Can’t find cause of Missfire 😡😡
#2461489
03/05/18 02:50 PM
03/05/18 02:50 PM
|
Joined: Nov 2011
Posts: 653 montana
pjc360
OP
mopar
|
OP
mopar
Joined: Nov 2011
Posts: 653
montana
|
As many of you already know I’ve been battling a nasty missfire on my 300hp crate 360 magnum for months now and I can’t for the life of me figurenout what’s causing it. Engine is low mileage, 30k miles, my problem cylinders appear to be cylinder 6 and cylinder 8. Cylinder 6 is almost always carbon fouled, with what looks like dry sooty residue like what you’d see on the inside of a chimney. I get 155 psi of cranking compression on all 8 cylinders with engine warm and carburetor wide open. I thought it was ignition at first, ive put a brand new firecore 50 electronic vacuum advance distributor in it, new cap and rotor, new Taylor plug wires, new ngk bkr5e plugs. New MSD Digital 6 ignition box, coil wires and trigger wires routed on opposite sides of the truck. Pick up coil wiring is the correct polarity, I’ve triple checked that. Timing is 18 initial and 34 total all in at 2500 rpm. I’ve verified the accuracy of my balancer with a piston stop, it’s dead on. I’ve checked timing with two different lights, both are the same. I’m running a brand new crane cams lx91 e-core coil. I’be pulled the valve cover on passenger side, rocker arms and valve springs and push rods appear to be fine, nothing is loose or broken. I’ve watched the valve train with engine idling, everything looks normal. I’m at a complete loss here. The missfiring starts around 2400-2500 rpm in every gear, even in nuetral. I can get it to clear up a tad bit on the highway if I stand on it pretty hard and then back off a little bit I think that’s because the plug in cylinder 6 gets cleaned off a little bit and starts firing better? Idk why it will clear up a little sometimes when you poor the coals to it. Carburetor is a brand new Quickfuel super street series 680cfm vacuum secondary carb. Intake is the Mopar Dual plane M1. Could this be a fuel issue? Is it getting too much fuel and missfiring because of it? Below 2500 rpm it runs super good and feels completely fine. It isles nice and has 20hg of vacuum at idle. If this is a fuel issue, why is it only affecting cylinder 6 mainly and sometimes cylinder 8? I mean I believe it could be a fuel issue at this point, I just don’t understand why only 6 and sometimes 8 are being affected. I don’t think my carb settings are all that crazy rich ether. I live in Montana so I’m 3500 feet above sea level. I’m running 65 main jets and 75 secondary jets, I’m running .031 idle feed restrictors and .065 idle air bleeds, I’m running .033 high speed air bleeds, a 9.5hg power valve with .049 pvcr’s. I have the floats set at the bottom of the sight glasses, all 4 idle mixture screws are about 3/4- 1 full turn out from seated. I get 6 psi of fuel pressure from my mechanical fuel pump. I’m just at a loss here and have no idea what’s going on or how to pin point what’s going on. Should I step down a couple notches on the idle feed restrictors and step down on the jetting a little bit and see what happens? Last night I lowered my floats from being half way up the sight glasses to the bottom and that appeared to slightly make it a little better but it’s still missing at those rpm’s.
|
|
|
Re: Can’t find cause of Missfire 😡😡
[Re: 71birdJ68]
#2461532
03/05/18 04:07 PM
03/05/18 04:07 PM
|
Joined: Aug 2006
Posts: 2,505 God's Country Maryland
GODSCOUNTRY340
master
|
master
Joined: Aug 2006
Posts: 2,505
God's Country Maryland
|
You've covered just about everything except the valve seals. Pop the valve springs off number six and replace both intake and exhaust valve seals. While you have the springs off wiggle the valve stems and see if there's too much play there. They should move just enough to feel, if there is a lot of side movement in them that's your problem.
I love the smell of Deer guts in the morning, it smells like... VICTORY!
|
|
|
Re: Can’t find cause of Missfire 😡😡
[Re: pjc360]
#2461543
03/05/18 04:29 PM
03/05/18 04:29 PM
|
Joined: Jul 2011
Posts: 956 Chicago
PurpleBeeper
super stock
|
super stock
Joined: Jul 2011
Posts: 956
Chicago
|
Hmmm... these are good things to check, but an intake leak would either suck oil (oily plugs) or air (white plugs). Valve stem seal/guide would suck oil (oily plugs). Chimney soot sounds rich for sure.
Too much fuel or too little air.....EGR problem? "Maybe" lean the carb out a little bit...maybe #6/#8 are just more prone to showing a rich condition? Really, really dirty air filter? Cam not degreed & again #6/#8 are more prone to the rich issue.
I'm grasping at straws here obviously. If you've done all the ignition stuff, then the fuel delivery system is the next place to look.
70 Roadrunner convt. street car
440+6, NOS, 4-spd, SS springs
'96 Mustang GT convt. street car
'04 4.6 SOHC, NOS, auto, lowered
"Officer, that button is for short on-ramps"
|
|
|
Re: Can’t find cause of Missfire 😡😡
[Re: justinp61]
#2461581
03/05/18 05:43 PM
03/05/18 05:43 PM
|
Joined: Jan 2003
Posts: 2,094 central texas
krautrock
top fuel
|
top fuel
Joined: Jan 2003
Posts: 2,094
central texas
|
If that doesn't help buy/borrow a wide band O2 meter and get the carb dialed in. It sounds like it could be going lean causing the misfire.
same thing i've been thinking. the carb is already a couple steps lean on jetting from baseline. the transition is pretty rich too. sounds like the miss is where the mains start to come in. i would put some 70 jets in and go drive it.
Last edited by krautrock; 03/05/18 05:44 PM.
|
|
|
Re: Can’t find cause of Missfire 😡😡
[Re: pjc360]
#2461603
03/05/18 06:24 PM
03/05/18 06:24 PM
|
Joined: Jan 2003
Posts: 1,043 Slidell, La.
doctor_mopar
super stock
|
super stock
Joined: Jan 2003
Posts: 1,043
Slidell, La.
|
With all you say, I am thinking it must be a problem/ defect with the carb going rich on the secondary venturi that is on the side that is nearest feeding 6 and 8 . If you have a dual plane intake , just follow the runners. I am just throwing it out there where I would be looking at this point !
------------------------ It doesn't matter what you do.........As long as you look good doing it !
|
|
|
Re: Can’t find cause of Missfire 😡😡
[Re: pjc360]
#2461800
03/06/18 12:51 AM
03/06/18 12:51 AM
|
Joined: Aug 2011
Posts: 2,857 Pattison Texas
CSK
master
|
master
Joined: Aug 2011
Posts: 2,857
Pattison Texas
|
weak valve springs, they cant keep up with rpm
Last edited by csk; 03/06/18 12:51 AM.
1968 Charger COLD A/C Hilborn EFI 512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim 2020 T/A 392 Stock 11.79 @ 114.5
|
|
|
Re: Can’t find cause of Missfire 😡😡
[Re: CSK]
#2461816
03/06/18 01:11 AM
03/06/18 01:11 AM
|
Joined: Aug 2006
Posts: 2,505 God's Country Maryland
GODSCOUNTRY340
master
|
master
Joined: Aug 2006
Posts: 2,505
God's Country Maryland
|
If you're sure it's rich try a different carb and see if it clears up. Even though that is a new carb it could be defective internally causing it to be rich in those two cylinders.
I love the smell of Deer guts in the morning, it smells like... VICTORY!
|
|
|
Re: Can’t find cause of Missfire 😡😡
[Re: justinp61]
#2461865
03/06/18 02:50 AM
03/06/18 02:50 AM
|
Joined: Aug 2003
Posts: 954 garnett kansas
rhad
super stock
|
super stock
Joined: Aug 2003
Posts: 954
garnett kansas
|
just throwing out a wild idea,some kind of casting problem with the intake causing the cylinders next to each other to misfire/run rich,like maybe not enough air because of a internal restriction,yeah that probably not it,but it sounds like the normal stuff aint it so the oddball thing might be!
my ladder of success is missing some rungs
|
|
|
Re: Can’t find cause of Missfire 😡😡
[Re: justinp61]
#2461986
03/06/18 01:40 PM
03/06/18 01:40 PM
|
Joined: Feb 2003
Posts: 16,146 Mesa, Arizona
dart4forte
I Live Here
|
I Live Here
Joined: Feb 2003
Posts: 16,146
Mesa, Arizona
|
Set the rev limiter on you Digital 6 to 0. My 408 had a problem with a pop on the hit and immediately after gear changes at the drag strip, it wouldn't do it on the street. I changed plugs, wires, distributor pick up, coil, valve springs and was in the carb several times. Finally at the suggestion of a member here I reset the limiter on my Digital 6 to 0 and the pop went away. The limiter was set at 7200 and I was shifting at 6400, it would pop even with the limiter set at 9900.
If that doesn't help buy/borrow a wide band O2 meter and get the carb dialed in. It sounds like it could be going lean causing the misfire. Th rev limiter ha a been a problem on some of the digital boxes. You may want to change out the box.
“So if it’s on the internet it must be true”
Abe Lincoln
|
|
|
Re: Can’t find cause of Missfire 😡😡
[Re: DAYCLONA]
#2461995
03/06/18 02:00 PM
03/06/18 02:00 PM
|
Joined: Jan 2003
Posts: 1,043 Slidell, La.
doctor_mopar
super stock
|
super stock
Joined: Jan 2003
Posts: 1,043
Slidell, La.
|
Dayclona really has a very likely answer there ! Even though I have known the firing order by memory since I was 12 years old, I have mixed up two plug wires like that, and the car ran a lot like that ! Sure deserves a double check !
------------------------ It doesn't matter what you do.........As long as you look good doing it !
|
|
|
|
|