backfiring through exhaust

-
M

moparguy

Well-Known Member
Joined
Aug 20, 2006
Messages
57
Reaction score
0
Could a bad coil be causing me to backfire under a load. Have checked my timing. Have a new distributor, cap, rotor, wires,spark plugs and ecu. With my timing light on every now and then the light misses the mark. Runs great unless i gas it going up hill with the rpm's low (under a load). Havn't changed coil, could it be my coil? Would like to eliminate the possibility of it being a problem with the ignition side before moving to fuel issues.
 
Are you sure you haven't crossed a plug wire? It's a simple and inexpensive check to do.
C
 
I'm sure. Runs great if i keep the rpm's up, but that is hard to do in town.
 
fouled plug from too rich at idle??? what carb?? what igntion components?? what is the timing?
 
My plugs are new. Have 750 cfm holley, runs a little rich, stock ignition components, but also new. Other than my coil, have 12 degrees of intitial timing. Plugs don't look fouled. Is it normal for my timing light to miss the mark every now and then? That is why a suspect something on the ignition side.
 
if it has a lopey cam, it can move. what is your vacuum at idle in gear? you may have too big of a power valve. electric fuel pump??? if so what kind? doesn't sound like a coil.
 
Has about a 480 lift cam, high volume mechanical fuel pump. You don't think it could be a weak spark?
 
Does everyone think i would be wasting my time with a new coil?
 
Rev it up in the dark and you may see a spark jumping from cracked wire or plug. Fits the symptom anyway.
Darkness serves as a diagnosic tool ? LOL
 
moparguy said:
My plugs are new. Have 750 cfm holley, runs a little rich, stock ignition components, but also new. Other than my coil, have 12 degrees of intitial timing. Plugs don't look fouled. Is it normal for my timing light to miss the mark every now and then? That is why a suspect something on the ignition side.
Click to expand...

What is your engine size? 318's like 7 degrees initial and 360's 9 or 10. IMO 12 is a bit much. It is normal for the timing light to miss the mark every now and then if the timing chain has stretched. Sounds to me like your chain has stretched and you're running the cam retarded. Put a timing light on the beast and have a long, careful, look at the timing marks. If the mark seems to be moving at all or skips entirely from time to time and wavers a bit before settling down, change the timing chain. They are the Achilles heel of smallblocks.
 
grumpuscreature said:
What is your engine size? 318's like 7 degrees initial and 360's 9 or 10. IMO 12 is a bit much. It is normal for the timing light to miss the mark every now and then if the timing chain has stretched. Sounds to me like your chain has stretched and you're running the cam retarded. Put a timing light on the beast and have a long, careful, look at the timing marks. If the mark seems to be moving at all or skips entirely from time to time and wavers a bit before settling down, change the timing chain. They are the Achilles heel of smallblocks.
Click to expand...

Thats what I would think too, but if ot has a .480 lift cam, I would almost wager that the timing chain is a double roller or double setup with steel gears. On a stock small block, the nylon gears sometimes wear down or come off, causing the timing chain to skip a tooth. The weak link type chain can also stretch on top of that. However, in this case, witht the aggressive cam, I would say that it has a better timing setup and is probably not the case. I know the comp XE line of cams, like my 268, like around 14-16 degrees of initial timing. 11 degrees initial gave me a slight backfire through the carb, but when I gave it 15 or so, it went away and runs great.

Check into what your cam likes best, especially with that carb cfm. What is your engine size and cam manufacturer?

Could be a slightly worn cam lobe, but I would expect coughing at all rpms and a rough idle, so I doubt it.

Look into the carb a bit more and make sure your cam has its sweet spot of timing.

Good luck and we are here to help. Happy New Years to all!!!!!:toothy10:
 
Now that I think of it, a backfire through the exhaust, rather than the intake, would be more along the line of electrical (plug wires crossing, weakening ecu, dying charging system, ect) or fuel system(too much fuel making it into the exhaust, carb, ect.).

Just a thought.
 
Usually if you get a backfire through the exhaust it can be caused be the ignition momentarily cutting out or a spark that is to weak to burn all the fuel under heavy load. Id change the coil as long as you're sure that all the wiring etc. is 100%.

Terry
 
I just had a back fire issue with my 360. I used the timing light on each wire untill I found the cylinder not firing each time. Needed a cap. Good Luck. John
 
You must log in or register to reply here.
-

Back
Top