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I need help with no idle , starts up fast but go's to a fast idle ( 2500 rpm ?) and then dies. I have been working on this jeep on and off form about ten years and owning the Jeep since 78. I have remove / replaced  and move everything and now I'm putting back . I must have something wrong some where. put on headers and glass packs with duel exhaust on a 258 s6. any thought's.

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Classic Jeep, Bob S...Begin by checking the fuel supply.  Test at the engine/carburetor side of the fuel filter:  1)  confirm the pressure coming from the pump (stock 1972 258 inline six fuel pump is 4-5 PSI at 500 engine rpm), and 2) determine whether there is enough volume of fuel flowing from the filter.  Fuel volume should be one quart in one minute or less at 500 rpm using a brass Tee and fittings from the fuel line into a steel container.  Make certain the fuel filter is not clogged.  If pressure is okay but volume low, consider the fuel filter or sock at the fuel tank pickup.  Testing requires a safe hose, a Tee, a pressure gauge and a metal can kept away from engine heat.  If the problem is not fuel pressure or volume, move to an air leak. 

When you installed the header, the intake manifold gasket may have developed a leak.  If a new gasket, it may not be sealing properly.  A quick check for a manifold leak with the engine running (idling if possible) is to spray a less volatile penetrant like WD-40 (kept away from the exhaust manifold) along the edge of the cylinder head/intake manifold junction.  If engine speed picks up, you have a vacuum leak.  Do the same around the base of the carburetor, you may have a vacuum leak there.

Other sources for a vacuum leak are the brake booster check valve (if so equipped), the brake booster diaphragm or the evaporative emissions vacuum circuit.  If equipped with power brakes, the brake pedal should have vacuum assist after shutting off the engine, at least for a few pumps of the brake pedal.  If brake assist requires that the engine is running, you have a vacuum leak at the diaphragm, a hose or the check valve.

If these issues are not the problem source, you need to rebuild the carburetor and install a new needle, seat and gaskets.  Not clear whether you have a 1- or 2-barrel carburetor, stock or otherwise.  (If an aftermarket Weber or Holley, that's a whole issue in itself.)  If stock YF Carter, rebuilding can make a dramatic difference if you set every specification to factory.  At the magazine, I have a complete step-by-step for rebuilding the later two-barrel BBD with good results:  https://www.4wdmechanix.com/Rebuilding-the-Two-Barrel-BBD-Feedback-Carburetor?r=1.  This is the later feedback carburetor version of the Carter BBD.  Although the BBD is distinctly different than a Carter YF one-barrel, you can glean useful tips from the build.

Moses

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