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Join Date: Nov 2003
Posts: 5
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Car back firing thru intake.
Car starts after about three trys. Aslo under full throttle car runs great but under part throttle or just cruising along the car will back fire thru the intake and i'm afriad to give it gas becasue of the back firing . Car has 964 cams and is a 1978 sc! New top end. I have played with the mixture and have only made it worse. I feel it is running overly rich but not sure.
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11-18-2003, 01:59 PM
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Join Date: Apr 2003
Location: Las Cruces, NM
Posts: 2,459
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If it back fires it might have already busted the cis box, which would explain why it's still persisting. If you already installed the pop valve in the air box then try redoing the timing, might be too retarded.
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2007 Mazda 3 hatch 1972 Porsche 914 roller with plenty of holes to fix 
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11-18-2003, 02:07 PM
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If I remember correctly, if it backfires out the exhaust, it is running to rich. If it backfires thru the intake, it is too lean. The first thing that I would do is to find someone with an exhaust analyzer and make sure that the FI is set somewhere between 2% and 3.5% at idle. Once you have done that, check the distributor timing. If the problem persists, you might have a camshaft that is not timed correctly. Resetting the cams involves some dissassembly, try the easy things first! One other thing that could be causing the problem is a bad coil or plug wire. With the car in a dark place, open the engine cover and start it. Look for blue/white "lightning bolts" that may be jumping from the plug wires to a ground. Usually if an engine runs really poorly, it is the coil wire that is shorting. If it only has an occasional skip, then it is probably a plug wire.
Good Luck! Fred Cook '80 911SC coupe 
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11-18-2003, 02:43 PM
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Join Date: Jun 2003
Location: Camarillo, Ca.
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Did you check your distributor?check if it is loose or is advanced\retarded.
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Aaron. F.S. 1965 Solex engine w carbs/cleanerBurnham Performance https://www.instagram.com/burnhamperformance/ |
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11-18-2003, 03:18 PM
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Green Skull 006
Join Date: May 2002
Location: Rhode Island
Posts: 2,040
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Too ruck?
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S Reg 823 R Gruppe 246 1955 pre-A Carrera Speedster...x 1974 leichtbau..."Sascha" "It makes me sad. Our cars were meant to be driven, not polished" - Ferry Porsche while surveying a PCA Parade concours field. |
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11-18-2003, 03:35 PM
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Join Date: Oct 2003
Posts: 3,019
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If you can see any arcing from plug wires at all should they be replaced? A couple of mine spark a little every now & then- would magnecores be a good idea?
thanks |
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11-18-2003, 04:13 PM
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Car was running okay just when cold it was popping thru intake then I richened it up and it got worse instead of better. Now it is back firing more and is eating gasoline like mad! Also when not on gas the car was very sluggish so I thought that maybe it needed to be richened up alittle. Now it is alot worse and is hard starting takes three turns of key to run and under anything other then full throttle pops out the intake like crazy.
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11-18-2003, 04:25 PM
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Senior Member
Join Date: Feb 2000
Location: Lacey, WA. USA
Posts: 25,310
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Intake popping is often the result of a lean running condition. If you richened it and made the problem worse, then I'd guess you have an intake leak ("vacuum" leak). A blown air box would be an example of this. Intake popping in an air box that has no popoff valve would blow the air box.
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Man of Carbon Fiber (stronger than steel) Mocha 1978 911SC. "Coco" |
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11-18-2003, 05:07 PM
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Nah Jim, itta cayar, notta pikkemup tooruck.
 Fred and Sup gave good starting points, do 'murican 78s have OXS (brain fade, otter know that one) ? |
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11-18-2003, 05:10 PM
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Zeus (god of thunder and lightning)
