Thoughts on why this is happening??

[Stangboy67]

At full throttle I'm getting a shuddering and what sounds like backfiring up through the carb after I reach about 3000 rpm. This occurs in 1st and 2nd gears (havn't tried it in 3rd,4th or 5th). If I ramp it up in 1st at part throttle (half), it is fine up past 5k. Otherwise the car runs fine in normal driving around town or pedal to the floor below 3k.

This is a daily driver and the problem has kind of appeared in the last few weeks. I have made no changes to the car what-so-ever. My first thought is the timing or I need to change those old plug wires. I was hoping to get a few ideas before I dig in to save on time and effort. Unfortunately my time is rather limited lately.

Vital engine related stats: 90' version of 5.0HO with stock longblock except for Edelbrock Performer intake and 600CFM elec choke carb, Pertronix ignition with about 12 degrees base timing.

TIA for your help

 

[ozstang65]

Running out of fuel? Check pump and filter conditions.

If it were electrical based I would think that throttle position wouldn't play a part in the breakdown. Unless you still have the vacuum advance hooked up. If it is still hooked up, check that the advance plate is moving freely.

 

[mikec35]

If it is doing it under a load it's probably your plug wires or a bad plug. Do the after dark test with someone in the car holding the break while in gear, put it under a slight load and look for the spark.. If not the wires, I would check the plugs and timing.

 

[ForceFed70]

Sounds like fuel filter to me.

 

[BlwnGazkit]

Check fuel delivery, timing advance, plugs and plug wires. I would start with the timing, it sounds like it's breaking up above 3k. If you're not running enough initial advance or the distributor isn't advancing enough under load then you have your exact situation.

 

[rbohm]

it sounds to me like your ignition system is breaking up under load. a fuel system problem would manifest itself under a greater range of circumstances.

 

[BlwnGazkit]

it sounds to me like your ignition system is breaking up under load. a fuel system problem would manifest itself under a greater range of circumstances.

AGREED

 

[Stangboy67]

Thanks guys! I'll dig into the ignition side of things this coming week (hopefully) and see what I come up with.

 

[2nd Mustang]

From my past experiences, bad plug wires will cause a high speed miss and/or a rougher than usual idle. If other things suggested don't turn up anything, I would also suggest checking for a vacuum leak. Good luck.

 

[6T6coupe]

Check your ignition first. It could be a bad wire or timing. If that checks out, then look at fuel delivery.

The fact that it occurs at WOT seems to indicate a fuel starvation issue. It might be float level, fuel pressure, or power valve - or maybe a combination of these.

But don't monkey with fuel until ignition is fully checked out.

 

[thehueypilot]

What are your plugs gapped at? The spark could be blowing out at the higher rpm's causing the high speed miss. Reduce the plug gap and see if it goes away.

 

[Stangboy67]

Just in case this can help someone...

A follow-up. Before I could get to checking into the things you all mentioned, a couple of other strange "electrical" related things began happening, for instance, the Temp guage giving false readings.

So I thought I must have a wiring problem and attacked the body and chasis grounds to the battery, then proceeded to cleanup any underhood electrical connection I could find. All problems disappeared including the above 3K rpm miss I was having. New plug wires are going in anyway this weekend though as they have many years on them. I guess after 8 years of daily driver status, this needs to be added to the maintenance list.