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Slight backfiring after setting timing to "manual" specs


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5 replies to this topic

#1 211

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Posted 14 December 2005 - 02:42 PM

So after getting my motor back together I decided to hit it with a timing light.

According to the book it should be 8* BTDC @ 800 RPM (or something like that).
My idle was already high (1100) so I turned it down to spec which is 800-900. Also, my timing was sitting at about 12* BTDC.

So now with my timing set to 8* (+/- 2*), I notice it backfires just slightly. Only happens when I "stab" the throttle and coast for a bit. Doesn't sound loud at all, in fact it's a really quiet, almost rattling tone; sounds a bit like when you kick up road sand and it bounces off your wheel well.
Regardless it's backfiring now and it didn't so much before.

Does it make sense that it'd backfire being 4 degrees advanced? Or is this now a carbeurator thing now that the timing is "correct".

160k mi (almost), I always thought you were allowed a few degrees advancement with higermileage vehicles to compensate for wear and timing belt stretch, things like that.

#2 jacobs

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Posted 14 December 2005 - 04:28 PM

I think you'll find the backfiring is due to the A.I.R. part of the emissions system. My Subaru has backfired for about 150,000 miles now and I just ignore it although I can stop it by blocking off the hoses feeding fresh air into the exhaust.

#3 Subarian

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Posted 14 December 2005 - 04:34 PM

I run 4 degrees additional initial advance (12 BTDC) and I haven't had any backfiring issues. I agree that it's probably not related to timing.I have removed the AIR from my engine. You also should have an anti-backfire valve. Make sure the vacuum to it is connected. It's probably located on the passenger side in front of the wheel well.

#4 211

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Posted 14 December 2005 - 05:18 PM

I run 4 degrees additional initial advance (12 BTDC) and I haven't had any backfiring issues. I agree that it's probably not related to timing.I have removed the AIR from my engine. You also should have an anti-backfire valve. Make sure the vacuum to it is connected. It's probably located on the passenger side in front of the wheel well.


Kay, bare with me, what's the AIR and what is it's primary purpose? Will I notice any positive affects if I plug it off? Also, as far as vacuum lines and anti-backfire valves; can you be a little more specific? If I were to stretch the vac lines from this car end to end, I could reach the Subaru Manufacturer from here (it's a rat nest).
Is the valve part of the manifold or connect to the manifold in anyway? You said pass side, in front of the wheel well. That's not the big black plasic canister next to the radiator with 3 or 4 hoses coming out the top is it?

#5 Subarian

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Posted 14 December 2005 - 05:33 PM

I know, the vacuum lines are a pain. But on the passenger side, mounted on the inner fender well and in front of the wheel well, there's a device with a couple of vacuum lines running to it. I can't remember exactly how it's supposed to work, since I replaced my stock carb a long time ago, but its purpose is to prevent backfiring when you shift.

The AIR system introduces fresh air into the exhaust ahead of the catalytic convertor to help it burn off hydrocarbons. On each head, next to the exhaust port, there is a tube that's about an inch in diameter. These run up to reed valves fed through the air cleaner. Sometimes those reed valves go bad.

On mine, I just blocked those ports off. I used a nickel (maybe it was a quarter) under the flange and bolted it tight. I cut off the tubing, because there was no where for it to go with my aftermarket carb, but you could try blocking it off that way and see if it fixes the problem. If not, just unbolt it and remove the coin.

Oh, and the canister is your evaporative canister. It takes vapors from the fuel system and recycles them into the intake. That's not going to have anything to do with backfiring.

#6 Subarian

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Posted 16 December 2005 - 12:20 PM

Did you ever figure out the problem? I took a pic of my anti-backfire valve. It's not in the original location, because I'm using an air filter box from a fuel injected motor. Normally it would sit right where the filter box is.

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