Hey guys I have a 77 ranger xlt with a 351 modified engine. A motor craft 2 barrel carb and a backfiring problem that is driving me nuts. so far a have replaced the wires and plugs, fuel filter and pump, ignition control module, pcv valve, checked the egr valve, and tried adjusting the carb. i got this truck a couple months ago and am trying to fix all that the P.O. had jerry rigged.

symptoms:
1. backfires going around 40 mph down the road
2. backfires after i have been going 65 or 70 mph down the interstate for a 5 miles or so. after is starts backfiring going down the interstate it sounds like something starts knocking under the hood.
3. wants to cut off when i push the brake at a stop light sometimes.

all of the backfiring sounds like it is coming through the carb. i know that is not good at all.

any help would be appreciated. I am thinking it is a vacuum leak somewhere but i just can't find it.

also i just noticed this week that when i crank my truck up and it is cold i hear a rattling sound coming from underneath it near the back. but when it warms up it stops.

 

My '77 Ranger XLT did the same thing and blew up both mufflers a couple weeks back. The rotor had a slop in it and kaboom pop pop. I had another 400 sitting in the shop and put that rotor on and still had an issue. Bought a brand new rotor when I made it to town and she runs like a top. I think it was $10.

 

 

Put a timing light on the plug wires, start with #1 and work your way around the set to #8 and watch the light to see if you are dropping fire. If you are; and you have replaced all that you say; it maybe time for a replacement dist.

 

aight guys here is an update to my situation.

1. I have a rattling noise coming from inside of my muffler. It started after the backfiring started.
2. I pulled some of the plugs and all of them looked the same. (see attached photo)
3. it seems like the truck only backfires going down the interstate after i have traveled about 4 or 5 miles and it is getting more and more constant. plus there is a knocking noise under the hood after it starts backfiring. It sounds like it is backfiring under the hood as well.
I replaced the rotor today and it is still doing it.
I have not done anything to the timing or any of those components since I have gotten the truck. that is still a little out of my league right now.

I have worked on adjusting the motor craft 2 barrel carb some. but the only thing i have adjusted are the two screws at the front bottom and the one going to the throttle. The two on the bottom I have turned out 2 turns.



It sounds like i have a vacuum leak somewhere near the back of the motor/carb. but for the life of me i can not find it. I am starting to think my truck is running hot for some reason. the temp gauge stays near the "al" in normal when I am driving it. Could that have anything to do with is backfiring through the carb or going down the interstate??

 

Perhaps something about the vacuum and the distributor advance mechanism? trying to think of causes that occur when once engine is warm... but come up blank.

Try a vacuum gauge tune: Classic Inlines - Tuning with a Vacuum Gage

But track down the vacuum leak before you do, as it would throw things out... I used starting fluid in a can AKA ether (dangerous), propane (also dangerous), and various gasses (usually dangerous) to locate leaks. I suppose someone here could suggest something safer. I'm from the '70s so I like it dangerous

Do that too!

Your potato quality pic of the spark plugs seems to be okay.

Noise from Muffler might be because something dislodged when your backfiring... or exhaust firing... I can't remember the correct terms, one is firing out the intake and carb, the other is into the exhaust system. Also, perhaps the air cleaner is rattling?

Sounds like it's time to get a shop manual and get hands dirty and do some learnin'! Even a basic haynes or chilton manual has decent info in it that one can read, especially the beginning chapters about safety and procedures.

 

It could be a valve sticking also