Blown Muffler

[FJ4013]

Rig: 1978 FJ40 2F

In the past 2 months I've had 2 backfires. Never had them before. The other day I had one that sounded like a bomb went off. Looked underneath and found this. A good friend who is an FJ40 guru thinks timing may be the cause. He's always been right and I typically take my truck to him for major fixes.

Just curious to know if anyone else has had this happen? The muffler is only a year old. The upside is the exhaust sounds a lot meaner now.

Blown Muffler.jpg

• FJ4013
• Apr 15, 2019

 

[Living in the Past]

Looks like unburned gas made it into the muffler and then hit the right mix of gas/air/temperature and exploded.

 

[FJ4013]

...

 

[FJ4013]

Looks like unburned gas made it into the muffler and then hit the right mix of gas/air/temperature and exploded.

Yup...that's the easy part to figure out. I'm looking for other solutions to look at besides timing. Go down the check list of items it might be. I have a new dizzy and a tuned carb so those two I can rule out. Aside from timing I'm not sure what else may cause this. Looking for more experienced users here to offer suggestions on what to check next.

 

[jm599]

Is it running okay when it’s not backfiring?

 

[JohnnyC]

you have a weber don't you... your running rich... most likely never adjusted correctly

advance the timing to about 12* and cleanout carb and adjust/jet it correctly

 

[4x4veteran]

Rig: 1978 FJ40 2F

In the past 2 months I've had 2 backfires. Never had them before. The other day I had one that sounded like a bomb went off. Looked underneath and found this. A good friend who is an FJ40 guru thinks timing may be the cause. He's always been right and I typically take my truck to him for major fixes.

Just curious to know if anyone else has had this happen? The muffler is only a year old. The upside is the exhaust sounds a lot meaner now.

Blown Muffler.jpg

• FJ4013
• Apr 15, 2019

Check for play in the distributor shaft. I know play on points will cause backfire. I haven't wore out an electronic dizzy yet.

That muffler looks a lot like the one on my mom's 61 Dodge when it blew out from a backfire.

The timing was ok, it was the stupid kid behind the steering wheel.

when I was 15 I had a friend that was a year older than me and he had an English Austin that we would ride around in.

As we went through town he would turn the ignition switch off and on making the car backfire through the glass-pack.

I thought that was funny so shortly after I got my drivers license I was waiting for my mom in a parking lot and thought I would try the backfire trick.

Sitting there with the engine idling I flicked the switch and "BOOM!"

Sounded and felt like the whole bottom of the car was blown away.

Of course being the lying little s**t I was, when asked what happened I said I didn't know, I was just sitting the with the engine running and it just happened.

So to make my winded story have a point other than showing what a little POS I was a sloppy dizzy shaft can cause a hiccup in the spark and a thus backfire.

Just saw where you said you had a new dizzy. Check the cap anyway.

 

[FJ4013]

Check for play in the distributor shaft. I know play on points will cause backfire. I haven't wore out an electronic dizzy yet.

That muffler looks a lot like the one on my mom's 61 Dodge when it blew out from a backfire.

The timing was ok, it was the stupid kid behind the steering wheel.

when I was 15 I had a friend that was a year older than me and he had an English Austin that we would ride around in.

As we went through town he would turn the ignition switch off and on making the car backfire through the glass-pack.

I thought that was funny so shortly after I got my drivers license I was waiting for my mom in a parking lot and thought I would try the backfire trick.

Sitting there with the engine idling I flicked the switch and "BOOM!"

Sounded and felt like the whole bottom of the car was blown away.

Of course being the lying little s**t I was, when asked what happened I said I didn't know, I was just sitting the with the engine running and it just happened.

So to make my winded story have a point other than showing what a little POS I was a sloppy dizzy shaft can cause a hiccup in the spark and a thus backfire.

Just saw where you said you had a new dizzy. Check the cap anyway.

Funny story. I remember doing the same as a kid. I replaced the points with an electronic ignition from Pertronix. I think the other poster has nailed it with my Weber 38/38. I leaned it out because it was running a bit rich. That carb most likely needs some attention so that is where I will start.

Thanks for the laugh!

 

[FJ4013]

you have a weber don't you... your running rich... most likely never adjusted correctly

advance the timing to about 12* and cleanout carb and adjust/jet it correctly

Yep you nailed it. A Weber 38/38. I don't know if that carb has ever had any attention given to it and a rebuild is probably in order. It's seen better days. Thanks.

 

[FJ4013]

Is it running okay when it’s not backfiring?

Yes. It runs great. And to clarify I've had only 2 backfires. The first one was about a month ago and the second one was the other day but that's when it blew the muffler apart. I was decelerating from a higher speed than normal so I could feel the compression when I down shifted and it was too high. I normally wouldn't have shifted so quickly from a higher rev but some idiot literally pulled right in front of me so I used the high revs to compression shift to slow me down even quicker because...well.....old brakes. That's when the muffler blew out.

 

[skidplate]

I had very similar happen to my' 75. I was running the Crane electronic ignition at the time and the little "magic" black box was faulty. The ignition would shut off then come back on and BANG! Went back to points and she runs fine now.

 

[45Kevin]

Yes. It runs great. And to clarify I've had only 2 backfires. The first one was about a month ago and the second one was the other day but that's when it blew the muffler apart. I was decelerating from a higher speed than normal so I could feel the compression when I down shifted and it was too high. I normally wouldn't have shifted so quickly from a higher rev but some idiot literally pulled right in front of me so I used the high revs to compression shift to slow me down even quicker because...well.....old brakes. That's when the muffler blew out.

I bet that guy has a brown stain on his underwear.

 

[jm599]

I recently changed a reasonably new dizzy cap because of intermittently running on 5 cylinders. 5 th plug would be wet. Back to full power now.
So somehow unburned gas got into your muffler....

 

[FJ4013]

I recently changed a reasonably new dizzy cap because of intermittently running on 5 cylinders. 5 th plug would be wet. Back to full power now.
So somehow unburned gas got into your muffler....

That's interesting but I'm lost. When you say the 5th plug was wet what do you mean and where was it wet? I'm always learning about my rig so excuse my ignorance here.

 

[jm599]

Cap may have had intermittent electrical short. So plug wasn’t firing I guess. Unburned gas evident on plug end. Plug wires were new too. New different cap fixed it. But you said yours was running okay.

If carb float was sticking and dumping gas I suspect it would have also been running rough too. But I am so far from being an expert best get their advice.

 

[4x4veteran]

Was going to say something profound but wth do I know?

 

[pjohnson]

Backfire is almost always timing related. Timing can also lead to excess fuel, due to rich mixture conditions and incomplete burning in the cylinder, making it's way down the exhaust and collecting in the muffler. Until it goes boom

That's my theory

 

[Trialsallday]

guys this same thing happened to me about a year or so ago. if you search I think I posted pics etc..
I found 2 things that seemed to take care of it. but I believe the main reason was a bad ground from my coil. I think it was grounded enough to run ok and idle ok but it would break up at higher rpms. I cleaned to surface of that ground contact and it suddenly reved smoothly and stopped smoking as well. so that unburnt fuel was making its was down then ignited.
secondly was I got a timing light and checked and adjusted my timing.

 

[Living in the Past]

That's interesting but I'm lost. When you say the 5th plug was wet what do you mean and where was it wet? I'm always learning about my rig so excuse my ignorance here.

Not sure I seen this answered. A wet plug would indicate it is either plug has a issue and not arcing hot enough to burn off all the fuel or something is causing it not to fire. Most likely plug wire or distributor related.

 

[dorotos]

Had this happen to me a few years ago right when I got my rig. Messed with the carb a bit, haven't had an issue since. Never looked at the timing like other people mentioned, but that could be your issue too.