I have a stock 1978 351M with about 50,000 original miles.
Once the car has warmed up I'm getting back firing if I tromp on the gas (when in park) or even on the highway if I take my foot off the gas quickly. I checked the static timing and is fine. Any thoughts would be appreciated. The cat. coverters were removed years ago.:-X23

 

Mine was doing the samething. I checked that timing 4 times and everytime I swear it was reading on spec. After ripping my hair out of my head and replacing a few things at the cost of a couple hundered bucks I finally decided to pull the front cover. Sure enough the chain had enough slack to move the timing around without jumping a tooth.

What I believe is happening to you is what happened to me. As the engine is idling or under power the chain is "tight" and you see the correct timing. When you rev and let it go, or take the engine from power to coast, the chain slacks up and you get the backfire.

I'd be willing to bet that if you replaced the chain your troubles will go away too.

 

Hey,

Is the vac advance hooked up / working?

Is the choke ok / not stuck?

Post if you need help testing / checking these items.

-Brents

 

too check timing i check with no1 plug and whatever plug is 180 opposite the check chain stretch

 

A buddy of mine had this idea also, it seems like a sound threoy. briskly Rev the engine back in forth from idle to 3500 rpm's while watching the timing marks. The light should have a nice smooth flow up and down the scale. If it dances around firing oddly between the high and low readings you can figure the chain has streched.

 

I would also check for vacumm leaks or malfuntioning emmissions equipment because an overly lean mixture or too much recirc. gasses will cause backfiring when you let off the gas in all instances.

400m

 

had similar prob myself .However you didn`t mention if it was backfiring out the exhaust or carb usually if it`s out the exhaust it could be timing, valve train probs , loose muffler,etc. backfiring out the carb could be lean condition.ps never did find out my prob, it just went away.

 

been away, just got back, thanks for all the ideas, I checked the timing, needed a little adjustmetn but still getting the backfiring through the exhaust

Wayne_J I monitored the timing while cycling through revs. seems smooth. Timing advance was ok.

400M I do need help on checking the vaccuum system and emmissions there's a heck of a lot of vaccuum hoses under there

ProFord I still have to check the 180 plug, forgot to do it last time.

 

On the back of the engine near the firewall you'll find a vacuum tree. Normally these have one or two ports that have a cover over it. Remove one of these covers and attach a vacuum gage. Compare your readings to these:

Engine Speed=Reading=Indication of Engine Condition
Smooth and steady idle (800 to 1200 RPM)=Between 17 to 21 inches=Engine is in Good Condition, but perform next test to be sure.

Open and close throttle quickly=Jumps from 2 to about 25 inches=Engine is in Good Condition.

Smooth and steady idle=Steady, but lower than normal reading=Worn rings, but perform next test to be sure.

Open and close throttle quickly=Jumps from 0 to 22 inches=Confirms worn rings.

Steady idle=Intermittent dropping back 3 or 5 divisions and returns to normal=Sticky Valves. If injection of penetrating oil into intake manifold temporarily stops pointer from
dropping back, it's certain the valves are sticking.

Steady 3000 RPM=Pointer fluctuates rapidly, faster engine speed causes more pointer swing=Weak valve springs.

Steady idle=Fast fluctuation between 14 to 19 points Worn intake valve stem guides.

Excessive pointer vibration at all speeds indicates a leaky head gasket.

Steady idle=Constant drop=Burnt valve or insufficient tappet clearance holding
valve partly open or a spark plug occasionally miss firing.

Steady idle=Steady 8 to 14 inches=Incorrect valve timing. It must also be remembered
that vacuum leaks and/or poor compression can result in a low vacuum reading.

Steady idle=Steady 14 to 16 inches=Incorrect ignition timing.

Steady idle=Drifting from 14 to 16 inches=Plug gaps too close or points not synchronized.

Steady idle=Drifting 5 to 19 inches=Compression leak between cylinders.

Steady idle=Steady below 5 inches=Leaky manifold or carburetor gasket, or stuck
manifold heat control valve.

Steady idle=Floats slowly between 12 and 16 inches=Carburetor out of adjustment.

Blipping engine speed=Quick drop to zero then return to normal reading=Muffler is clear.

Blipping engine speed=Slow drop of pointer then slow return
to normal reading=Muffler is choked or blocked.

A good vacuum gage and this chart can go a long ways to helping solve a lot of engine troubles.

 

Thanks for the check list,
the vacuum is all came out within the good noted overall around 18 inches at 800 rpm
range on the throttle when open and close quickly was around 0 to 24.5 inches
at 3000 rpm vacuum held steady
the one thing I did notice that as I when through the different tests that required movement of the throttle, the rpm would settle at a different point each time fluctuating . at one point it was idling at 500 - 540 rpm after opening throttle it would settle at a different spot with the idle fluctuating between 695 - 750 . It seems to change each time I opened / closed the throttle ... as low as 500 other times as high as 900rpm,
the engine label indicates a idle of 600 at 12 degrees BTDC

I also checked the plug that was 180 degrees (cylinder 6)and the static timing was right on the mark.

 

hello, first post here in an attempt to help.

i have had the same problem with my 351M and
when i was rebuilding my carb, i took the oportunity
to clean the egr passage in the intake manifold
with a round brush.lots of carbon build up in there.
i also cleaned the passages in the EGR Spacer plate.
Since i have done that, the problem never occured again.
This problem have caused me an exploded muffler !!!
it was sounding like a 12 gauge! i had fun with it though as i scared the people on the street with it heh heh


Hope this helps.

Sylvain

 

Thanks Sylvain, I am just in the process of pulling the carb for a rebuild, I will check and clean the egr system, I will update in a couple of weeks when i finally get it back together.