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Item Posts    Sort Order

Cylender #7 not firing

 
Shawns66Mustang
New User
Posts: 28
Joined: 01/08
Posted: 12/03/13 08:27 AM

I built a 302 to put in my 66 couple and have had nothing but problems with it. Recently using a temp gun I discovered that cylinder #7 isn’t firing. The header tuber on that cylinder is 170 degree and all the others are 650 degrees. I’m getting spark but changed the cap and rotor, swapped the wires and plugs, took the valves apart to make sure they weren’t binding up, adjusted the hydraulic lifter several times, and did a leak down test and a compression test which was at 140 same as the rest. I can see the valves moving up and down so I haven’t lost the cam totally.
Any ideas before I start tearing it apart???  

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MustangMan
Moderator
Posts: 261
Joined: 08/11
Posted: 12/03/13 11:57 AM

If your mechanical inspection shows no issues I'd lean towards no ignition to that cylinder. Check your distributor cap, rotor, and wires. It is possible the terminal in the cap is broken or missing or the wire is bad. Try swapping the wire (and the plug, one at a time) to another cylinder and see if the low temps follow the part or stay with cylinder #7.

HTH...
Mark
.  
Mark Houlahan
Brand Manager
Mustang Monthly Magazine

1965 FFR Roadster 427W/TKO-600 5-speed
1966 Mustang 289-4V/C4 Auto
1968 Mustang 4.6L Three-Valve/5R55S Auto
1990 Mustang 306/AOD
2002 Explorer Sport Trac 4.0L V-6/5R55 Auto
2005 F-150 4x2 4.2L V-6/4R70W Auto

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wayne712222
User
Posts: 127
Joined: 10/13
Posted: 12/03/13 12:50 PM

what year.. and what model 302.. or actually which cam..

there are 2 different cams for 302 motors if you built a late model..

one for EFI 5.0 and one for the HO 5.0

these have different firing orders... ground in..

the crank is the same..  i think there is a 4/7 swap..

i am also taking that you have a carb on the engine...

could you have a massive vacuum leak...

lastly.. there is an issue with the firing order of the last 2 cylinders on the drivers side on some motors... as they fire 90 degrees of crank rotation apart.

the wires need to be reclipped into the spark plug wire holders...

where it comes out of the distributor cap and over the valve cover.. the order on the drivers side needs to be in the clip..

7,5,6,8  this moves the wires apart so it does not cross fire..

what kind of ignition system do you have???

points???

electronic conversion..  

duraspark or other electronic ignition...

do you own a spark tester   gap type.. put it at the end of the #7 spark plug wire and see if you are getting spark there..

you can also OHM the spark plug wire from the inside of the terminal in the cap.. to the inside of the boot at the spark plug end... set digital volt meter to 200K ohms..  200,000..  read the wire resistance directly..  1,000 ohms per inch max..
so you should have a number on the meter smaller than the length of the spark plug wire in inches..   that wire is probably 20 inches long..  so you should have a reading less than 20 on the display.. probably 12 to 15.. as 20 is right at the edge of tossing it out..

if you have a fuel injected motor. you could also have similar issues or a clogged injector. broken wire.. loose connector..

so a little more info about your set up if its not a carb...

and do you have power brakes.. or a PCV that flows into the rear passage.. a truly massive vacuum leak might cause this also...

does it misfire at 2500 rpms.. or have a low temp on that header tube..

i had a forum member on a different forum that aimed at the HOT part of the tube. threw off his whole diagnosis..  thought he had a hot cylinder..  when he measured a different part of the tube. they all were the same temp..

not saying you are off.. but i would like more info.. and more testing...  

Post Reply
wayne712222
User
Posts: 127
Joined: 10/13
Posted: 12/03/13 12:57 PM

wow.. i just looked.. its a 4 wire swap...

50 Firing Orders Zpsf39900ca

i am not saying its your issue..  just wow...  

Post Reply
Shawns66Mustang
New User
Posts: 28
Joined: 01/08
Posted: 12/03/13 04:31 PM

I’ve replaced the cap & rotor and swapped wires and plugs, no change.
My motor is a 1971 302 with an Edebrock cam, heads, intake and carb, with a MSD 6A ignition system and distributor. I’ve checked the cam/crank firing order and I’m sure I have the 1-5-4-2-6-3-7-8.
I’m getting spark but it would appear that I’m not getting fuel and air.  

Post Reply
shyrgfuh3
New User
Posts: 25
Joined: 11/13
Posted: 12/03/13 05:44 PM

i would do this.

1. if you have a pcv valve plug it at the carb.

if it is connected to the rear of intake instead of carb then connect it to carb after car runs properly and not before.

2. if not fixed check your plug wire firing order again and again and again.

3. if not fixed switch to the 351 firing order.

4. if worse switch firing order back, then remove carb and stick a coat hanger down the port to see if there is a rag stuffed in it.

5. if no rag then plug ALL vacuum at the carb.

lower idle.

spray all over GENTLY with flammable brake cleaner using long nozzle especially around the port that feeds that cylinder where it meets the head. if the rpm increases there is a leak. if it does not increase the rpm then it may have a leak in that runner on the bottom side.

if no leaK IS FOUND THEN MOVE ALL THE WIRES on distributor and move them OVER 1 POSITION AND RETIME ENGINE TO 10 DEGREES BEFORE TOP DEAD CENTER WITH DISTRIBUTOR VACUUM DISCONNECTED.

check firing order again and again.

If not fixed turn both idle screws in 1 turn increments to see if both have an effect on idle. of one dies not you have an air leak or more likely the carb idle circuit is plugged on that side.

the drivers side screw operates the passenger side cylinders.  

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shyrgfuh3
New User
Posts: 25
Joined: 11/13
Posted: 12/03/13 05:45 PM

double post  

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wayne712222
User
Posts: 127
Joined: 10/13
Posted: 12/03/13 07:43 PM

hi bob.. i would like to know what the working end of the spark plug looks like..

is it wet with unburned gasoline...

is it black and sooted with slightly burned gasoline..

is is black and oily.. where that cylinder is not firing at all and pumping oil into the chamber on the what would be the power stroke. that would be oil fouling the spark plug.

3/4's of a turn after compression at TDC.. when the spark plug is out.. and you blow compressed air into the spark plug hole.. does it come out the tail pipe freely .. showing that there is not some blockage or crushed individual header tube..


timing light hooked over the coil wire.. as i don't expect anybody but bob and i to have a digital scope to look at ignition patterns..

does the timing light flash without any obvious dark times..   if one of the legs of the reluctor is broken or cracked.. you may loose spark on that one cylinder where it lines up.. and hence the reason for bob to ask you to turn the distributor and realign the spark plug wires in the cap..  so that  its firing on a different pole piece in the distributor..


does your cell phone take video.. take a movie of the engine as you operated it up from idle to about a steady 3,500 rpm and back to idle several times.. as seeing and hearing really makes it a lot easier to diagnose...

walk the camera around the engine compartment so we have a guided tour


post it to photobucket   and the links here..  

Post Reply
shyrgfuh3
New User
Posts: 25
Joined: 11/13
Posted: 12/04/13 05:15 AM

yup, yup and yup

who knows, maybe a rat climbed in your tail pipe and got stuck in the tube. good suggestion on the ex tube check.

you have a strange prob but in the end it will be something simple.  

Post Reply
Shawns66Mustang
New User
Posts: 28
Joined: 01/08
Posted: 12/05/13 08:19 AM

Up date.
Ok my PVC valve was going to the intake manifold so I blocked it off for now and started it up. The #7 header tube is now 430 degrees. I don’t understand why that would create suck a big problem?
The tubes on the driver’s side are 430 to 525 degrees and but the passenger side is 500 to 580 degrees. I guess I have one side richer than the other?  

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shyrgfuh3
New User
Posts: 25
Joined: 11/13
Posted: 12/05/13 11:30 AM

Ok my PVC valve was going to the intake manifold so I blocked it off for now and started it up. The #7 header tube is now 430 degrees.


ok, good you're now fixed.


xxxxx


I don’t understand why that would create suck a big problem?


lol, few people do which is why they connect their pcv to the intake and not the carb.

your pcv leaks air all the time.

this means the cylinder it is closest to will run leaner than the rest because that cylinder is not only sucking air and gas through the carb it is now also sucking air with no gas from your crankcase. this causes the mixture to lean out.

in most cases it will not lean a cyl out so much that it will not fire. this means that it is extremely likely that your pcv is also sucking a ton of air and possibly more than it should be.

once the pcv is connected to the carb, pull it put and plug it. if the rpm drops more than 100 between being in the eng and being plugged it is either bad or too big and you must get a smaller flow one. you can buy originals on ebay. used is usually ok.


xxxxx


the tubes on the driver’s side are 430 to 525 degrees and but the passenger side is 500 to 580 degrees. I guess I have one side richer than the other?


possibly, just try turning out the fuel mix screw 1/2 turn on the drivers side, yes the drivers side of carb is for the passenger side of eng as i mentioned above.

please send my complimentary set of faux wood handled steak knives at your earliest convenience.  

Post Reply
wayne712222
User
Posts: 127
Joined: 10/13
Posted: 12/05/13 01:08 PM

cool....  if you have an edelbrock carb.. AFB or AVS... there is a 3/8 nipple on the front of the carb for the PCV too hook up to..

with the PCV hooked back up and the idle speed reset to compensate for it..

do you own a vacuum gauge..  if so.. get it out...

hook it up to manifold vacuum.. warm the engine to full operating temp..

at idle.. is the idle speed good..???not to fast..

start turning the idle mixture screws in one at a time till you see the vacuum start to drop .. or the engine starts to stumble..  
then turn it out until you reach the highest vacuum..  

slowly turn it in till you see the vacuum start to drop.. take it back to the exact spot it starts to drop at..

do the other screw..  till you see the vacuum or idle speed start to drop.. back it out to get to the highest vacuum reading..  turn that screw in slowly until the vacuum starts to drop.. back it out to the spot where it just starts to drop

reset the idle speed . if its too high..  using the idle speed screw that touches the lever..

go to the first screw and go backwards and in again till it starts to drop..
get both screws to the spot where it just start to drop.. then turn them in a fraction of a turn till the vacuum drops 1 inch of vacuum for the adjustment of both..

this is LEAN best idle mixture.. and should get you the best fuel economy and you should be able to idle without any fouling of the spark plugs..


if you find that the idle mixture screws on one side or both sides  don't seem to adjust properly.. changing the vacuum readings.. you could have some clogged passages...  follow below.

do you also have access to compressed air.. ??

while counting turn the idle screws in till they just seat lightly.. don't MASH THEM in..  how many turns out are they.???   remove them and the springs.. do this one at a time.....  spray some carb cleaner into the hole you took the idle mixture screw out of.. then follow it with a blast of compressed air.. give it another shot of carb cleaner and another blast of compressed air..  put the screw back in and set it to the same number of counted you counted..

do the other screw...  

warning.. you will have to hold the throttle open to get the engine to start..  as you will have added too much carb cleaner.. it only takes a second or two of carb cleaner.. and try not to let it over spray onto the finish..

this should clean out any debris or dirt that MIGHT have gotten into the idle passages..   go back to the top and see if you can adjust the idle mixture .  

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