Back firing issue
 

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

MSD strikes again. Thanks for the summary. Generally popping in the exhaust is from running lean or an exhaust leak. And backfiring in the exhaust is from running rich. And backfiring through the carb is from running lean. But throw in electronic gizmos and all the usual scenarios are out the window.

Good post. In my experience, a strong spark condition will help make up for a weak fuel delivery condition, but a strong fuel delivery condition will not make up for a weak spark condition. 98% of engine performance issues are one or the other. :cool:

Where were your large wire power and grounds for the box connected ?
Blas

Always run them straight to the battery and run the distributor pickup and coil feeds down opposite sides of the block. And always use the noise suppressing capacitor to block leaking AC and to smooth out the DC feed.;)

Wired to the battery per MSD.

before you discard the box can you give us the numbers on the box. since Holley bought MSD they claim that their boxes have been pirated , they have posted some numbers. I will go back and check. All over the counter MSD parts are made overseas . thank you

Sorry, the box has been discarded and is no longer available.

BillK

Sorry, the box has been discarded and is no longer available. Since, the nock off boxes came to light, I buy all my MSD directly with a phone call to MSD in TX. This box was not a nock off.

BillK

Seen this too many times to count now.
6ALs are more trouble than they are worth. 7s and 8s are good.
There are many other options out there, ICE, Pertronix Second Strike, Crane etc. I have a CB Performance Black Box that has more output, and to higher RPM than a 6AL.

My car ate 6ALs like candy. I replaced two of them.

And did any of these MSD boxes, of which you speak, have a properly installed capacitor across the feeds to filter the power to the box? :confused:

Who knows. 13 years ago...

Well I say that because a leaky alternator will kill a MSD box that has no capacitor in short order. That tip doesn't get the press it is entitled to, but it's the truth. DC voltage spikes will also contribute to an early demise.:cool:

I had an MSD coil die right out of the gate (55 miles) and during the diagnosis process I spoke with Bob at ERA. He indicated he has not seen an MSD box failure that has the properly installed capacitor.

I installed the capacitor when I replaced the coil. Time will tell if it protects the 6AL-2 box.

John

Throw away that ancient msd!!
Do you cobra a favour and drop in a progression ignition.
You will be amazed at the tuning possibilities all done on the fly using your smart phone.

https://progressionignition.com/

FYI
The MSD capacitor referenced above
is part number 8830.

is there another capacitor ? the one referenced is listed by MSD as only for radio noise suppression.

That is it. If you speak to MSD Tech Support, they will suggest using it to protect your MSD box. It will stop electrical spikes from a variety of causes.

It is cheap insurance. I will advise in another 10 years if it worked well or not.

John

Capacitors are capacitors are capacitors, for the most part, their specs are just different. You can save a buck or two by buying a 26 kufd/25volt capacitor from Mouser but you want decent quality. And you can go up a bit on both numbers, it doesn't have to be exactly 26kufd. Before doing work on your car, remember to bleed the capacitor. To do that just disconnect your battery and hit the horn button (assuming your horn is wired to work without the key being on). You will hear the relay click and the horn might even make an audible burp noise. Mine does. That tells you your capacitor is holding but is now empty. You can leave your battery disconnected for quite a while and your capacitor will still hold a charge. Or, it can fail, like anything else, and it can fail open or closed. If it fails open then it's like it's not even there and it does nothing. If it fails closed then it's a short and will either blow the circuit breaker, if you have one, or burn something up or get so hot it fails open. MSD boxes are particularly sensitive to alternating current that is polluting your direct current feed to the box. A capacitor across the lines allows the AC to "take the shortcut" around the MSD box. I strongly recommend one. If you put your car's electrical system on an oscilloscope hoping to see a nice straight flat line at 13.7vdc you might be shocked at what you see. :cool: