[JBJarhead]

I bought a 71Mach1 with a 351W. It does not matter is cold or hot, or you step on gas or ease into it; it will feel like a miss. When it happens the tach drops as well. I initially maybe bad plug or two. I changed them all and none of them had any corrosion on them. Plug wires look new with no burn marks or inside wire showing. Carb and intake are newer and can not see anything out of place. Any ideas on what might be causing?

 

[crjackson]

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[rickyracer1983]

Just started doing this on did you change something? Might be a broken valve spring or burnt valve. Put a vacuum gage on it and tell us what that is reading/doing.

 

[Yadkin]

Always test the ignition wiring first because its the least dependable, cheapest parts and easiest to diagnose. Put your timing light on the #1 plug and observe the timing mark at idle, then when you blip the throttle, then at a higher RPM. Is the flash consistent? Is it advancing how it's supposed to? What is the initial timing at idle with and without the vacuum advance disconnected and port plugged?

 

[73 351Q]

check cap and rotor for cracks or burn marks inside. make sure firing order is correct, and that correct wire is on the correct plug. depending on the year of that windsor, it might have an egr plate on it, and they tend to leak vacuum. [71 mach 1 should be a cleveland engine, which has a different firing order]does it have points, or electronic ignition? if points, replace points and condenser with a new set and set the gap. i always use standard motor products points, they are very well made. if electronic ignition, check to see if the resistor wire under the dash has been removed so that you are feeding a full 12 volts to ignition. the 71 originally had a resistor to drop voltage to about 7 volts for the points. make sure the advance weights in distributor are free, and that vacuum advance is working and is hooked to the correct ported vacuum. what kind of carb does it have? do you know what year the motor is? if all that looks good, might be internal. timing chains on those old motors sometimes came with plastic teeth, timing chain might have jumped a tooth, or cam may have a flat lobe or two.

 

[Grimbrand]

I bought a 71Mach1 with a 351W. It does not matter is cold or hot, or you step on gas or ease into it; it will feel like a miss. When it happens the tach drops as well. I initially maybe bad plug or two. I changed them all and none of them had any corrosion on them. Plug wires look new with no burn marks or inside wire showing. Carb and intake are newer and can not see anything out of place. Any ideas on what might be causing?

After reading this, the line about 'tach drops as well' stood out to me. Do you mean it shows 'zero', or just that the RPMs on the engine fall, as it's sputtering? Because if the signal for the tach is being interrupted, that most definitely shows a significant ignition system problem. Maybe a bad coil or dizzy trouble. Since you just replaced your plug wires, I think it's safe to rule them out.

If it's just fluttering because the engine RPM is too, then that would make sense.

Once you can rule out ignition trouble, take a look at your carb. What kind is it? Modern gas turns to nasty crud in the bowls pretty fast, so if it's left to sit for long periods between drives, even a 'new' carb can have trouble. The additives in use these days are not very carb friendly, especially alcohol, which eats away at rubber seals. Over time, it evaporates, forming weird crystal structures and corrosion on the bowls that clog everything up.

 

[rickyracer1983]

The acc pump on the carb isn't working. When you move the pedal, you're suppose to get fuel from it or it leans the engine out and it miss fires.

 

[JBJarhead]

Thanks a ton, and will check each of these. I appreciate the insight!!