On the Rochester Monojet, the “plug gauge” referenced in the HP book is essentially the factory tool used to position the metering rod accurately in relation to the main jet during setup. Rochester never listed that dimension in the consumer-facing manuals because they assumed technicians would have the gauge in the dealer tool kit.
For most Monojet applications (especially the common M and MV series), the factory service spec translates to:
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Plug gauge thickness: 0.149? (some sources round to 0.150?)
This gauge is inserted between the metering rod arm and the adjustable stop with the throttle held in the specified position (usually wide open or with the choke off depending on model). Once the gauge is in place, the metering rod adjustment screw is set so the rod’s tip is correctly positioned in the main jet.
Why it’s not published in decimals in the HP book:
The original “J-number” tool (often J-26796 or similar) was only sold to GM dealers, so they listed the tool number, not the dimension. Rebuilders later back-calculated the measurement.
Here’s how you can set your Rochester Monojet metering rod depth without the factory J-tool, using a caliper or a simple homemade spacer.
Background
The metering rod on a Monojet controls the fuel flow through the main jet by raising and lowering in response to engine load and throttle position. Setting its height correctly ensures the engine transitions smoothly from idle/cruise to power enrichment.
Tools You’ll Need
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Vernier or digital calipers OR a piece of metal/plastic precisely cut to 0.149? (3.78 mm) thickness
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Small screwdriver (for the metering rod adjustment screw)
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Good light and a steady hand
Procedure
1. Remove the Air Horn (Top Cover)
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Disconnect linkages and carefully lift off the air horn to expose the float and metering rod assembly.
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Watch for the small metering rod spring — don’t lose it.
2. Position the Throttle
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Hold the throttle in the specified “power” position — this is usually wide open for most Monojet models, but double-check your year/model.
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The metering rod lifter should be in its highest position when you measure.
3. Insert the Gauge
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Slide your 0.149? spacer or set calipers to 0.149? and use the depth end.
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Place the gauge between the metering rod arm and the casting stop (or between the arm and a specified reference point per model — on most Monojet M/MV carbs, it’s the boss in the air horn casting).
4. Adjust the Rod Height
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Loosen the lock screw on the metering rod arm.
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Turn the adjusting screw until the rod tip just seats in the main jet when the gauge is in place.
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The rod should not be forced; it should just touch the jet seat.
5. Check Movement
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Remove the gauge and cycle the throttle.
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The rod should rise and fall smoothly with no binding and drop fully into the jet at idle/cruise position.
6. Reassemble
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Reinstall the air horn with a fresh gasket.
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Make sure all linkages, choke components, and springs are reconnected properly.
Tip for Homemade Gauges
If you don’t have calipers:
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Use a piece of feeler gauge stock and stack it to exactly 0.149?.
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Or cut a strip of aluminum or plastic to that thickness (you can check at a hardware store using their nut/bolt gauge or calipers).
