I just bought a subaru with an EA71, finally got it running and now it backfires when i shift, rev, go uphil or turn it off. The problem has gotten so bad that my muffler blew apart. I'm sure this has been answered before but i would appreciate any help.

I have the same issue, I turned the mix screw to lean it out as much as i could. it barely idles but it reduced the back back fire from Bazooka caliber bombs. to mere BB gun caliber pops.  I will have to live with it until I pony up the cash for a webber.  

I have two different threads on this board. First one kind of leads into the second one. The engine was running bad at the same time. Here's the links:

 

http://www.ultimatesubaru.org/forum/topic/142895-79-brat-sputter-start-issues/

http://www.ultimatesubaru.org/forum/topic/144805-ea71-backfiring/

 

For me, what ended up happening is I was misfiring one cylinder due to bad oil seals and not all the fuel was burning off, so I tore down the motor and rebuilt it. It's sitting on my engine stand right now, so I can't tell you if that got it all, but everything else on my motor looked great except the heads. So, I'm pretty confident I got it.

 

Good luck.

Edited May 29, 201411 yr by lvsarge

Backfire is the mixture igniting in the intake manifold and reversing through the carburetor. What you're describing is afterfire. A rich condition and extremely retarded (or advanced) timing can cause afterfire, since the air/fuel mixture makes it past the exhaust valve and into the exhaust system, where it is ignited by hot exhaust components. I'd say start looking for overfuelling from the carburetor (float/needle/seat, leaking jets), then check the ignition timing, paying special attention to the advance/retard functions moving the breaker plate/VR sensor correctly. Even a simple weak ignition system (at low throttle loads and high engine speed from shifting) can misfire and dump fuel in the exhaust.

Edited June 20, 201411 yr by djcommie

My 78 Brat use to do that.....so I ran a strait pipe out the side so I could watch the pretty light show.

Just a shot in the dark but I used to have an 84 that would do this. Turns out I had a small hole in my exhaust pipe before the catalytic converter. Somehow if you have a small leak in the exaust at certain points it causes a backfire situation. Mine would mostly pop when shifting gears or letting off accelerator. Scared the piss out of this one guy I backfired next to on the highway. I looked to my side when it happened, he screamed, then looked like he was going to cry, then he just pulled over to the side and put his head in his hands. Never saw him again. Got the pinhole diagnosed and welded the next day. Check for exhast leaks especially catalytic converter and forward to the engine.

second on the exhaust leak. mine used to afterfire like crazy, then I noticed the exhaust joint between the headers and the muffler was only half welded! I welded all the way around and now it runs fine. No pops or anything!

The coasting bypass vacuum modulator being misadjusted also affected mine, it would open too early, often during light deceleration, and make it afterfire bad enough to throw fireballs when the engine was cool enough to have the choke still on. Putting the factory exhaust back on mine and adjust that modulator pretty much cured mine.