What could cause backfire?

[hops]

Patient: 1980 Brat

 

Symptoms: When going down the highway and completely letting of the throttle to coast it backfires once. I'm talking one loud BOOOOM (gun shot) not a bunch of paapapapapapapapapap's.

 

After the engine heats up real good it's not as likeley to do it.

 

Thanks,

Klaus

[All_talk]

Hey hops

 

Crackling and popping out the exhaust on deceleration is normally a sign of a lean burn condition. The fact that it gets better after warm up would also conferm this. I’m assuming carburetor, right? With the throttle closed all the fuel comes through the idle circuit, you might try richening up the idle mix a bit. Is it all stock? If you have modified it, or removed some emissions equipment that could create the condition. I don’t know if the ’80 Subarus used a dashpot to hold the throttle plate open on high vacuum (a lot of cars did about the time), but if so you might check it for proper operation.

 

Another thought… Vacuum leak?

 

Hope this helps

Gary

[Numbchux]

My GL does this...disabling the ASV's helps (I think EA81's had them), I've come to the conclusion that it has something to do with my incredible lack of exhaust backpressure, which I think is also the reason it's got no power, I'll know more when I get my new exhaust on it. So for the meantime, I just don't make any sudden throttle changes, always very smooth, let the RPM's come down.

[northguy]

Look at the end of the exhaust for telltale signs of unspent fuel (excessive soot). You might also have a slight timing problem.

[jeffroid]

I've had exhaust leaks cause backfiring as well.

[hops]

The Brat has an EA71, weber carb, turbo muffler. The parts came off my '78 wich did not have the backfire problem.

 

The timing is right on ( checked it several times).

The valves are properly adjusted.

 

The problem is not a crackling/popping backfire.

 

The problem is one loud boom.

 

What are ASV'S?

 

Thanks,

Klaus

[archemitis]

mine sounded like a shotgun, fireball too. it was either a big hole in the exhaust just behind the first cat, or a loose timing belt.

[beauregaardhooligan]

That sounds like to *much* gas to me.

[moosens]

ASV= Air Suction Valve,I believe.

 

I've also gotten loud shotgun backfire with a hole in the exhaust and timing dead on.

[Numbchux]

yep, ASV's = Air Suction Valves, I'm not exactly sure as to their purpose, but they allow fresh air back into the exhaust, either to cool it down, or allow the cat to do a more efficient job, well, I disable mine and it turned my one HUGE shotgun shot backfire, into popping/crackling, at the worst. Also, i know I have a vacuum leak in the emissions system somewhere, the cap/rotor/plugs/wires all need replacing (but I just don't want to put any money into a motor that's going to gain donor status in less than a month), but the timing is DEAD on, and I've tried advancing/retarding it, and found that advancing it a couple degrees helped some also.

 

I hope this helps, I realize we're talking about slightly different engines here, so some of this info will need to be modified for your use, but it's a start.

[hops]

If I remember right it was also doing it with the original hitachi carb on it, but worse. If the weather holds up I will do some work to it this weekend. Hopefully I will be able to determine what causes the backfire. The exhaust gaskets are definitely something to look at and maybe replace. Airsuction valve, hmmm. How do you go about figuring out if it is malfunctioning?

 

Thanks for the great leads.

 

Klaus

[hops]

Come to think about it, I don't think I have a ASV Valve. At least I don't have anything hooked into my exhaust.

 

Klaus

[85Sub4WD]

You guys forgot about the Anti-Backfire valve on the fender wall - there is also a coasting bypass system on the 1980 engines - I could see either one being a likely cause.