double pumper") throttle body rather than vacuum secondary (ie, often 4160).  There are two modes of operation for mechanical secondaries:

  • progressive -- the secondary shaft only starts to open after the primary shaft has opened to 40-45º.
  • 1:1 -- the secondary shaft opens at the same rate and the same time as the primary shaft.

The following link shows the operation of a double pumper carb with progressive secondary configuration:

Holley Carburetor Tuning Secondary Circuit - Summit Racing Quick Flicks

There are 2 holes in the primary shaft where the secondary throttle shaft connector link is located.  The upper hole is for progressive operation while the lower hole is for 1:1 operation.

For street-driven vehicles, the progressive setup is preferred because it has more precise part-throttle control for better drivability.  If the Gann TB9-8 is not available, use the Holley 112-118 throttle body instead.

Be sure to have your local licensed propane conversion shop inspect your work BEFORE you run any fuel through your system.

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