Backfire... loud... any damage?

[mimregi]

edit: apparently, according to another thread, I may mean afterfire here. unclear to me...

So the short version of the story is that my car backfired VERY loudly on the way to work this morning and now has a clearly different sound. I was close to work so I kept driving (if not for the difference in sound, it was running pretty well) and took a peek. A tertiary glance gid not reveal an elmer fudd-esque banana peeling tail pipe or any other obvious damage... should I be concerned? Obviously I need to get the muffler off to take a closer look, but it is my daily driver, so I need to make a call on whether or not it is safe to drive.

The longer version of the story... the car has dual weber DGVs which are long overdue for some love (I was actually intending to post some time this week about syncing them and some other questions I have, I have never done that kind of work before). The car has been idling rough but more or less running pretty well at speed. Typically, it would just idle a bit rough, get progressively worse, let out a little cough and then be fine for a bit before repeating the cycle. It did that this morning, no cause for alarm, but the "cycle" went quicker and I actually got the little cough twice. After the second, we're running fine again, it starts to degrade slowly, I downshift to corner and BOOM. It sounded (to me at least) like a shotgun.

I had some muffler work done a few months ago. The work seemed ok (admittedly I should have given it a more careful inspection or just done it myself), but I wasn't inlkove with the way it sounded -- it was just a bit tinnier than I had gotten used to, but I chaled this up to the fact that before the work, the muffler had a hole in it.

Also, the other day on the highway, the chrome muffler tip shot off when I threw it into third (wasn't happy about that). I'll be damned if that has anything to do with anything, but I mention it here because I am an amateur at this and have no shame -- if it is even POSSIBLY related, I would rather include it.

Anyone have an opinion regarding the drivable safety of my poor car? I know he is just acting up for attention -- daddy has been so busy lately, I haven't been able to spend quality time with him, just back and forth to work... no bath time, no carb cleaning, not even the new stereo I promised... kids.

Thanks in advance!

-m

Quick info:

>'73 240z, Weber DGV carbs

>starts right up, generally runs great except for the aforementioned idling, which started maybe a month ago

>Has blown white smoke when revved from time to time in the past, though that has never been associated with any recurring issue

>Used to have a pretty bad gas smell (which is but one reason I suspect the car is running rich), mostly went away after the muffler work.

>At about the same time as the rough idling started, I noticed that it had a very low tolerance for the choke, meaning that a full choke, once the engine was running, would typically stall the car out. I couldn't idle at more than 30-50% choke without slowly stalling out (and when cold of course, at no choke it would also stall out). Obviously related is that the time to get it from cold to not needing choke at all to stay running was notably shorter during this period.

[Arne]

Chances are that the explosion damaged some of the baffles in the muffler. Check the muffler shell to see if it looks swelled, also.

My best guess is that other than damage to the muffler, the car is probably safe to drive.

[zeiss150]

I believe that kind of an "explosion" out of the exaust is a sign that the timing is "off" The engine is firing the intake charge while the exaust valves are open. The explosion is suposed to happen with both valves closed. Check your timing is most likely off. If I remember correctly (and thats a big if) you need to advance your timming a little. Just a shot in the dark. I hope it helps.

Matt-

[mimregi]

So after some further inspection, there is a split in the seam of the muffler, which explains the added noise. I drove it home from work yesterday (14mi in the rain). The car is running better, though not necessarily great, which is to say it is not coughing or firing anymore but still a bit rough.

I'm off work next week, so I think it is time to finally bite the bullet and rip the carbs off there, clean them out, get them adjusted and synced and do a full tune up (in addition to replacing the miffler -- it sound like a mustang now...).

Thanks!

[beandip]

address the carb issues , you are running way rich is my input. Unburned fuel collected in the muffler and detonated. I wouldn't do anything with the muffler until you fix what is wrong with your Webbers . Not a timing issue .

[Arne]

address the carb issues , you are running way rich is my input. Unburned fuel collected in the muffler and detonated. I wouldn't do anything with the muffler until you fix what is wrong with your Webbers . Not a timing issue .

Agreed. Un-burnt fuel is almost always the cause of explosions like this. Don't fiddle with the timing until you know the carbs are right.

[mimregi]

Just ordered the stuff to do the carbs today in fact =)

Many thanks again for the input! I'm looking forward to the job, it will be my first. Don't be shocked if I am posting more questions here next weekend!

-m

(btw Arne, the steering wheel looks great!)

[Maddos]

I had a similar problem with my 4.1 ford transit when the fuel pump was on the way out.