Back firing issue

[bkozlow]

A widely held opinion by members of this site appears to be when someone asks for help in diagnosing a problem, that the person on finding the solution should communicate the solution to all, and rightly so. Sharing the knowledge increases all our knowledge. Well, I am going to present a recent problem I had. I didn't present the problem on the site but will now and go through each of the steps I took. I will at the end inform all as to what the solution was.

PROBLEM

Engine has about 450 miles, it started to backfire in the mufflers at RPM's less than 2000, over that it ran very smoothly with no back firing. It was running on the rich side.

PROCEDURE 1

DO THE OBVIOUS---Check the timing
RESULTS--Timing right on

PROCEDURE 2
Determine if there is a pattern to the miss firing
RESULTS--85% of the backfires were coming from the 5-8 cylinders (drivers side) eventually it appeared it was numbers 7 or 8 cylinder that were causing the problem. Determined via sound, flames and a heat gun which cylinders were involved.

PROCEDURE 3

Tighten the header bolts to stop air leaks
RESULTS--While no leakage around the header flange was found all bolts were tightened, some leakage found where the mufflers connect to the headers--this is a Coupe so you have 4 mufflers and they use as the originals an interference fit-no gaskets Take off the mufflers and re-assemble using NAPA #35958 EXHAUST PUTTY. Leakage at the mufflers solved, problem continues.

PROCEDURE 4
Question is the header wrap covering hiding a broken weld letting air into the system. Remove the wrap and inspect.

RESULTS-- No leakage spots found.

PROCEDURE 5
Thinking it still must be an air leak replaced all the header bolts with Mr Gasket header bolts with hex key in the bolt head, these bolts allow you to tighten them when normal wrenches or sockets do not work well, # 34420G

RESULTS--Back firing on the passenger side all but vanished--I must be on the right track--WRONG

PROCEDURE 6
Replaced exhaust gaskets (Mr Gasket) with REMFLEX gaskets, but before I did this, I cut the header flange between cylinders 6-7. On the coupe each pair of cylinders has it's own muffler. I did this hoping to reduce any warpage in the header flange.

RESULTS--NO CHANGE PROBLEM CONTINUES

PROCEDURE 7

Closed idle screws on the rear barrels of the Holley carb to see if leaning out helped, checked pump pressure 6 1/2 pounds and float levels fine.

RESULTS--NO CHANGE PROBLEM CONTINUES

PROCEDURE 8

As stated, I felt it was cylinders 7-8 that were causing the problem, this header was also where the O2 bung for the sensor was installed, removed the sensor and replaced with a plug, thinking it was causing a hot spot igniting the fuel.

RESULTS--NO CHANGE PROBLEM CONTINUES

PROCEDURE 9

Replaced all the spark plugs--running rich confirmed by plug color, inspected all ignition wires, nick found in one wire, replaced all wires, checked firing order twice OK, and made sure no cross firings.

Replaced the coil with a new one.
Replaced the cap and rotor on the MSD Billet distributor, lubed it and made sure the advance was working correctly.

RESULTS--NO CHANGE PROBLEM CONTINUES

PROCEDURE 10

Ordered all new inners for the distributor to rebuild it but before I did that there was one other procedure I wanted to try--replaced the MSD 6AL box even though it only had 450 miles on it.

IT WAS THE BOX!

While some may have gone through a different process I find if you approach each problem in an orderly way you can generally solve the problem, but lets remember to finish of the problem by reporting what the solution was.

BillK