Hello,
I have a brand new rebuilt, 700 miles, '73 390 FE, with an Elderbrock 1406 carb.
I have an issue, I just can't solve.
When the engine is cold, it runs ok. The more it warms up, it starts to spit, sputter and backfire.
Yesterday, I had the timing fixed, and carb adjusted; and It ran outstanding. I thought the issue was resolved.
However, today as it warmed up, it started running very rich again and rough, it didn't spit, sputtering and backfiring as much as before. Then, the engine dies. You let it cool down, the automatic choke closes up, and you can run it again until it warms up and the choke begins to opening up again. Same progress.
It has an elderbrock Streetmaster 390 intake. I was told, that may be the elderbrock 1406 was not a good fit for 390 FE.
Any ideas, of what the issue might be?

thank you for your help!

 

What kind of ignition system are you running?

 

Have you checked for vacuum leaks?

 

Hello, supposed to be an MSD Ignition and I don't seem to have any vacuum leaks.

 

Put a vacuum gauge on it and see what it reads when stumbling.Something is leaning out the carb calibration. The EDEL 4V intake was known for a very thin carb pad sidewall casting and would suck outside air there. EDEL came out with a carb mtg plate to alleviate that problem. Also check carb mtg torque and gasket(s).

 

I haven't been able to check the vacuum yet. However, I noticed my #2 plug had fuel in it. So, I changed out the plug. I ended up changing the spark plug wire as well. Because, when I tried to remove the plug wire, the boot was in my hand, and the ignition terminal for the spark plug remained on the spark plug.
So, I changed the plug and wire. Drove it for a while, about 24 miles. then it started again. this time, just very sluggish, knock a little, blogging down, lose power, and it would die on me, there was a slight smell of gas. Limped it home (would have to let it sit for about 15mins each time). Got home, checked #2, again; broke the plug wire, same as before (thank goodness I have extra) and the plug had the smell of gas again.

Thank you

 

Sounds like the first thing you need to do is install a set of quality plug wires.

 

I don't know how reliable MSD products are, but I had a very similar problem a few years ago and it turned out to be a bad Acel coil.

 

Ignition Oscilloscopes are your friend.

I don't know whether the older units are used often anymore in the shops (not very likely), maybe they have one in storage? And some old guy to interpret the traces too?

Anybody know how well the modern digital lap-top scope types work with old school iron?

Anyway spooling your engine up against an ignition oscilloscope is well worth the effort and will save MUCH time, part$ and headscratching. They will sniff out defective ignition components almost instantly. Find somebody who has a Sun or Allen unit and knows how to use it. I bought a Heathkit (remember them?) for $40 and immediately found issues. Major issues. I had used all brand new parts right? My superior sense of deductive reasoning sez "it has to be good ... all new parts!" ... Or so I thought. It don't work that way.

In most cases genuine OEM Motorcraft ignition parts are going to be way better, and more reliable, than some aftermarket mix&match scratch &dent assemblege of "stuff" from golly knows where made of golly knows what. High voltage is strange stuff.