1975 Chevy 350 Engine Missing

1975 k20 4x4 with 350.

Engines idles great, and runs great when mashing the throttle, however, when backing off the throttle or trying to maintain constant speed on highway (light throttle pressure) the engine misses and backfires.

I recently (~ 6 months ago) put new plug wires, plugs, distributer cap, and coil on engine and set the timing to 6 OTDC.

Ran fine until just a few days ago.

One note - the plugs sit real close to the exhaust manifold and due to this the end of the plug wires (where they connect to the plugs) seem to suffer from heat damage, is this normal?

Thanks in advance,

[bejay]

usually there should be heat shields on the manifolds to deflect some of the heat off of the plug wire however they may be missing but it should be corrected if its already damaged the new wires.
if equiped with an egr valve you might try removing vacum line going to egr valve and test driving to see if problem goes away this valve is bolted to the intake and will have a vacum line going to it and will be on the passenger or right side of the carb.
another thing you can check is to unplug vacum advance vacum line and test drive and see if problem has went away if it has likely a bad pickup in distributer assuming the timing is set at 6 degrees btdc.
does it backfire out of the exhaust or the intake?

I will give those ideas a try.

I am pretty sure it is backfiring out of the exhaust, I get the snap/crackle/and pop that sounds like a harley motorcyle.

[billys68ss]

Why do you have the timing set at 6 degrees? Is that the OE spec? most 350's I have ever worked on were 10-13 degrees. Altitude will play a factor in this im sure. I would suppose it depends on your location. When you set the timing, did you disconnect and plug the vacuum advance?
Billy

The book I have (Haynes) recommends the timing be set on 4 degrees btdc for everyday use and 6 degrees btdc for heavy loading. I set it at 6 because I use the truck mainly to haul firewood and lawn equipment.

I unplugged the vacuum advance line and that seemed to minimize the missing.

[billys68ss]

With that you could possibly have a bad diaphragm in the vacuum advance. If the spark plug boots are damaged then you probably should replace the wires. If the wires are the least bit damaged then you should definately replace them. I have to say I learned something from this post. I just checked out my Chilton manual and it tells me that in 1975 (on passenger cars) the timing is to be set at 6-8 degrees BTDC. But in 1973 (which is where I must have gotten my previous info) the 350 was suggested at 10 degrees BTDC. Oh well, I guess it helps to go back to the book once in awhile and not rely on whats in my head. LOL. I figure the change could be attributed to the fact that 1973 was the last of the higher compression engines in the early 70's.
Billy

Replaced plug wires and fixed the problem. The old wires were burned up pretty bad. Wrapped the new ones in electrical tape to try to minimize any heat.

Got it running real good, which is good, cause my other car (95 camry) was stolen monday night. They took it right out of my driveway!!!