Carburetor/ timing

[Anthony C.]

Hi , after doing rebuild on 75 Q jet carburetor do you have to reset timing ? Also when marring carb back on manifold how tight should the 4 bolts be .. thank you

[Mark E.]

Re: Carburetor/ timing

Anthony,

Rebuilding the carburetor does not affect ignition timing. However, it's a good practice to consider rebuilding a carburetor as part of a major tune-up which includes checking the ignition system (plugs, points, condenser, rotor, cap, cables, coil, timing), filters and other service items. However, check and adjust one thing at a time so it's easier to diagnose a problem.

Q-Jet hold down bolts are often over tightened causing the throttle body and air horn to warp. I believe the torque specs are in the manual, but just a tad more than snug is sufficient. Check again after a couple of heat cycles. If there's a leak, check for warping rather than trying to over-tighten them. Some of the older Q-Jets (maybe not Corvette) used slotted head screws for the forward hold down bolts. My theory is this was to encourage using a screwdriver to avoid over tightening with a ratchet.

[Duke W.]

Re: Carburetor/ timing

Carburetors were installed at St. Louis so the torque spec is in the AIM... should also be in the CSM or COM.

Duke

[David M.]

Re: Carburetor/ timing

Per Cliff Ruggles of Cliffs high performance: carb to intake fastener torque is ~120-169 inch pounds (lb-in) or 10-14 foot pounds (lb-ft). I use a 1/4" drive ratchet. I get all 4 bolts snug then just a bit more. I will recheck all carb fasteners after a few heat cycles.

Check for other possible sources for vacuum leaks before adjusting anything.
Verify the fuel line doesn't leak. Fuel line nuts shouldn't be over torqued. Always use a flare wrench. Always use two opposed open end wrenches for proper support.

The air cleaner wing nut is another source of over torqueing. The threaded rod will pull up on the throttle plate and warp it. So just snug on the air cleaner. Always use an air horn gasket between the cleaner & carb.

Initial start up setting: Very gently bottom out both air fuel screws. Then back out 1.5 full turns. Hook up a vacuum gauge to a direct intake manifold nipple. Start engine. Adjust the air fuel screws to obtain the highest vacuum reading possible at curb idle.

Adjust the timing to achieve the highest vacuum reading. Alternate back n forth between the two until the gauge stabilizes at its highest point. Take note of the timing setting using a timing light. Road test readjust as needed.

[Duke W.]

Re: Carburetor/ timing

Any engine with ported vacuum advance will show increased idle speed and vacuum as initial advance is increased because the optimum total idle advance for a low overlap cam engine is about 25 degrees, which is about what the OE setup is on the non-emission controlled L48 antecedent base engine, the '67 327/300.

Figure out what you want for a spark advance map, OE emission controlled, or the '67 327/300 with full time vacuum advance and a more aggressive centrifugal curve.

Then go through the carburetor idle speed/mixture adjustment procedure.

Duke