How to solve an exhuast backfire?

[Polyphemus287]

Hi all

This is my first post on the forums, but I have been a longtime unregistered lurker. Recently, I acquired a 1983 c10 with the 4.1 six cylinder. The engine would appear to be completely stock, except for perhaps a lack of vacuum lines.

The problem: The trucks is having is a small backfire from the exhaust. It idles fine, but if the engine is given some throttle under load, or without load it I get small pop upon lifting from the gas pedal.

This problem started after I gave the truck a tune up and fixed a couple of oil leaks which included:
Plugs (gaped properly)
new dist. cap
new rotor
fuel filter
air filter
oil change
new valve cover gasket
new fuel pump & line
I replaced some of the old remaining vacuum lines with new hose, and replaced some of the old vacuum lines running to the carburetor that had been plugged with bolts, with new rubber plugs.

Prior to the tune up the engine did not run as smooth, as it does now. The engine shakes less etc, than it did prior to my tune up. Prior to the tune up I drove it for an hour, with no backfire or any other issue than a shake at idle. It was a little short of power in my opinion, but this could just be because of its age.

Strangely, the old plugs that were removed had a white tint to them. This leads me to believe that the truck was running lean prior to the tune up. To my understanding a backfire out of the exhaust is caused by unburnt fuel flowing through the exhaust and igniting. My initial impression is that my tune up somehow resulted in the engine gaining more fuel flow than before.

At this point, I am stumped. Perhaps misguidedly, I took a can carb cleaner and tried to find vacuum leaks but could not find any. I am not sure where my problem lies, but my initial impression is that it has something to do with the carburetor or the engine's timing.

Any ideas or input would be greatly appreciated

 

[smoothandlow84]

Are you sure that the firing order is correct, since you replaced the plugs? Just a thought......because I had two wires crossed and it caused a backfire. Dumb mistake on my behalf...but it happens.

 

[350runner]

A pop on deceleration usually means the engine is running lean. Hints the white plugs. Check for vacuum leaks. If none are found try having the carb rebuilt or jetted to provide the correct a/f fuel mix. Or out could just be mixed up plug wires.

Sent from the dust in front of you!

 

[Polyphemus287]

Smoothandlow,

I have not checked the firing order for the engine yet, but given how it idles I am pretty sure it is correct. When replacing the plugs I did them one at a time to prevent myself from messing up the firing order. I will double check it first thing tomorrow though. Thanks for the reply

 

[Polyphemus287]

350runner,

I sprayed carb cleaner around all the vacuum lines, carburetor, and intake manifold without any change in engine rpm. Admittedly, It was almost impossible to check for leaks on the bottom of the manifold, but I sprayed the hell out of it with no change.

As far as having the carb rebuilt, is there any way to check it for proper operation? I just got a quote from a local napa for a rebuilt carb @ 489 dollars and rebuild kit for 35. I would try to rebuilt it, but man from my alldata schematic it looks extremely complicated. I know it has the e2se 2 barrel on it, which to my understanding one cannot adjust the air fuel ratio.

Would it be easier to fix this situation with an aftermarket carburetor with adjustable mixture screws?

Thanks for help

 

[350runner]

Before you start digging into the carb trace your steps. Ignition, what could have gone wrong? Bad new plugs? Old plug wires finally gone bad?

Sent from the dust in front of you!

 

[Georgeb]

Before you start digging into the carb trace your steps. Ignition, what could have gone wrong? Bad new plugs? Old plug wires finally gone bad?

Sent from the dust in front of you!

2X^

 

[87scotty]

Might be worth giving the timing a check sounds like you may have fixed a vac leak causing your white plugs always do the easy checks before replacing parts

 

[Polyphemus287]

Thanks to all for input,

This morning I retraced my steps.

Here are my results:
Firing order is correct
New plugs are in good shape, gapped correctly
Both dist cap and rotor are installed correctly with no visible cracks etc.

I sprayed almost an entire can of carb cleaner all over the engine including vacuum lines, carb gaskets, intake manifold etc. Never even got the slightest change in engines rpm. The underside of the manifold on this six cylinder is a pain to spray into, but I did the best I could.

I am still stumped by the whole thing.

87scotty,

I do not have a timing light to check the engine timing, but I can pick one up if need be. Is it possible to diagnose it as a timing issue by loosening the distributor and moving it slightly to see if the condition gets better or worse?

 

[350runner]

If you have or can borrow a multi meter you can ohm out the spark plug wires.

Sent from the dust in front of you!

 

[Polyphemus287]

350runner,

I forgot to mention in my original post that the spark plug wires are new as well. My apologies for that. However, I suppose it is possible for them to be bad out of the box. I do have a multimeter, but I am not sure it reads ohms. I will check them anyways and report back

 

[87scotty]

Yes you can see by turning the distributor but your definetly going to want to get it timed right

 

[Polyphemus287]

350runner,

I tested the plug wires, with the readings ranging between 1.5-2.1 ohms. I compared this to an old set of wires which had readings ranging from 6-9 ohms. This leads me to believe the plug wires are in good shape, but I could be wrong. Any input on that would be helpful.

87scotty,

I think I will make a mark on the distributor before making any adjustments just so I can resort to back to a baseline incase things go afoul.

My only question is which way retards the timing and which is advancing the timing?

 

[87scotty]

Counter clock advances I believe but your not gonna hurt anything turn in one way or another! Whatever speeds it up is advancing and what slows it down retards I'd try advancing it a lil see if it fixes it timing may have been set with a vac leak and that'd through it off

 

[350runner]

Those plug wires are good. The lower the ohms the better. With your new plug wires this lean issue is more profound. Try retarding the timing and see what happens. Also what octane gas are you running. Might be a good time to switch to the highest available. It's worth a try.

Sent from the dust in front of you!