The Stovebolt Site Forums The Shop Area Engine Shop exploded muffler- '56 gmc 270 ci six

Well, I got my '56 on the road this week, passed inspection Monday. This truck has run very little in the past 23 years. 93 k original miles, engine not changed except carb rebuild. Cletis helped me with a little initial antifreeze overflow, and generally the truck has been running well, except for unexpectedly dying at low idle and being hard to restart, as well as popping on acceleration (when lugging a little). Today I drove city traffic about 8 miles with a high idle(throttle pulled a bit), worked for four hours with the truck shut down, no gas smell, tightened some bumper bolts, then went to start up. As soon as the engine caught, there was such a big backfire? boom that my ears hurt- I could feel the concussion and felt the engine go rough, so I immediately shut it down, went to look at the carb, seemed fine (except I did not look down at the butterfly throttle valve), and then I noted the exhaust pipe seemed low. To my amazement, the muffler was blown open like a sardine can, and the heavy strapping holding it up had been blown apart too. This muffler is 2 feet long and 7 1/2" in diameter! A voice floated in from the street- " everyone all right in there?" "Yes- just a backfire" says I. The truck ran, loudly, and seemed to tend to stall when giving gas. I tied up the remnants of the muffler with the strapping, drove about a block, had very little power and a lot of noise, and decided a tow was my best option. I had been thinking about checking the timing, and had tried to increase the idle by opening the idle adjust screw a turn on Tuesday, with no change, so I left it. It seems to me there must have been gas/air in the muffler to do that explosion, and I am really looking forward to the collective wisdom of the folks in this forum to keep my next muffler from a similar fate, and to get a sense of what damage may have been done. The tow guy suggested a muffler with a larger outlet. The muffler is under the front of the bed, and a short tailpipe ends high before the rear axle. The engine side pipe is 2 1/2 " in diameter, and the outlet pipe is 2". The bends on the engine side probably bring it closer to 2" or 2 1/4" there.

I blew up a muffler once in a '62 Impala. Coil went bad so no ignition, leading to overload of fuel. Then BAM!!!! Replaced coil & muffler and all was fixed. Also same problem in '82 C20, but mufflers survived backfire.
So I would say probably coil. Maybe points, but definetly an ignition problem. Your exhaust should be more than big enough for a 6cyl & fine for a V8.

Maybe coil going south would account for subsequent rough running and low power. The truck was 6v, was converted to 12 by po. If I get a 12v coil, do i remove the ballast resistor? edit: answered my own question: no. ballast resistor is part of ignition circuit.

I think, when you shut down at a faster than normal idle you pumped the exhaust system full of gas/air mixture--then on restart you have an ignition source and BOOM.It is surprising that it could happen after an extended shutdown.
I think correct the idle/low speed carb problem and I would check the point gap/dwell.The points may be at the stage of closing up due to rubbing block wear as to cause intermittant spark.
A lot of "seem to be" carb problems are corrected by attention to ignition system.

Bob-that has some plausibility, as the muffler was tilted up toward the back (20 degrees or so) Gas and air must be heavier than air, and the blowout was more prominent on the down end-it opened wide there and did not on the high end. I am following both pieces of advice and checking ignition including coil. The inside of this new muffler is blackish- indicating a rich mixture, I think, so I should clearly correct the idle problem too.

When I got my '53 running after 25 years of standing still, I also lived through a fine big BOOM when it first started to fire. Without the wisdom of this site, I concluded that during all those years mice and related vermin had plugged the exhaust system as well as filling every other orifice available. I ended up working on the motor for a few weeks before trying to fire it up again. By that time I had a new standard assortment muffler installed and it hasn't exploded since.

I would consider that those years of weed control duty might have led to a plugged exhaust.

I blew out (exploded) the new muffler on the 270 engine in my 54/55 GMC 350 COE. We had the timing set incorrectly. It started and ran fine after statically setting the timing.

I have not yet checked with Midas Muffler to see if they will honor their warranty?

In my experience with any gasoline engine if the muffler blows apart it was caused by a back fire in the engine. Talk to the guru's in your local Muffler Shops.
Normbc9

Update to all- organbuilder, you were right ten years ago, when I first fired up the engine, mouse nest stuff blew out all over the garage. The mice had departed before the several hundred mile journey from Vermont. I had since put on a new exhaust system. Right off a couple days ago I found that I had failed to set the idle mixture screw- after three carb rebuilds over the years, I confused a thought of the instruction with doing it- instead of screwing it in until it stops then backing out a turn, I had left it Five turns out- a little rich. That supports Bob's theory- a rich and high idle pumping unburned stuff into the muffler. The dwell was 45 degrees- I reset to 32, and the timing which was right for that 45 dwell was off for 32. I reset the timing but left the ball a little below the needle mark as the engine would almost die when I went to the mark. I did not disconnect the vacuum advance to time the engine, as the manual did not mention that, but maybe they assumed everyone would know that you should? I put on a new muffler. I have not yet gotten a new coil, and I took it out for a test drive (had started and run like a champ for all the testing) and the engine promptly stalled under power going downhill and would not re start. The carb seemed wet with gas, so I left the truck parked on the road for now. This is the same symptom I had once before the muffler blew up- stalling and not restarting. My plan is a new coil and to more carefully rebuild the carb again. edit: will also try a vacuum test and some carb adjusting before a carb rebuild. What do folks think?

Tim, Meineke said that was not a warranty repair, and I had not even asked. I had gotten the parts over the counter, and brought the rusty pipes for them to duplicate. I just wanted to know what another muffler would cost.

normbc9- Yes you are correct, and your reply prompted me to go to Wikipedia and I found that contrary to my long held belief that a backfire was a cylinder explosion audible thru the exhaust, a backfire is an explosion in the exhaust. Engine gurus call an explosion in the intake a backfire (usually not pretty) and an explosion in the exhaust an afterfire, but for general purposes a backfire is an explosion in the exhaust system, which is what blew out my muffler.

At 9pm was a little hard to start, managed to get it going, warm up, and then backed into the driveway, ran engine for a while- running a little rough and stumbles then catches on sudden throttle opening.....

Thanks to all you here who advocate methodical testing. I pulled out my vacuum gauge and at idle it evenly pulses a half an inch at 20 inches- which makes sense as the engine has a little hiss that I've been told indicates a burned valve. The cranking test was a pulse between 7 and 9, so I tightened the carb to the intake manifold, and it went up to 15 inches at cranking! Runs much better now- no stumble. Will test drive later.

Update- test drive was still rough, so i tested compression and cylinder #2 had a compression of 72 #, and the rest were between 152 and 158#. I put compressed air in the cylinder with both valves closed and you could easily hear the rush of air sound coming from the muffler. My theory is #2 exhaust valve was burned and the explosion happened with it open and broke off some carbon or something that was helping the seal...Will pull the head to find out and fix it but have to wait awhile to heal from back surgery happening Monday.