Occasional backfire in exhaust

SPARKEY2

MEMBER
1978 DODGE TRUCK
5.9L
V8
2WD
AUTOMATIC
58,000 MILES

I have a 1978 Dodge Lil Red Express with the high performance 360 engine. Under acceleration and deceleration everything is fine. When I am simply trying to hold a steady speed (45 mph or so) I get an occasional backfire in the left stack. There is no rhythm to it, Just an occasional pop. The last time I had this I changed the plugs and everything got better. This time changing the plugs did nothing. I also changed the air filter with no change.
Can you tell me the correct timing for this vehicle?
Also, what else could be causing this occasional backfire?
Thank you

5 Replies

PATENTED_REPAIR_PRO

MECHANIC
1,853 POSTS

You may have a spark plug wire shorting out temporarily on the exhaust manifold, the raw gas building up in the combustion chamber, then when the spark plug wire makes a good spark again, POW. So try new spark plug wires.

SPONSORED LINKS

Was this

answer

helpful?

Yes

No

+1

Tuesday, September 18th, 2018 AT 9:48 AM

SPARKEY2

MEMBER
3 POSTS

I put new plug wires on yesterday and the backfire now seems better, but the truck seemed to have a very tiny skip once in a while. Any idea on where I should head next?
I still wonder if the timing could be off? I cannot find out any information on where the base timing should be set for this truck.

Was this

answer

helpful?

Yes

No

Wednesday, September 19th, 2018 AT 4:33 AM

PATENTED_REPAIR_PRO

MECHANIC
1,853 POSTS

I would try a new cap and rotor next.
The timing should be adjusted while in neutral, timing advance disconnected and plugged and set to first column California and second federal.

Image (Click to make bigger)

Was this

answer

helpful?

Yes

No

Wednesday, September 19th, 2018 AT 12:51 PM

SPARKEY2

MEMBER
3 POSTS

Yesterday I actually tried to find part numbers for both the cap and rotor but with no success. My distributor has been changed to a Mallory Unilite Performance Distributor Part # 4757901. I called Mallory and they gave me part # 409 for the cap and part # 335 for the rotor. I was able to find the rotor but not the cap so I emailed them to confirm the cap # and I am still waiting for a reply.

Are the timing settings for a standard 360 or the high performance Lil Red 360?

Also, I took the truck for a pretty good run today. The popping is almost gone, just a very tiny pop once in a while but it does sound like there is a tiny skip, again once in a while. Also, there is a pretty good flat spot when taking off especially when not fully warmed up. The carburetor was professionally rebuilt in 2015 and I run Startron + Techtron all the time. Trying to keep the ethanol at bay.

Was this

answer

helpful?

Yes

No

Wednesday, September 19th, 2018 AT 3:16 PM

PATENTED_REPAIR_PRO

MECHANIC
1,853 POSTS

The flat spot may be because of one of two things; a vacuum advance that does not work, because of a busted vacuum advance diaphragm or the accelerator pump on the carburetor not working. The busted vacuum advance diaphragm can be tested, by removing the vacuum hose from the carburetor or intake manifold, ten lighting up a cigarette and blow smoke into the vacuum hose. If you see smoke pouring out around the distributor cap, then the vacuum advance diaphragm is busted. You can see if the carburetor accelerator pump is working with the engine off, look down the carburetor air horn into the barrel as you snap the throttle open quickly. You should see a steady stream of gas down into each front barrel if the accelerator pump is working correctly.
The timing is for the 360-1 or 360-3 which I truly did not understand what that -1 or -3 meant. I do not think the problem is due to the ignition timing off anyhow, because it appears to be just misfiring on one cylinder randomly. You may want to check all the spark plugs for one that is getting a bit wet and smelling of gasoline and install a new spark plug and keep an eye on it if the misfire comes back and it ends up smelling like gas again. Or getting oil fouled, from a bad valve guide seal or piston oil control ring gunked up with carbon.