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Old 08-28-2012, 05:22 PM
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'72 BB consistent muffler explosion

I have a 72 402 BB engine. The engine was totally rebuilt 5 years ago and I installed new rod bearings, rings, mild performance cam with timing gears, lifters, and ground the valves and seats. When the engine was fired up it ran like a fresh motor, not issues. It has a stock points ignition and a 4 barrel Qjet.
The engine then sat for 3 years due to unexpected issues in life. I recently rebuilt the carb to insure no stale gas/varnish issues and fired it up. It runs but with consistent muffler explosions. I insured that the distributor was installed correctly ( #1 at TDC with rotor pointing to #1 tower). Timed it to 12 degrees BTDC , no change, still exploding. Rotating distributor full travel, no change. Vacuum gauge at 15 in (in late timing indicator). Ran a compression test, 110 psi across the board. Anyone have a clue as to what the issue could be?

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Old 08-28-2012, 05:30 PM
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Timed with vacuum plugged to vac advance pot?
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Old 08-28-2012, 06:54 PM
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I disconnected and plugged the vacuum advance.
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Old 08-28-2012, 06:55 PM
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Possibly an exhaust valve sticking or leaking. It may only be one cylinder causing it. You might start the engine and pull each spark plug one at a time until the popping stops. This would give you an indication which cylinder is the issue. Just an idea.
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Old 08-28-2012, 07:29 PM
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Exactly my next thought S10.
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Old 08-28-2012, 08:08 PM
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Exactly my next thought S10.
Great minds think alike. LOL
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Old 08-28-2012, 09:01 PM
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I'll give that a try. Just curious wouldn't a sticking/leaking valve be detected by the compression test? All cylinders were at 110 psi.
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Old 08-28-2012, 09:08 PM
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If you got 110 across the board I'd try a new compression test gauge. You'll never see it perfect like that.
And 110 ish sounds low. Should be more up towards 140 to 150.
Did you adjust the idle mixture screws for the highest vacuum using a vacuum gauge.
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Old 08-28-2012, 09:21 PM
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Quote:
Originally Posted by DaleR View Post
I have a ’72 402 BB engine. The engine was totally rebuilt 5 years ago and I installed new rod bearings, rings, mild performance cam with timing gears, lifters, and ground the valves and seats. When the engine was fired up it ran like a fresh motor, not issues. It has a stock points ignition and a 4 barrel Qjet.
The engine then sat for 3 years due to unexpected issues in life. I recently rebuilt the carb to insure no stale gas/varnish issues and fired it up. It runs but with consistent muffler explosions. I insured that the distributor was installed correctly ( #1 at TDC with rotor pointing to #1 tower). Timed it to 12 degrees BTDC , no change, still exploding. Rotating distributor full travel, no change. Vacuum gauge at 15 in (in late timing indicator). Ran a compression test, 110 psi across the board. Anyone have a clue as to what the issue could be?
Double check the firing order to be positive the plug wires are correct at both the cap and at the plugs. Number 5 and 7 are next to each other and are often swapped accidentally (see drawing below). Even if you think they're right I'd want you to be absolutely sure before going any deeper.

Be sure the cap is spotless inside and not cracked. No cross firing plug wires allowed either. Removing one plug wire at a time while the engine is running will tell you if this is isolated to a particular cylinder as has been said already.

FWIW, the compression test showing 110 psi may be low, but this also depends on the cam timing. A 'lumpy' cam will tend to pump lower than a mild/RV cam.

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Old 08-28-2012, 10:14 PM
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get rid of the points. My 65 acadian blasted off a couple mufflers when my battery was low and I turned on my head lights just after down shifting on a hill
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Old 08-29-2012, 07:35 AM
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A less shocking way of checking for a misfiring cylinder is to squirt a little water on each exhaust port. The water on the good ones will crackle and disapear real quick. The bad ones will do a slow fizzle.
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Old 08-29-2012, 09:28 AM
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'72 BB consistent muffler explosion

Check the points to see if they are burnt. If so replace the points and condenser. Check the cap for any corrosion, best to use a cap with brass terminals. If reusing wipe out the inside with a clean rag. Maybe even use a blow dryer on the inside. Try more initial timing with the vacuum gauge hooked up to see if vacuum increases also. Maybe 16-20 initial and enough mechanical to have 36-38 total. Have you checked how much mechanical you have now and how much total? If advancing causes engine to ping retard just enough to stop pinging.
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Old 08-29-2012, 09:21 PM
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make sure you have a good set of points and condensor, rotor and cap etc. if it still does it, do a the leakdown test. that will tell youif you havea leaky valve. may be just a weak valve spring as well. especially if it sat all that time without turning over. also, just for giggles, try running it at night at operating temp, with the hood open. see if it misfires from one plug wire to another. oh, by the way, if you are gonna take a plug wire off one at a time with it running, take the wire off at the distributor end-less chance you will get zapped! it does sound like it is a little low on compression at 110.
good luck
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