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Registered:
October 2003
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backfire when letting off throttle
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Sun, 26 September 2004 10:32
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the engines a 13t with twin 40 sidedraft mikuni's, jetted at 140 main and 200 air. the backfiring occurs when i let off the gas and can't figure out what's wrong. leaning out the idle screws a little lessens the occurence but does not completely remove it? any ideas? timings at 15 deg btdc, and the vacuum advances are plugged up
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Location:
Baulko Hillo
Registered:
April 2004
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Re: backfire when letting off throttle
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Sun, 26 September 2004 10:40
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I'm just throwing up an idea here and perhaps someone can answer this, but could a large buildup of carbon on the valves cause something like this? I mean, it would get very hot and could ignite the fule couldn't it?
Feel free to completely shoot me down in flames if this is ridiculous..... 
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Location:
Land of Oz
Registered:
June 2004
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Re: backfire when letting off throttle
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Sun, 26 September 2004 10:48
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Hi,
There was some discussion about this on another forum a few weeks ago. I think it the car was running very rich or something like that. Can't remeber now.
However, since my engine has been tuned and all fixed up, it gurgles when decellerating - moreso, if you're using the engine as a brake. I love it. It used to do that years ago, then faded away as the carbies leaked and were badly in need of overhaul - but the gurgle is back! 
| AE86slut wrote on Sun, 26 September 2004 20:40 |
Feel free to completely shoot me down in flames if this is ridiculous.....
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Hmmm.. I'd just like to shoot you down for the hell of it, but can't think of any devastingly witty repartee, so I guess I'll let you go this time. 
seeyuzz
river (with the gurgling 18RG)
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Location:
Baulko Hillo
Registered:
April 2004
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Re: backfire when letting off throttle
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Sun, 26 September 2004 10:59
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Your mercy has been noted River.....
I guess I don't mean it as a reason for the above problem, but is it possible?
As for the actual problem, running very rich would prolly rate pretty highly on the list of causes.
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Location:
Land of Oz
Registered:
June 2004
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Re: backfire when letting off throttle
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Sun, 26 September 2004 11:06
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Hi,
Well a full-on backfire is unburnt fuel being ignited in the exhasut system, isn't it? So the two things you need for this is unburnt fuel and some form of ignitor.
Running rich would give you the unburnt fuel. Bits of hot coke/carbon deposits and other shit from the engine could supply the ignition. But, if that's the case, wouldn't a nice damn hard drive clean out some of the deposits? Then a tune-up on the carbs help with unburnt fuel?
I'm not sure, but I wonder if poor timing could also be a factor?
seeyuzz
river
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Location:
Dandenong Ranges, VIC
Registered:
July 2004
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Re: backfire when letting off throttle
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Sun, 26 September 2004 11:31
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cant you get fuel additives that clean all the shit off the back of your valves? like some of the Nulon or Wynns stuff? never tryed it myself but its gotta be worth a shot for like $5 a bottle...
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Location:
Adelaide
Registered:
August 2004
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Re: backfire when letting off throttle
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Sun, 26 September 2004 11:43
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The old STP additive is the way to go,
Idle the engine until warm, remove spark plugs, pour about 25ml stp into each cylinder and let sit for twenty minutes,
spark plugs back in, fire up, lots and lots and lots of white smoke, slowly trickle more stp down carb, very hard to keep enging running while u are doing this but persist.
When youve got half a bottle of stp left, pour it into fuel tank.
Keep idling until smoke clears, change oil and spark plugs.
Should be like new!!
They used to give this stuff away at the pump in the 60's
[Updated on: Sun, 26 September 2004 12:21]
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