72 base 350 rough idle/miss
07-05-2018, 10:14 PM
#1
Instructor
Thread Starter
72 base 350 rough idle/miss
So I’ve been chasing a miss and or rough idle on my car. I’ve change out nearly everything and it won’t go away. New plugs, wires, coil, rebuilt distributor, rebuilt carb, new full stock exhaust system. Car is all stock. What is interesting is that this seems most noticeable out of the passenger side exhaust. If I pull the number 4 plug wire it seems to go away. Compression test and leak down are fine. Car is set to idle at 700 in park and 550/600 in drive. The miss is very random. Also I did the Seafoam treatment, half in the tank and half sucked thru the vacuum line thinking I may have sticking valves. This car sat for over ten years before I ended up with it. Any advice would be appreciated.
Last edited by DarrenX33; 07-05-2018 at 10:17 PM. Reason: ADDING content
07-05-2018, 11:39 PM
#2
Racer
Next thing is to start looking for vacuum leaks. Start by plugging lines from the carb and manifold. Also get some carb cleaner spray, and watch for rpm increases when sprayed at the base of the carb, vacuum fittings, etc.
07-07-2018, 12:56 AM
#3
Instructor
Thread Starter
So doing that seems to have pointed me to a leak between the master cylinder and the booster. Also something going on when the PCV is connected. But with all lines disconnected I still get a stumble when I got it at 1500 rpm. And the exhaust smells like it does when you have pre ignition.
07-07-2018, 02:44 PM
#5
Dr. Detroit
Member Since: Mar 2012
Location: New Braunfels Texas
Posts: 9,963
Received 3,892 Likes
on
2,564 Posts
Stick a vacuum gauge on it........does the needle bounce? Is it steady?
Jebby
Jebby
07-15-2018, 09:36 PM
#6
Instructor
Thread Starter
Vacuum gauge bounces a bit between 18 and 19.
I also found a bad front seal in my brake booster.. Just fixed that.
Here's the plugs....
I also found a bad front seal in my brake booster.. Just fixed that.
Here's the plugs....
Last edited by DarrenX33; 07-15-2018 at 09:48 PM. Reason: ADDING content
07-16-2018, 02:23 AM
#7
Team Owner
Do you have a timing light with an inductive pickup which clamps over a plug wire? If so, just connect the timing light to alternator terminals (+12 vdc and ground) and to the coil. Clamp the induction pickup on a plug wire, pull the trigger and watch the light pulses. A firing "miss" will result in no light pulse. Just go thru the plug wires to find which cylinder (or cylinderS) is causing the problem. If you find that it is random and can happen on ALL plug wires, you probably have a bad condenser or a bad ground for the distributor.
Distributor electrical ground results from conduction between distributor housing and engine block via the dizzy hold-down clamp. If there is paint on engine block or distirbutor housing which blocks electrical transfer through that clamp, you will have ignition problems.
Distributor electrical ground results from conduction between distributor housing and engine block via the dizzy hold-down clamp. If there is paint on engine block or distirbutor housing which blocks electrical transfer through that clamp, you will have ignition problems.
The following users liked this post:
ZRXGreen (07-18-2018)
07-16-2018, 10:15 PM
#10
Race Director
Your problem is the oil on the plugs
the problem is ether oil through the rocker arm studs or oil through the valve seals
my bet is the studs
Until that’s fixed your problem and not go away
If you see any shiny or black on the threads or anywhere in the conductor you’re leaking oil into the cylinder
the problem is ether oil through the rocker arm studs or oil through the valve seals
my bet is the studs
Until that’s fixed your problem and not go away
If you see any shiny or black on the threads or anywhere in the conductor you’re leaking oil into the cylinder
Last edited by pauldana; 07-16-2018 at 10:34 PM.
07-16-2018, 10:43 PM
#11
Instructor
Thread Starter
Your problem is the oil on the plugs
the problem is ether oil through the rocker arm studs or oil through the valve seals
my bet is the studs
Until that’s fixed your problem and not go away
If you see any shiny or black on the threads or anywhere in the conductor you’re leaking oil into the cylinder
the problem is ether oil through the rocker arm studs or oil through the valve seals
my bet is the studs
Until that’s fixed your problem and not go away
If you see any shiny or black on the threads or anywhere in the conductor you’re leaking oil into the cylinder
07-17-2018, 12:39 PM
#14
Instructor
Thread Starter
07-17-2018, 07:35 PM
#15
Race Director
I don't know,Jerry but if he rebuilds these for a living he probably shimmed it.. this makes the idle more stable
07-17-2018, 09:56 PM
#16
Instructor
Thread Starter
OK, plugs back in and checked all plug wires with the timing light. 2 is not firing. All others look fine. I tried the following to get 2 to work..
1) plug swap 1 to 2, no change
2) new plug wire, no change
3) tighten up plug, no change
4) change cap, no change
5) change rotor, no change
6) change timing, no variation of timing brings 2 back.
7) change capacitor, no change
8) watched for spark on plug while grounding it. no spark until you hit about 1500 rpm, then spark is there but looks weak. Perhaps my spark is weak all around.
Completely stumped.
1) plug swap 1 to 2, no change
2) new plug wire, no change
3) tighten up plug, no change
4) change cap, no change
5) change rotor, no change
6) change timing, no variation of timing brings 2 back.
7) change capacitor, no change
8) watched for spark on plug while grounding it. no spark until you hit about 1500 rpm, then spark is there but looks weak. Perhaps my spark is weak all around.
Completely stumped.
Last edited by DarrenX33; 07-17-2018 at 09:58 PM.
07-18-2018, 01:45 AM
#17
Team Owner
What plug gap are you using? If you have a stock points ignition system, you need to be set at .035". If you have points system, but have gone to a "hi energy" coil, you still need to gap the plugs at .035". If you have an electronic ignition system which is intended to get 12vdc, and you hooked it up to the stock resistance wire, there's not enough energy to jump the increased plug gap (usually .045") until your engine is up to 2500 rpm or higher. In that case, you need to run 'normal' wire to the coil instead of the resistance wire.
Your ignition system doesn't have enough energy to jump the gap you have set on the plugs or the plug gap is too large for the ignition system.
I really haven't come up with any other possible reason for only one plug not firing (that you haven't already eliminated from your "swap" testing). But, there is always the "something else" category which a lot of strange sh:+ falls into and which no one can imagine is possible. So I hope your problem lies in one of the two alternatives I suggested.
Please keep us informed of what you are doing and the results of any tests...or success! We can all learn something from "weird" problems like this. There IS an answer to your problem and when you finally figure it out, it will be simple to fix.
Your ignition system doesn't have enough energy to jump the gap you have set on the plugs or the plug gap is too large for the ignition system.
I really haven't come up with any other possible reason for only one plug not firing (that you haven't already eliminated from your "swap" testing). But, there is always the "something else" category which a lot of strange sh:+ falls into and which no one can imagine is possible. So I hope your problem lies in one of the two alternatives I suggested.
Please keep us informed of what you are doing and the results of any tests...or success! We can all learn something from "weird" problems like this. There IS an answer to your problem and when you finally figure it out, it will be simple to fix.
07-18-2018, 06:55 AM
#18
Instructor
Thread Starter
I have the stock points with stock resistance wire. I’m currently using a PS20 coil which comes with the Crane electronic system. At one point I was going to go with that but didn’t. I swapped in the coil a while back when I though I had an issue with my old one. Plug gap is .035.
07-18-2018, 10:53 AM
#19
Race Director
i wonder if you should not pull distributor and clock it one spark plug over and then move all the wires and see if the problem persists .....if it moved with the reclocking we know its in the distributor cap and wires..if its still with the #2 plug maybe you have a grounding problem from a screwed up helicoil, the spark plug has to be grounded....as i think about this why not run a grounding wire to the spark plug and wrap it around the hex part of the plug, if it fires you have a grounding issue
07-18-2018, 07:48 PM
#20
Team Owner
Hmmm.... Not sure I'd want to have a ground wire with 15-20K volts on it.. The idea is a good one, though. Maybe wedge some solid metal piece between the metal plug body and the cylinder head to assure a good ground to that plug.