its a 77 f150. basicly stock as far as i know. have owned the truck for a few years. and recently it started doing the backfire/pop issue. it has progressivly gotten worse.. much worse. everything on the ignition side is new. dist, coil, ign mod, relay, plugs and wires got replaced a few months back along with cap and rotor. drove it for the first time today in a week or so and it is running like crap, gotta keep the rpm low and put around, this issue came out of nowhere. i have a carb rebuilt kit coming and should be here friday but i dont think the carb is the issue. i looked at it when i got home from work and noticed that the gasket between the exhaust mani and downpipe was blown out and actually moving around at idle. could this be the cause? any ideas would be great thanks.

 

Every bad exhaust manifold gasket I've ever heard makes a quiet ticking while going down the road. It's almost like bad rocker arms, except quieter. Does your truck do this? The exhaust/intake manifold gasket might not even be bad. You might just have to re-torque your manifold bolts and make sure everything is tight.

The backfiring issue can be caused by improperly set timing, burned up intake valves (Backfiring through the carb), burned up exhaust valves (drawing in air through the exhaust, backfiring through exhaust), improperly adjusted rocker arms, sticking valves, etc.

Generally, backfiring through the carburetor occurs when the intake valve is open when the particular cylinder fires. And, since you said it's progressively gotten worse, I'm leaning towards the fact that you may have burnt up intake valves.

However, fix your intake/exhaust manifold gasket before doing anything. If the gasket is blown out, you could have a vacuum leak there as well. Vacuum leaks can cause the weirdest problems with just about any vehicle.

 

i had backfiring issues on my 300 it ended up being the AIR pump was still being belt driven sending too much air to the carb some how and backfiring thru the exhaust.. i have no cats either if that matters.. took the belt off for the AIR pump and it stopped.

 

Start with a compression check before you do anything. No sense in putting a new intake/exhaust gasket in, just to remove the head for a valve job

 

the truck dosent have an air pump. i am leaning twords valves too but wouldnt the vavles be a constant issue not an intermediate issue.. this started as a very minor very intermediate problem that has since gotta much worse. but the truck will idle and accelorate fine when it wants too. thinkin about running some seafoam through the crankcase to loosen everything up. heard it works great or it will make it worse.. guess thats the only way to find out.. the intermediate thing about it kinda says that the timing woundnt be causeing it. and the only vac line on the truck is brand new.

 

I would lean towards carbon on the valves then. Take the hose off the brake booster, and stick it in the sea foam can. Start the truck. It's going to smoke like crazy, and you may have to hold the throttle to keep it running, but that will clean the carbon out quickly

 

how much sea foam do you think i need to use? i think that would be better than into the crankcase since it wont have a chance to break up anything and take a chance clogging the oil pump pickup.

 

I'd let it suck the whole can through. I can't stress upon you how much smoke is going to come out the tail pipe....

 

oh ive seen the videos on youtube. i know its gonna be bad. my truck is on the top floor of the parking garage so i hope the fire dept dosnt show up...

 

Check simple things... you mentioned having only 1 vacuum hose and it being new. Are all non-used vacuum ports closed off? Check the PCV hose as well as the power brake booster hose. If the transmission is automatic and has a modulator, check that hose as well.

Grab the air filter and try to rock back and forth. Our six cylinders are bad about the carburetor loosening. If the filter [carburetor] moves, use a 1/2" wrench and tighten the two nuts at the base of the carburetor.

With the engine idleing, grab different areas of the wiring harness and wiggle to see if the engine "acts up". Be very careful of moving parts!

 

MOV062 - YouTube

Hopefully it doesn't sound like that. You can hear the popping when I step on the gas.

This was caused by a lobe on the cam that went flat. It came out of no where and started getting progressively worse, causing poor performance (obviously).

 

If it is poping/ backfiring through the carb timming is off, or it is running to lean. If it is poping/ backfiring through exhaust it is timming, or running to rich. My uncle had a problem similar to this it ended up being air mixture screw. If it backfires through the carb it could also be a vacuum leak.

Good luck.

 

update.. this is pissin me off.. tried the seafoam. im sure it did some good but truck still runs the same.. watched the video that was posted and is not the same thing. deff more of a backfire, after i ran the truck i parked it jumped out and popped the hood. there was fuel all over the base of the carb. not sure if this was caused by a backfire through the carb or the carb just needing rebuilt but i have never seen it do this before. so i pulled the carb and it is now in pieces on my desk. this isnt my first carb rodeo just so you guys know. i cant seem to find anything wrong with it. no bad gaskets jets are all clear, nothing was gummed up, just some carbon build up in the throat but nothing out of the ordinary. everything was tight when i removed it. im in a good habit of checkin bolts on the truck every few weeks to make sure nothing is rattling loose. i do for sure have a leak inbetween the intake and exhaust mani but its not causeing a vacuum, its cause an exhaust leak.. i think im gonna get a compression tester and recheck tdc and drop the distributer back in just to be sure the timing is good. thoughts??

 

Intake and exhaust is the same gasket on the 4.9. I think you have found your problem

 

the one im talking about is between the two. directly under the carb where the two manifolds come together