'72 ls5 problems
#21
Pro
Thread Starter
Does the shop have old test equipment like the Sun Engine Analyzer Scope and someone that can use it?
http://performancetechnician.com/pdf...ion-System.pdf
http://performancetechnician.com/pdf...ion-System.pdf
I'll be checking it out!
#22
Pro
Thread Starter
GOOD IDEA Will try it tonight and let you know...
#23
Le Mans Master
They like our cars and many of us are antiques. Impressive tool though, and lots of information to interpret. Good luck!
#25
Instructor
Many good suggestions given. A couple of things I would try. I didn't see where you indicated what RPM the misfire was occurring. 40 MPH can be very different to an engine as far as load is concerned if you are in 1st gear or high gear. I would turn the idle stop screw in until you can hear the misfire consistently, hopefully below 3K RPM. Once you hear the misfire, take the timing light and clip it onto each plug wire and then stick the strobe end of the timing light to your ear. Many times I could HEAR the strobe not flash before I could see it. Plus it helps if you are working in a bright environment. If it is just one cylinder, this may help you isolate which one.
Next, I would set the idle stop screw to hold the idle just above 1k RPM. Remove the PCV valve but leave it plugged into the hose. Take the breather hose or single filter, out of the other valve cover. Cover both valve cover openings with your hands. If you start to feel a suction on your hands, you have an intake gasket leak into the lifter valley. Heads that have been angle milled can cause this. Also, a mismatch with intake port and intake manifold can cause this. I wear nitrile gloves when working on cars and it makes it easier to detect the suction. Even a small vacuum leak will suck the glove into the opening. Check the throttle shafts on the carb. If you can move the shaft around (not rotating the shaft) you may have a large enough vacuum leak to cause a misfire that would seem random. If you have power brakes, make sure you disconnect the booster. A ruptured diaphragm can leak vacuum as well. If the misfire can be isolated to a single cylinder and no vacuum leaks or ignition issues can be found, Then I would take a strong look at the valves, valve guides, springs, etc. A broken spring, badly worn guide or valve seat/face can also cause intermittent misfires. That's all I can remember off the top of my head.... HTHs.... Good luck
Woody
Next, I would set the idle stop screw to hold the idle just above 1k RPM. Remove the PCV valve but leave it plugged into the hose. Take the breather hose or single filter, out of the other valve cover. Cover both valve cover openings with your hands. If you start to feel a suction on your hands, you have an intake gasket leak into the lifter valley. Heads that have been angle milled can cause this. Also, a mismatch with intake port and intake manifold can cause this. I wear nitrile gloves when working on cars and it makes it easier to detect the suction. Even a small vacuum leak will suck the glove into the opening. Check the throttle shafts on the carb. If you can move the shaft around (not rotating the shaft) you may have a large enough vacuum leak to cause a misfire that would seem random. If you have power brakes, make sure you disconnect the booster. A ruptured diaphragm can leak vacuum as well. If the misfire can be isolated to a single cylinder and no vacuum leaks or ignition issues can be found, Then I would take a strong look at the valves, valve guides, springs, etc. A broken spring, badly worn guide or valve seat/face can also cause intermittent misfires. That's all I can remember off the top of my head.... HTHs.... Good luck
Woody
Last edited by txfatboy1; 06-20-2018 at 10:53 PM. Reason: correct spelling
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rogergcam (07-12-2018)
#26
Drifting
check your wires at the plug ends I bought a set of msd wires once and one of the plug ends was disconnected but close enough to spark it took me almost a month to figure it out ,, I never though a new set of wires would be defective and I used the water trick at night but never sparked because it was insulated
Last edited by forman; 06-21-2018 at 06:36 AM.
The following users liked this post:
rogergcam (07-12-2018)
#27
Burning Brakes
check your wires at the plug ends I bought a set of msd wires once and one of the plug ends was disconnected but close enough to spark it took me almost a month to figure it out ,, I never though a new set of wires would be defective and I used the water trick at night but never sparked because it was insulated
The following users liked this post:
rogergcam (07-12-2018)