66 Mustang with exhaust backfiring issues

[bridgemute]

Something else to check out.Do you have any leaks in the exhaust system?No matter how small,they can let fresh air into the system and that will also lead to backfiring or popping in the exhaust system if the engine is running even a little bit rich.
Good luck.Have fun.Be safe.
Leo

I feel somewhat confident that I don't have an exhaust leak, the reason I say this is because I replaced the exhaust header gasket and it was absolutely obliterated with tears and cracks and I was so confident that this was the issue...but with a new gasket it made absolutely no difference.

 

[bridgemute]

Before I remove the cylinder head, I'd like to know if anyone has seen a broken exhaust baffle create my present issue? Cylinders 1, 3, and 6 exhaust into the passenger side exhaust pipe.

I re-checked compression and cylinder 1 was actually low at 105, while the others were at 140-150. However the engine idles smoothly. The backfiring will pop once in awhile during idle but it gets worse as the engine heats up.

My plan right now is to remove the muffler on that side to see if the backfiring goes away.

 

[bubba22349]

Having that much difference in compression on cyl. #1 compared to the others is going to cause you trouble at any RPM above idle and as soon as you put a load on the engine in gear. Look for a weak (pressure) or collapsed valve spring too. Good luck

 

[bridgemute]

I pulled the bad rattly muffler and the backfiring seems to have gone away. I have new mufflers on order...so ill be waiting for a bit on 100% verification.

Muffler surgery found some broken baffles that may have been restricting flow until maybe I broke it all loose manhandling the thing when removing it. There was alot of loose rusty crap.

I'll update here as soon as I verify.

 

[chad]

"...seen a broken exhaust baffle create my present issue?…"
no, but the opposite - cuz a backfire can B high pressure OUTSIDE of the cyl., and blow out baffels, from that.

I don't think U hafta go to the expense (X & $) of head removal 4 this (yet).
F/U w/bubba;s ideas 1st…

"...I would put in a Duraspark II if anything…"
U may have it now, can't tell U cuz U haven't IDed the dizzy 2 me yet. It seems like if it ran fine w/the 1st 2100 U got compatable carb/dizzy sys. Sitting 4 a yr could have gummed up the lifter - that allowed the valve to operate inproperly, allowing gas to stay in those 2 cyl…getting fired off later as accumulated (a la the 2 sooty plugs) & raw gas igniting in the hot exhaust, (U said it started after pushing the car hard) blowlng out the baffels?

 

[bridgemute]

That head swap may be a project for the future. The distributor I am currently running is the original Load o matic hooked up to the Autolite 2100 carb vacuum. Somehow I have gotten it to run 10 years no problem until this backfiring issue has started. The tag on the carb is: C5MF C D 6BA. I do have a duraspark on order but that was an upgrade I have been wanting to do for a while now. If this muffler change out doesn't work I'll be pulling the valve cover back off and inspecting the valves and lifters closer. No. 1 cylinder did have low compression (found during a re-test) at 105 vs. the rest being at 135-150.

 

[chad]

...The distributor I am currently running is the original Load o matic hooked up to the Autolite 2100 carb vacuum. Somehow I have gotten it to run 10 years no problem until this backfiring issue has started….No. 1 cylinder did have low compression (found during a re-test) at 105 vs. the rest being at 135-150.

Readers of the site have seen ALL KINDA reports on results of mix'n match with prts frm either side of the '68 date - total 'no go' to "almost totally fine". Just thought I'd push it (thru 22 posts & 2 wks) due to seein other's stumbles around the issue.

If it is a lifter doesn't some type of "additive" poured right in there create an easy 1st step/try B4 a full head removal? (say Marvel Mistery Oil, Rust Buster, Penetrating Fluid, Kero?)

 

[CZLN6]

Howdy Again bridgemute:

Your "C5MF C D 6BA" Autolite Is from 1965 and made for a Mercury car. "C" = the decade of the sixties, "5" indicates the year 1965. "M" indicates Mercury, and "F" refers to an engine part. Now we need to venturi size casting from the side of the float bowl to help narrow engine application. The "C" and "D" indicated running design changes. "6BA" is a date code "6" = the sixth month, June. "B" = the second week of June, I'm not positive on the "A". it could be the day of the week.

On the carbs and distributor, the LoM oem distributor relies on a carb with a Spark Control Valve (SCV) to send a load sensitive vacuum signal. Your 2100 carb is designed to send a ported vacuum signal to the distributor with centrifugal advance. They do not mix and match well. A mismatch can be crutched in a variety of ways but none work well in all instance. So, my advice is to proceed to the DuraSpark distributor matched to your 2100 and check thoroughly for vacuum leaks.

Keep it coming.

Adios, David

 

[bridgemute]

As crazy as this sounds, the new muffler on the passenger side completely solved the problem.

 

[chad]

"...exhaust backfiring issues…" solved.
Not if carb/distrib R 'mismatched' (over that change yr '68). It might not backfire( fuel explosion @ wrong time or place) but for other issues -
it's a sometimes yes, sometimes no (at best) for MPGs, performance, starting, & other issues…

Y not upgrade for performance (execration & pep, MPGs & all above reasons) as David offers in last post?

 

[bridgemute]

I have all Duraspark parts in hand ready to go into the car