Posted By: 440Jim
High RPM Misfire -- SOLVED! - 07/07/05 12:32 AM
Last weekend, I lost in 3rd round due to an ignition misfire. This post tells the story, some background to the problem, and my initial attempts to solve it.
Another $30 win
To make this post more complete, here is the previous trouble shooting I did:
This is what I checked on Sunday:
- Spark plugs all looked good
- Spark plug wire resistance matched previous measurements in April 2005, including coil wire
I checked the plug wires as they were attached to the distributor cap
- Distributor cap looked good, inside and out, terminals good, rotor good
- All wiring connections were checked for tightness
- I did the MSD check from the instructions, removing distributor pickup wires and shorting/breaking connection.
It sparked a plug I put on the coil output during that test, so the coil apprears OK too.
- I measured the distributor (stock) magnetic pickup and it was 345 ohms, about right.
- Distributor reluctor gap was 0.006", good. There is a little play in the distributor bearings, but not much 0.002"
- The fuseblock has a separate circuit for the MSD low power (turn on) wire, and the fuse didn't seem tight when I pulled it out. I reinstalled it fully, but I don't know if it was really making a bad connection or not. The main power wires from the MSD go directly to the battery terminals.
Unless I found a loose wire, I didn't expect this to find the problem. It runs fine at low rpm, but over 4000-5000 it acts up badly. Next time I work on it, I will check more wires under the dash for chafing. I will put it back together (plugs etc) and try running the engine again. Then if it still has a problem, I will put the old alternator back in. This was a new Power Master 100 amp, 1-wire alternator I put in just before going to the track. Remember, it was running fine for 5 passes, then it started. Something must have happened, a part went bad, etc.
-----------------
On Tuesday after work, I took the MSD box (Digital 6) to Speed Unlimited (over an hour drive each way) and they tested it on their MSD display. He varied the RPM, tested the rev limiter, etc. and it worked fine. I got home and put it back in the car, installed the spark plugs etc. But it was too late in the evening to run the engine in the neighborhood with open headers, especially since I need to take it up to 5000 rpm to check things. I even took a spare distributor and pluged it in, then spun the distributor by hand, it fired the plug just fine. I am suspecting the coil is going bad, I checked the resistance (for what that is worth) and it showed 0.2 ohms and the catalog says 0.02 ohms (Blaster HVC #8252). I will try it with the old coil and the Blaster 2 (round, red) coil from the Cuda for comparison.
-------------
Today is now Wednesday. I tried the Blaster 2 coil and that didn't help. I replaced the magnetic pickup and that didn't help. I put the old Delco alternator back in (63 amp) and it now would run smoothly past 6500 rpm. This was with the fan off (see background topic). With the fan on it still misfired. At least I was back to where I was a few weeks ago, turning the fan off to run, turning it on while on the return road.
While the motor was all warmed up and running smoothly, I checked the rev limiter (MSD Digital 6) against the tach. It was real close at 5000, and 6000 rpm, but when set at 7500 it limited about 7100, and setting it at 7600 it limited at 7200. I left it there as I cross the finish line at 6900-7000.
Another $30 win
To make this post more complete, here is the previous trouble shooting I did:
This is what I checked on Sunday:
- Spark plugs all looked good
- Spark plug wire resistance matched previous measurements in April 2005, including coil wire
I checked the plug wires as they were attached to the distributor cap
- Distributor cap looked good, inside and out, terminals good, rotor good
- All wiring connections were checked for tightness
- I did the MSD check from the instructions, removing distributor pickup wires and shorting/breaking connection.
It sparked a plug I put on the coil output during that test, so the coil apprears OK too.
- I measured the distributor (stock) magnetic pickup and it was 345 ohms, about right.
- Distributor reluctor gap was 0.006", good. There is a little play in the distributor bearings, but not much 0.002"
- The fuseblock has a separate circuit for the MSD low power (turn on) wire, and the fuse didn't seem tight when I pulled it out. I reinstalled it fully, but I don't know if it was really making a bad connection or not. The main power wires from the MSD go directly to the battery terminals.
Unless I found a loose wire, I didn't expect this to find the problem. It runs fine at low rpm, but over 4000-5000 it acts up badly. Next time I work on it, I will check more wires under the dash for chafing. I will put it back together (plugs etc) and try running the engine again. Then if it still has a problem, I will put the old alternator back in. This was a new Power Master 100 amp, 1-wire alternator I put in just before going to the track. Remember, it was running fine for 5 passes, then it started. Something must have happened, a part went bad, etc.
-----------------
On Tuesday after work, I took the MSD box (Digital 6) to Speed Unlimited (over an hour drive each way) and they tested it on their MSD display. He varied the RPM, tested the rev limiter, etc. and it worked fine. I got home and put it back in the car, installed the spark plugs etc. But it was too late in the evening to run the engine in the neighborhood with open headers, especially since I need to take it up to 5000 rpm to check things. I even took a spare distributor and pluged it in, then spun the distributor by hand, it fired the plug just fine. I am suspecting the coil is going bad, I checked the resistance (for what that is worth) and it showed 0.2 ohms and the catalog says 0.02 ohms (Blaster HVC #8252). I will try it with the old coil and the Blaster 2 (round, red) coil from the Cuda for comparison.
-------------
Today is now Wednesday. I tried the Blaster 2 coil and that didn't help. I replaced the magnetic pickup and that didn't help. I put the old Delco alternator back in (63 amp) and it now would run smoothly past 6500 rpm. This was with the fan off (see background topic). With the fan on it still misfired. At least I was back to where I was a few weeks ago, turning the fan off to run, turning it on while on the return road.
While the motor was all warmed up and running smoothly, I checked the rev limiter (MSD Digital 6) against the tach. It was real close at 5000, and 6000 rpm, but when set at 7500 it limited about 7100, and setting it at 7600 it limited at 7200. I left it there as I cross the finish line at 6900-7000.
Now get hold of MIKESDUSTER and help him out. 
)