Fox Misfires Under Hard Acceleration - Carburetor Experts Needed
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Was the problem there before replacing "the plugs and everything"? If not, I'd take a closer look at the new stuff.
I can't help that much help on carbs, but since we're doing things the old-fashioned way, I'd read the plugs. Shut off the car at WOT at the RPM you've gotten backfires and pull the plugs. Figure out if the fuel is right. If not, adjust your carb/jet as necessary. Just based on what I've read over the years, a 650 DP seems like a big carb on a 302, but this is really not my area.
If the fuel is fine, you could should start looking to the ignition system. While you pull the plugs, check the gap. I think stock gap was in the .050-.055" range. If you have a weak ignition system, you might be able to compensate by closing the gap to the .030 range. MSD boxes fire multiple sparks and can help clean up some misses. What coil are you running? How does your motor manage timing advance? What is total timing?
I can't help that much help on carbs, but since we're doing things the old-fashioned way, I'd read the plugs. Shut off the car at WOT at the RPM you've gotten backfires and pull the plugs. Figure out if the fuel is right. If not, adjust your carb/jet as necessary. Just based on what I've read over the years, a 650 DP seems like a big carb on a 302, but this is really not my area.
If the fuel is fine, you could should start looking to the ignition system. While you pull the plugs, check the gap. I think stock gap was in the .050-.055" range. If you have a weak ignition system, you might be able to compensate by closing the gap to the .030 range. MSD boxes fire multiple sparks and can help clean up some misses. What coil are you running? How does your motor manage timing advance? What is total timing?
Yes it'll pop slightly after hard acceleration threw exhaust the hard popping is thru the carb multiple times during hard acceleration but won't do it going slow or idiling just happened one day started going crazy
Both but harder threw the carb ignition timing is at 14 it doesn't misfire or backfire idiling only when I get on it hard or WOT
6 psi idiling it'll pop and backfire WOT or over 2k rpm right around on the aggressive side
If someone hadn't butchered a perfectly good car, the computer would tell you what is wrong. Now you have to do it the hard way and dig your way through a pile of possibilities.
Backfiring out the intake is either a valve stuck open or a lean mixture or spark plug wire(s) connected to the wrong cylinder(s). Check compression on all cylinders and then look for vacuum hoses loose, cracked, or misconnected..
Sticking valves: If a intake valve is bent, has a bad spring or is misadjusted, the engine will sometimes backfire through the intake. Use a vacuum gauge connected to any convenient spot on the intake manifold. Run the engine at 1000 RPM & look for 18-21 inches of vacuum with a steady needle. A problem intake valve will make the vacuum gauge needle sweep 5-10 inches.
Lean fuel mixture breaks out into several sub categories:
A.). Vacuum leaks
B.) Poor fuel delivery due to bad fuel pump, clogged filter or bad fuel pump wiring. Look for low pressure or fluctuating pressure.
Vacuum leak due to slipped lower intake manifold gasket...
Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.
Vacuum gauge indications:
Backfiring out the intake is either a valve stuck open or a lean mixture or spark plug wire(s) connected to the wrong cylinder(s). Check compression on all cylinders and then look for vacuum hoses loose, cracked, or misconnected..
Sticking valves: If a intake valve is bent, has a bad spring or is misadjusted, the engine will sometimes backfire through the intake. Use a vacuum gauge connected to any convenient spot on the intake manifold. Run the engine at 1000 RPM & look for 18-21 inches of vacuum with a steady needle. A problem intake valve will make the vacuum gauge needle sweep 5-10 inches.
Lean fuel mixture breaks out into several sub categories:
A.). Vacuum leaks
B.) Poor fuel delivery due to bad fuel pump, clogged filter or bad fuel pump wiring. Look for low pressure or fluctuating pressure.
Vacuum leak due to slipped lower intake manifold gasket...
Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.
Vacuum gauge indications:
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Along with what jrichker said you will have to study up on carb and ignition tuning.
The part that doesn't make any sense is the build is fresh less than 6 months old heads checked no warpage no broken springs bc I checked them I pulled valve cover off there's no dead lifter or excessive play in the valve train from the intake up is all new less than 6 months including arp head bolts this recent backfiring started randomly one day it passed inspection and everything months ago no vacuum leaks on any hoses at all need to pull the plugs and take a look I have a 200$ fuel pressure regulator from jegs I have stock fuel pump and stock fuel filter that I didn't change I richened it with mixture screws that didn't help aggravating tbh does not do any backfiring or missing during low speed or idiling
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I will refer back to posts #11 and #12 it's a learning curve that will be hard to explain what to do here
Fuel level
Air/fuel mixture adjustments
Vacuum advance adjustments
Ignition, vacuum leak checks
Loose wires
Fuel level
Air/fuel mixture adjustments
Vacuum advance adjustments
Ignition, vacuum leak checks
Loose wires
The mixture screws at the base of the carburetor only affect the idle and off idle mixture. You have to dig a lot deeper to find the main mixture metering system.
Yea, power valves, squirters, main jets, float level, it's all part of the myth that a carb is easier than efi.
It's been so long since I messed with a carb and old ignition systems.
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