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Unread 06-23-2013, 01:59 PM   #1
brtncstm160
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1975 FSJ Cherokee 
 
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Engine stumble/exhaust backfire and muffler blowout

So i recently did a cam and lifter replacement and everything has been going fine for the last 200 miles. Over the last two weeks however when driving home from work the engine will start to stumble as if its dying as im driving down the road, its happened once. Last night though it stumbled once then backfired really really loud out the exhaust pipe. Pulled over checked under the hood everything seemed fine. Kept going down the road and started to stumble again then blew out the muffler. Split it wide open. Engine is timed to 10 BTDC. Am i having a fuel mixture problem causing the backfire? Could the engine stumble be a short somewhere? Or is the backfire linked to the engine stumbling? Thanks for any answers and advice.

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Unread 06-23-2013, 02:56 PM   #2
RockRollin
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Not sure if a 75 had a cat originaly, could be a suspect. I had a similar experience with a 77 Delta 88, turns out the cat was partially blocked due to rich condition, finally built up enough heat and BOOM. Gutted the cat and split the muffler. FWIW
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Unread 06-23-2013, 04:04 PM   #3
super98lsc
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fuel is pooling in the exhaust then igniting, check mixtures read all 8 plugs to see if one looks odd then check compression.
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Unread 06-23-2013, 04:06 PM   #4
super98lsc
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also what ignition are you running? Could be the stumble is a loss of spark which will load up the motor with fuel then BAM the spark returns or the heat in the manifolds lights off the raw fuel.
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06-Corvette z51-6spd,b&b,K&N low miles.
77-Corvette Stingray 496 BBC/TH400 built 667hp+
===================================
00 WJ 4x4, 4.7 stroker, turbocharged,..lots more..
01 WJ 4x2 4.7v8 Limited (needs valve job)
90 Grand Wagoneer 401 Edelbrock Heads, Lunati .510/.527 FT Cam, Edelbrock RPM Air Gap, Holley HP 750, Thorley Headers, HEI, Electric Fan w Digital controller,4" lift 31x10.50 Goodyear Duratrac's on Pro-Comp Wheels,new interior, seats,seals,Alpine and JL full system, Remote HEI, Clifford 2 way with remote start,windows,tailgate.
96 Cherokee Country 4dr/2x2 White/Tan 3" Lift Sahara 18" Wheels 32.5"'s 99up head and intake- for sale soon!
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Unread 06-25-2013, 02:07 PM   #5
brtncstm160
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i did a cdi upgrade when i rebuilt the engine about 3 months ago. Hasnt been any rain and hasnt been driven in any water since the rebuild. It definitely was the original muffler dont think this one has a cat. To me it sounds like what youre describing with the loss of spark.
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Unread 06-25-2013, 03:50 PM   #6
super98lsc
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Could be a faulty cdi unit or coil,
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06-Corvette z51-6spd,b&b,K&N low miles.
77-Corvette Stingray 496 BBC/TH400 built 667hp+
===================================
00 WJ 4x4, 4.7 stroker, turbocharged,..lots more..
01 WJ 4x2 4.7v8 Limited (needs valve job)
90 Grand Wagoneer 401 Edelbrock Heads, Lunati .510/.527 FT Cam, Edelbrock RPM Air Gap, Holley HP 750, Thorley Headers, HEI, Electric Fan w Digital controller,4" lift 31x10.50 Goodyear Duratrac's on Pro-Comp Wheels,new interior, seats,seals,Alpine and JL full system, Remote HEI, Clifford 2 way with remote start,windows,tailgate.
96 Cherokee Country 4dr/2x2 White/Tan 3" Lift Sahara 18" Wheels 32.5"'s 99up head and intake- for sale soon!
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Unread 06-26-2013, 12:00 PM   #7
brtncstm160
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sounds about par for the course, ive replaced the coil and module once already.
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Unread 06-26-2013, 06:47 PM   #8
brtncstm160
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Not sure how it happened but it seems the ignition coil mount was super loose so it was bouncing all over the place. possible cause? also does anyone know if its possible to get a replacement "resistance block"? The little thing that goes inline before the ignition coil on the positive side. Mine is quite old and the cables are quite loose. i feel this could be an issue as well.
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Unread 06-27-2013, 05:27 PM   #9
olliehopnoodle
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The coil does need to be grounded, so yes that could contribute.
I don't know what a 'resistance block' is. I would need to see a picture.
What coil did you use? If you used an internal resistance coil and you still have your resistance wire that can cause issues too.
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Unread 06-27-2013, 06:13 PM   #10
brtncstm160
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The white block in front of the ignition is what i was referring to as a resistance block. It was on there before i did the upgrade and im pretty sure i read somewhere to have that before the coil to not burn it out or something. Not sure about what coil it is just a normal 12v canister style thats as much as i know.

EDIT: The upload of my photo keeps failing. am i to new of a member to post photos?
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Unread 06-27-2013, 06:20 PM   #11
brtncstm160
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https://www.google.com/url?sa=i&rct=...72465047714552

i guess its just called an external resistor. but the points on mine are quite loose any chance that could be causing the faulty ignition?
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Unread 06-27-2013, 07:58 PM   #12
olliehopnoodle
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What do you mean the points are loose? No brand name or part number for the coil?
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Unread 06-27-2013, 10:21 PM   #13
super98lsc
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the white block is the ballast resistor that old points/condenser systems used. Like ollie said shoot us some photos and part numbers from the coil, cdi unit, cap and distributor if that ballast resistor is shorting to ground it will kill spark among other things.
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06-Corvette z51-6spd,b&b,K&N low miles.
77-Corvette Stingray 496 BBC/TH400 built 667hp+
===================================
00 WJ 4x4, 4.7 stroker, turbocharged,..lots more..
01 WJ 4x2 4.7v8 Limited (needs valve job)
90 Grand Wagoneer 401 Edelbrock Heads, Lunati .510/.527 FT Cam, Edelbrock RPM Air Gap, Holley HP 750, Thorley Headers, HEI, Electric Fan w Digital controller,4" lift 31x10.50 Goodyear Duratrac's on Pro-Comp Wheels,new interior, seats,seals,Alpine and JL full system, Remote HEI, Clifford 2 way with remote start,windows,tailgate.
96 Cherokee Country 4dr/2x2 White/Tan 3" Lift Sahara 18" Wheels 32.5"'s 99up head and intake- for sale soon!
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