Technical Rice Crispies -- SNAP - CRACKLE - POP [ in the exhaust notes ]

Ideas anyone on what might be the cause?

My 56 Merc ambulance -- 312 Y-block -- Holley 1850 4bbl -- man choke -- automatic trans

Ocurrin on decell or coastin downhill or between shifts --

RUNS fine -- GREAT acceleration -- smooth cruisin

started bout a month ago and gittin more prevalent as time goe by and miles driven increase

 

More unburned fuel getting into the exhaust on decel -- i.e. now running richer. Something has changed / is changing. Check your carb, is your throttle plate closing completely on decel? Has the idle mixture changed?

Some say an exhaust leak will do that, I don't know but check that your exhaust system has not loosened up or got a hole in it near the exh header.

 

Coasting and decelerating are high vacuum conditions and will exacerbate any tendency towards a lean condition in your motor. The sound you mention is called "lean pop" and is common on Harleys that were jetted lean from the factory for emissions reasons. As long as your plugs look normal - a tan color with minimal residue - its probably not causing any damage. If your plugs are bone white your motor is running too lean and you'll need to rejet the carb a little richer to prevent internal damage from combustion chamber conditions that are too hot for safe engine operation.

The sound can also be caused by running ethanol in a vintage engine. Today's fuel is hard on Holley 4-bbl carbs like yours. It will eventually cause the accelerator pump diaphragm to crack and leak on the outside of the engine which can cause a fire. It also causes the power valve diaphragm to crack and leak fuel, but it goes down into the intake plenum and causes a rich condition. They are both inexpensive and are easily changed even by a carburetor novice. I recommend doing it at least once a year or at the first sign of fuel mismanagement at the carburetor.

In any case, check your timing and look for a vacuum leak as these are the most likely the causes of the the sounds you mention. One place often overlooked as a source of vacuum leak and improper timing is the vacuum advance diaphragm. This often causes the engine to lope like it's got a high performance cam at idle. Another place you can get a hard-to-detect vacuum leak is at the vacuum modulator in your transmission. Whatever the cause, an engine diagnostic vacuum gauge will tell the story here. Oh...and be sure you've got your distributor vacuum line connected to the "manifold vacuum" port on the carburetor base and NOT to the "ported vacuum" port.

Good luck...and keep up your good work. HOWEVER...as a previous resident of LA I'd highly recommend that you get MORE AND BIGGER LIGHTS for the back of your car! I lost my teenage daughter to a driver who ran into her car while it was parked on the side of the freeway with the flashers on...after he crashed through a police car behind my daughter with all its lights lit up. Be very careful, okay. The world is full of people who simply don't give a damn.

 

Could also be a slight leak in the exhaust manifold to head gasket.
Sucking a little air in on decel.

 

SO sorry to hear bout yer daughter MissysDad1 sounds like nuthin but a brick wall, and maybe not even *that* woulda saved her from the S.I.D. that killed her that's sumthin on my mind every time I stop to render aid -- my sincere condolances for yer tragic loss

to that end, not so noticeable in the action pic posted, the back of the Merc actually LIGHTS UP a LOT more then shown there -- I use just what I believe necessary for the "situation" as I don't want passing drivers to be "distracted" by my lights and slow down to "LookieLou" in the pic, I was only usin my arrow bar [ to the left ] and the left signals and revolvin turret lite -- I *have* a LOT more thankee kindly --