'87 22R backfire with exploding muffler trick

'87 22R, 4 spd, 146K miles, 2x4. I see similar posts, but this ? doesn't concern black smoke or a New muffler, and may/may not be the same kind of problem...idk

My truck has had a slight intermittent backfire since I have owned it, for ~16K miles, usually in between shifting gears. It had always been very minor, and I had not been able to make it completely quit. I have set the timing, valves, tune-up etc., but it never did stop it completely. It never was loud or "bad" or often before, and it always starts right up and runs beautifully.
Also, when I first got the truck, it got a new budget exhaust from the cat. converter back. That repair was never great, and the muffler only lasted @ a year. For the past year the muffler has been on borrowed time...until this Tuesday.
The muffler finally separated from the forward pipe, got louder (obviously), and at the same time, the backfire quickly got worse. That afternoon I checked that the timing was at 0 degrees TDC,(Yes), then made a couple of short, noisy trips, <10 mi. ea. On the 2nd trip, while shifting to 3rd, it backfired so bad the muffler exploded. Afterwards, the backfire got so bad that anytime I shifted or took my foot off the gas it would backfire just as bad as when it blew out the old muffler.

The factory service manual points to several checks concerning the timing and the carburetor, for muffler afterfire and/or backfiring, but I am a lil lost on where to start or what to suspect. There was/is no smoke to speak of, gas mileage seems a lil low but ok, and the exhaust doesn't seem to have an odor of raw gasoline. The muffler shop here said $300 for a whole new exhaust awhile back...that included a cat. converter, but only b/c it was "rusty". The converter isn't new but doesn't rattle, isn't rusted thru, and doesn't seem to be overheating or glowing...
She definitely needs a new muffler now, but I am not driving it or fixing the exhaust until I can fix the backfire issue.

Thanks for reading, and for any helpful advice.

Did you set the ignition timing at idle to 12 deg BTDC with the vacuum advance connected? Disconnecting the vacuum advance will give you 0 deg advance. Sorry if you know this, but you didn't describe how you set the timing.

Check that choke is closed when the engine is cold and opens when hot. Clean the choke linkage if anything is sticking.

Check the hoses attached to the carburetor for cracks, loose fit to nipples, anything that may cause a vacuum leak.

Try running some Chevron Techron cleaner through the fuel system. Clean the carburetor throat. May need to rebuild the carburetor (internal vacuum leak).

Disconnect the fuel line, crank the engine, and check for sufficient fuel flow. If low, replace filter, then fuel pump if necessary.

Check the timing chain for slack if your timing is not stable. It may stretch.

Thanks Brian. I will follow your advice and check those things out.
I set the timing per the factory manual, with the vacuum disconnected at 0 degrees TDC. All the manual says beyond that is "check timing further at 12 degrees BTDC." It didn't call out a procedure for that, but I will check it at 12 degrees with the vacuum connected.
I will also check out the other things you suggested. I am leaning towards a carburetor issue, just because it is a little hiccup-y when its cold, and it has had the persistent minor backfire. Also it didn't occur to me before, but I believe there was a backfire in the carburetor at one point. I had changed the air filter, and several months ago with the air cleaner off I noticed a part of the air filter had melted. I will check it out, and hopefully find A good cause for the problem.

Vacuum leaks in older engines are by far the most likely source of problems. Hoses seem tight, but they're hard and don't hold a vacuum at the connections. Also may crack. Buy some bulk hose and replace everything attached to the carburetor or intake manifold. The leak may still be in the carburetor as you said. Your engine is known to stretch timing chains. Even if the timing is stable, if everything else fails, look to the timing chain and sprockets (damper and tensioner also). Replace them all.

Backfiring when you let off the gas is an important symptom, but I can't make heads nor tails out of it. Could be almost anything on the list. It will make sense when you find the problem.

A good course of action. Guarantee the vacuum lines are original. I am remiss that I haven't really ever checked out the carb too thoroughly, with everything else I have done to it. I have cleaned it and the linkage before with carb cleaner, it just ran so well I hated to mess with it.
I have used Techron or Marvel's in the gas before and it did seem to help performance and the backfiring. Also, it does burn a little oil, like a 1 1/2 qts every 6,000 miles. No black sooty smoke though.
I agree it could be anything, and the book leaves me kind of ambivalent.
I am going to follow your advice and start ruling things out.
BTW are there any other symptoms of a sloppy timing chain? And are there any good sources for 22R carbs besides the local box stores?
Thanks again for your help!

In your engine, it is not easy to determine if the timing chain is not stretched. They usually are to some extent. If they are moderately stretched, you can hear a tapping or rattling noise from the front of the engine on cold start-up. Toyota recommends that the chain be replaced fairly often. If it is really bad, it beats the hell out of the guide and even trashes the cover and causes a coolant leak into the engine oil. Check out the following links:

http://www.lcengineering.com/LCNewsletter/2013/OCTOBER/timingchaintech.html
http://www.yotatech.com/f116/beginners-22re-timing-chain-failure-replacement-head-gasket-replacement-212688/

As far as sources for carburetors, NAPA is my store of choice. Rockauto.com has them. If you feel up to it, the cheap way to go is with a carburetor rebuild kit.