Topic: 390 saga continues BACKFIRES

[Matt1968]

I have my 390 running now BUT,  I am getting backfires out of my exhaust.  At idle it's popping and backfiring.  I know if it backfires at the carb its a specific issue and if you backfire at the exhaust its a different issue.

Can someone educate me please.  Are there any FE specific items I need to look at?

Matt

[67gtasanjose]

ALWAYS, with such a problem, begin with the basics. Not really an "FE ONLY" sort of a thing.
1.) Check and double check Firing Order. This is it 90% of the time.
2.) Make sure the dwell angle of your points is correct and NOT moving around. You need a dwell meter to accomplish this while it is running. (assuming you have points ignition).
3.) lastly, if all above is OK you have valve(s) leaking some. A compression test would then be in order to determine the cause.

[WT8095]

More basics:

• Check the plug wires to make sure they are securely attached on both ends.
• Check the condition of the plug wires.
• Check the routing of the plug wires (crossfire).
• Check the inside of the distributor cap for moisture or other foreign material.

Beyond basics:

• Check the timing chain for excessive play (have to remove the fuel pump from the timing cover).

[cobrajetchris]

I had a problem similar to this on my FE and it about drove me crazy. I checked ALL of the basics as listed above and many more only to find out it was the mufflers causing the problem that disguised itself as a backfire and was not an engine problem at all. The noise had the identical sound. While the engine is idling in part take a rubber mallet and bang on the mufflers and see if that changes anything. The mufflers looked like new on the outside but the guts where all rusted out causing the problem. I disconnected the mufflers and problem solved.

[67gtasanjose]

What Chris just posted made me think of another scenario:

WITH a THERMACTOR SYSTEM, the Smog Pump could be injecting AIR into the exhaust if things are not connected or working correctly. The AIR should only be entering the exhaust ONLY DURING ACCELERATION, if at other times (Idle or deccelerating) it will produce a backfire effect in the exhaust system, MOST PRONOUNCED BACKFIRING WHEN DECELERATING, to the point where it can blow up the exhaust system (usually the mufflers).
The AIR should be diverted away from the downstream (exhaust) when connected and operating correctly. The backfiring in this scenario would be caused from the addition of AIR which helps (any) unburned fuel to detonate. BOOM-BOOM BOOM

[J_Speegle]

Guessing this is a 68 390?

Also would check the vacuum hose routing since the diaphragm could get the wrong signal/vacuum at the wrong time

Basics and one thing at a time

I'm guessing your using the original point style of ignition. Before I rebuilt my Boss I had a bad backfiring issue once the car warmed up to the point of it just shutting down. Cool down and it would fire right up.  Issue was the Pertronics

[Matt1968]

What Chris just posted made me think of another scenario:

WITH a THERMACTOR SYSTEM, the Smog Pump could be injecting AIR into the exhaust if things are not connected or working correctly. The AIR should only be entering the exhaust ONLY DURING ACCELERATION, if at other times (Idle or deccelerating) it will produce a backfire effect in the exhaust system, MOST PRONOUNCED BACKFIRING WHEN DECELERATING, to the point where it can blow up the exhaust system (usually the mufflers).
The AIR should be diverted away from the downstream (exhaust) when connected and operating correctly. The backfiring in this scenario would be caused from the addition of AIR which helps (any) unburned fuel to detonate. BOOM-BOOM BOOM

No thermactor on my engine..

[Matt1968]

Guessing this is a 68 390?

Also would check the vacuum hose routing since the diaphragm could get the wrong signal/vacuum at the wrong time

Basics and one thing at a time

I'm guessing your using the original point style of ignition. Before I rebuilt my Boss I had a bad backfiring issue once the car warmed up to the point of it just shutting down. Cool down and it would fire right up.  Issue was the Pertronics

Yes 68.....yes single points

[Bob Gaines]

Did you have the distributor out?

[Matt1968]

http://www.concoursmustang.com/forum/index.php?topic=11315.0

Yes dist was out recently.  Please see original post.

3 years ago I parked it.  At that time it ran incredibly well.   Took the dist out to oil prime the engine.

Now backfiring every 5 seconds.

[WT8095]

http://www.concoursmustang.com/forum/index.php?topic=11315.0

Yes dist was out recently.  Please see original post.

3 years ago I parked it.  At that time it ran incredibly well.   Took the dist out to oil prime the engine.

Now backfiring every 5 seconds.

Did you cover the tailpipes? Do you have any cats on patrol where the car was stored? 

[Matt1968]

Anyone in the Toledo OH want to check this thing out?

[Bob Gaines]

http://www.concoursmustang.com/forum/index.php?topic=11315.0

Yes dist was out recently.  Please see original post.

3 years ago I parked it.  At that time it ran incredibly well.   Took the dist out to oil prime the engine.

Now backfiring every 5 seconds.

Logically if it ran well and then after removing and reinstalling the distributor it started backfireing , the distributor is the source of the problem and in this scenario the most likely thing is that the distributor has been re installed incorrectly.

[67gtasanjose]

Anyone in the Toledo OH want to check this thing out?

I'm 2 hours away in NE Ohio, do you have a trailer?

[svo2scj]

HI Matt

Give me a call 517 861 7630  (North of Ann Arbor)  It sounds like you know car pretty well -do you think you need a second set of eyes, work together and just confirm correct?

Can you take the time to work on car in one day if planned (or plan a visit and a return if you want to work on it)
Would you need timing light, fresh plugs - have floor jack and stands (clutch) ?
Let me know if you can only work weekends OR if you are avail weekdays (stays light until 9:30).   

Mark Haas
P.S.  I'm not a certified tech, but never met an engine I couldn't start !No cost no obligation - other than a follow up post to inform all.  (good or bad) LOL