65 289 spark plug gap/popping!

Hipo

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Jul 8, 2019
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I have a 65 mustang with 289, C4 automatic.
The engine has a little bigger cam, larger valves, tri-y headers, 4bbl, dual exhaust with flowmaster X.

The spark plugs are gapped at .0035 AC Delco.

When I rev the engine and let off it pops. Would a different gap help this problem or possibly help take
advantage of the mods the engine has from stock? More power? Would it hurt anything?

I played with the timing and it helped some, but it still pops.

any ideas appreciated! Love this forum!
 

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mattbaker302

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Apr 18, 2019
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I have a 65 mustang with 289, C4 automatic.
The engine has a little bigger cam, larger valves, tri-y headers, 4bbl, dual exhaust with flowmaster X.

The spark plugs are gapped at .0035 AC Delco.

When I rev the engine and let off it pops. Would a different gap help this problem or possibly help take
advantage of the mods the engine has from stock? More power? Would it hurt anything?

I played with the timing and it helped some, but it still pops.

any ideas appreciated! Love this forum!
like a backfire? if so, then I would make sure the carb isnt to rich. I dont know why the sparkplugs would have anything to do with it unless they werent gapped correctly. timing is another thing to double-check id start with that then go-to carb.
 

rbohm

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Apr 12, 2002
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is the popping coming from the carb or the exhaust? since you played with the timing and got a change i would say that your timing is off. start by setting your timing at 10 degrees btdc at idle, and work from there.

as for the plug gaps, if you have the stock ignition, then set them at .035-,040. if you have a later model or aftermarket electronic ignition thenset the gaps at .045
 

Hipo

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like a backfire? if so, then I would make sure the carb isnt to rich. I dont know why the sparkplugs would have anything to do with it unless they werent gapped correctly. timing is another thing to double-check id start with that then go-to carb.
like a backfire? if so, then I would make sure the carb isnt to rich. I dont know why the sparkplugs would have anything to do with it unless they werent gapped correctly. timing is another thing to double-check id start with that then go-to carb.
Mattbaker302,
Yes backfiring. Maybe edelbrock can sell me some different size jets as there’s no other adjustment on the carb. Thanks for your input.
 

Hipo

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Jul 8, 2019
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is the popping coming from the carb or the exhaust? since you played with the timing and got a change i would say that your timing is off. start by setting your timing at 10 degrees btdc at idle, and work from there.

as for the plug gaps, if you have the stock ignition, then set them at .035-,040. if you have a later model or aftermarket electronic ignition thenset the gaps at .045
Rbohm,
its backfiring/popping out the back exhaust. Thanks for the recommendation!
 

rbohm

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Apr 12, 2002
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Rbohm,
its backfiring/popping out the back exhaust. Thanks for the recommendation!
then what you are experiencing is called after burn. usually caused by a rich fuel mixture that keeps burning well after the exhaust valve opens. you likely need more timing and/or less fuel. my advice is to attend to this ASAP as the result can be exhaust valve burning in the long run.
 

Hipo

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Thanks again! I’m going to contact Edelbrock and see what i can do to lean out the carb. Most likely a re-jet to smaller jets will help. Like I mentioned before I adjusted the timing and got rid of some of it. It running too rich could be why it’s a dog off the line. I appreciate your help!
 

rbohm

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yo4u dont need to contact edelbrock, your local speed shop should have the jets and metering rods to make the changes. start by seeing what you have in the carb now, my thought is a 99 jet, not sure about the metering rod. but you can get matched pairs. i went to a 92 or 92 jet in my old grand marquis with the matching metering rod.
 
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Hipo

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Jul 8, 2019
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yo4u dont need to contact edelbrock, your local speed shop should have the jets and metering rods to make the changes. start by seeing what you have in the carb now, my thought is a 99 jet, not sure about the metering rod. but you can get matched pairs. i went to a 92 or 92 jet in my old grand marquis with the matching metering rod.
pretty sure it’s a model 1406. Whatever it came with is what it has. I’ll try .45 on the plug and more timing. If that doesn’t work I’ll try going down a notch on the metering rods. Thanks!
 

bartl

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Another couple causes of exhaust "popping" on deceleration is insufficient spark advance at closed throttle, typically the result of using timed (ported) vacuum versus full manifold vacuum for vacuum advance combined with a LEAN fuel mixture. Lean mixtures take longer to burn than rich mixtures and if you don't ignite it early enough, combustion will still be taking place when the exhaust valve opens to expel the gases. Another cause is poor control of reversion by the headers. Reversion is when the exhaust pulse reverses direction as the piston travels down on the intake stroke during intake and exhaust valve overlap. If the header primaries for 2 cylinders are connected, one of which is on the exhaust stroke and the other on the intake, reversion can "pull" hot exhaust up to meet fuel-laden mix and cause ignition in the port. Other causes can include small exhaust leaks at the header flange that cause fresh, oxygen-rich air to contact unburned gases and result in spontaneous combustion. IMHO, the spark advance issue is the most common and where I'd look first.
 

rbohm

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Apr 12, 2002
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pretty sure it’s a model 1406. Whatever it came with is what it has. I’ll try .45 on the plug and more timing. If that doesn’t work I’ll try going down a notch on the metering rods. Thanks!
in teh 1406 the primary jets are replaceable, as are both sets of metering rods. in fact the edelbrock performer carbs are MORE tuneable than the 4150/4160 holleys
 

Hipo

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Thanks for all the replies and help. bartl actually touched on what it ended up being. The simplest fix that I’m pretty embarrased about. So I switched the vacuum on the carb from ported to timed and it immediately stopped the popping problem. The vacuum hoses were in the wrong place. Ugh! I’m just glad that fixed it. Thanks again!
 

mattbaker302

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Apr 18, 2019
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Thanks for all the replies and help. bartl actually touched on what it ended up being. The simplest fix that I’m pretty embarrased about. So I switched the vacuum on the carb from ported to timed and it immediately stopped the popping problem. The vacuum hoses were in the wrong place. Ugh! I’m just glad that fixed it. Thanks again!
haha well, it's always the simple things that we miss. glad ya figured it out
 
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