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mcdragracer Samba Member
Joined: December 30, 2007 Posts: 714 Location: Cally
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Posted: Mon Jan 18, 2016 12:15 pm Post subject: 911 not firing on #4 need some help |
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I have a friends 76 911sc with 3.L, it has a new rebuilt engine, ( only has about 25 min running time on new rebuilt) #4 is not firing due to I pulled the ignition cable off the cap and doesn't altar the idle, all cylinders have 120lbs cranking compression, #4 has only 4% leak down, I swapped injectors, plug and plug wires to that cylinder, still nothing.
Spark plug is firing on that cylinder when I check outside the car, I checked the spray pattern on that injector and 2 others, they all spray the same and have all new injectors, car idles great on 5 cylinders and no external intake leaks detected.
Is there something funny about how the fuel distributor delivers fuel to a cylinder when its running, do I have maybe have a slightly tight valve when its running even though I have good leak down and compression, I'm a little stumped here.
Also what plugs do you recommend, has WR8DC in it right now.
(Has stock cis injection, K jet tronics). |
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Thorshammer Samba Member
Joined: September 08, 2009 Posts: 1 Location: Lakeville, MA
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Posted: Mon Jan 18, 2016 3:29 pm Post subject: Re: 911 not firing on #4 need some help |
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This is a good one. Remember that the resistance at the plug gap goes way down when there is no compression or mixture present (like when you check it out of the chamber) So, the only way to really check this is a 10.00 spark plug checking tool that has an adjustable gap. Set the gap to 8mm, and ground one end. If it jumps 8mm, you have enough available spark energy to fire the plug. If this is okay....
Is there anti seize on the plug threads. I have had instances where too much anti seize will cause a misfire.
Maybe tight valves, yes, it will show up at idle.
Does the pipe get hot? Do you have access to a 5 gas analyzer?
I would strongly consider an intake leak causing a vacuum leak if the spark energy is okay |
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mcdragracer Samba Member
Joined: December 30, 2007 Posts: 714 Location: Cally
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Posted: Tue Jan 19, 2016 12:39 pm Post subject: Re: 911 not firing on #4 need some help |
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I think I have one of those spark plug checking tools, i will check into that, I don't have access to a 5 gas anymore, but I do have a wide band and just welded the bung in on Sunday just haven't had a chance to start up the engine again. I have a temp gun and will check the header when its running.
I do have VERY minimal antiseize on the plug, although I swapped out with 3 other plugs and still no fire in #4, i did spray around with carb spray and didn't detect a leak, the engine does idle smooth at idle and doesn't hunt, also when it rev's up it's still on only 5 cylinders, I'm gonna try your suggestions this week and see what I come up with, I'm kinda leaning torwards maybe a slightly tight valve it's just that the it has custom headers and close to the lower valve cover, gonna be a pain to remove cover. |
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mcdragracer Samba Member
Joined: December 30, 2007 Posts: 714 Location: Cally
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Posted: Sun Jan 24, 2016 5:49 pm Post subject: Re: 911 not firing on #4 need some help |
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Update: Valves ok, spark ok, no intake leak found, fuel mixture real close.
What I did was switch lines on top of the fuel distributor and swapped #4 line over to #3 on the fuel distributor head, the problem now went to #3, #4 fires fine now so I suspecting low fuel pressure out of the fuel distributor head for that line feed, when the engine is running I crack the banjo bolt head and the fuel doesn't seem to come out as strong on that one, thus suspecting low fuel pressure out of the head on just that one.
So my question is, can that be fixed, do they make a rebuild type kit for them, I know they have a screen inside each individual head hole, I did remove the fuel distributor, removed the pin made sure it slides real smooth, I set the sensor plate per book spec's, gently blew out where the screens are, the fuel dist itself looks pretty clean, I did not split it apart just yet. |
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mcdragracer Samba Member
Joined: December 30, 2007 Posts: 714 Location: Cally
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Posted: Tue Feb 02, 2016 1:33 pm Post subject: Re: 911 not firing on #4 need some help |
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OK found the problem, it was the fuel distributor, got an exchange unit and this thing runs sweet now!!!! |
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DRACO A5OG Samba Member
Joined: September 03, 2016 Posts: 10 Location: SoCal
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Posted: Wed Sep 07, 2016 9:27 am Post subject: Re: 911 not firing on #4 need some help |
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Congrats on locating the culprit. Break a Leg! _________________ 1985 Porsche 911 Carrera Targa |
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