What could cause high RPM misfire?
#1
Team Owner
Thread Starter
What could cause high RPM misfire?
Went to the track again yesterday and made a few passes.. On some of the passes, the car would misfire at high RPM's...
I looked down at the Air/Fuel gauge and the A/F was in normal range..
Pulled all the plugs and checked them. All of them had about the same shade of grey with no damage or indication of a lean condition..
The entire electrical system including ignition (MSD Billet Distributor, MST Blaster coil, MSD 6AL and MSD 8.5mm wires) and Painless switch panel and wiring harness are NEW.. Followed all the plug wires and none of them were anywhere close to the headers etc...
Checked the timing and my total timing is set to 36 degrees where it should be. I have no idea what else to look for
Any ideas?
P.S. The misfiring doesn't seem to slow the car down. Ran my normal ET's... I still need to find out what causes the misfiring..
I looked down at the Air/Fuel gauge and the A/F was in normal range..
Pulled all the plugs and checked them. All of them had about the same shade of grey with no damage or indication of a lean condition..
The entire electrical system including ignition (MSD Billet Distributor, MST Blaster coil, MSD 6AL and MSD 8.5mm wires) and Painless switch panel and wiring harness are NEW.. Followed all the plug wires and none of them were anywhere close to the headers etc...
Checked the timing and my total timing is set to 36 degrees where it should be. I have no idea what else to look for
Any ideas?
P.S. The misfiring doesn't seem to slow the car down. Ran my normal ET's... I still need to find out what causes the misfiring..
#2
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How's the voltage at the MSD box?
This may also be of some interest
Above quote is from http://members.aol.com/blue317/techtips.htm#misfire
This may also be of some interest
August edition - High-RPM Misfire - Suggested MSD Updates:
If you are using a MSD ignition system we strongly suggest you install the updated #8862 shielded magnetic pickup harness which connects the distributor to the ignition box. This shielded harness which includes a ground wire, greatly reduces the possiblity of induced voltage to the trigger circuit.
It is also suggested by MSD to twist the magnetic pickup wires coming out of the distributor or your pickup on your crank trigger to reduce outside voltage (RF) that can mistrigger your ignition. Install the distributor pickup harness as far away as possible from any wires, especially the primary coil wires and sparkplug wires.
We also suggest installing the MSD #8830 RF filter in the constant power supply wire and ground when using a MSD ignition box to filter any unstable voltage to your MSD Two-Step rev control or other electronic accessories.
These two inexpensive "fixes" cured a misfire gremlin that plagued us (and showed up in the tach at high rpms) when we increased the battery voltage in our '57 Chev. We were only able to locate the problem using a lab scope to detect stray voltage near the distributor pickup wires! This common problem was also confirmed after a frustrating weekend, by the MSD tech department.
If you are using a MSD ignition system we strongly suggest you install the updated #8862 shielded magnetic pickup harness which connects the distributor to the ignition box. This shielded harness which includes a ground wire, greatly reduces the possiblity of induced voltage to the trigger circuit.
It is also suggested by MSD to twist the magnetic pickup wires coming out of the distributor or your pickup on your crank trigger to reduce outside voltage (RF) that can mistrigger your ignition. Install the distributor pickup harness as far away as possible from any wires, especially the primary coil wires and sparkplug wires.
We also suggest installing the MSD #8830 RF filter in the constant power supply wire and ground when using a MSD ignition box to filter any unstable voltage to your MSD Two-Step rev control or other electronic accessories.
These two inexpensive "fixes" cured a misfire gremlin that plagued us (and showed up in the tach at high rpms) when we increased the battery voltage in our '57 Chev. We were only able to locate the problem using a lab scope to detect stray voltage near the distributor pickup wires! This common problem was also confirmed after a frustrating weekend, by the MSD tech department.
#3
Team Owner
Thread Starter
Originally Posted by 76custompaint
How's the voltage at the MSD box?
This may also be of some interest
Above quote is from http://members.aol.com/blue317/techtips.htm#misfire
This may also be of some interest
Above quote is from http://members.aol.com/blue317/techtips.htm#misfire
#4
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Another thing I found, not sure how frequent this one is..
Found at : http://www.lcengineering.com/newslet...30837e50ef61a3
Another problem you may run into is fuel siphon from the accelerator pump. At high rpm the nozzle may “flutter” causing fuel to leak by and leading to a high rpm misfire. This concern can cause some issues in tuning the carb. To check for this concern first remove the air cleaner assembly, start and warm up the engine, hold engine at race rpm and look down the barrels (MAKE SURE THAT YOU ARE WEARING SAFETY GLASSES WHEN YOU DO THIS) to see if the nozzle is dripping/spraying fuel into the engine. If it is then you can start by lowering the float level slightly or tightening the acceleration spring or even compensating for this issue by reducing the main jet size.
#5
Le Mans Master
some possible "could be's"
bad spark plug (internal crack you can't see)
bad ignition wire
bad coil
(new doesn't mean they can't still be bad!)
float level in carb too low
also, for what's it's worth, over the winter I replaced (as part of a big engine bay clean-up project) the spark plugs, ignition wires, distributor cap, rotor, rebuilt the carb, and rebuilt the distributor, etc, etc.
Not long after getting her running again I discovered a high rpm miss.
The spark plugs I was running were the same kind I had always run - AC R45's.
What i ended up doing to fix the miss issue was swap them out to a non-resistor plug - the NGK B4. As soon as I did that the miss went away.
My firend who is EXTREMELY knowledgable on motors suggested that as the first thing to try and it works.
His thereoy was that the new wires were higher supression wires than the older ones and along with the standard resisitpr spark plugs it created just enough of a voltage drop to cause a miss at higher rpm's. Remember, back when these older cars were new the spark plugs were not resisitr plugs like most of them are now.
I'm not saying this is your problem, but if you try everything else and it doesn't fix the issue it's worth a try to throw in some non-resisitor plugs and see what happens.
EDIT:
opps while I was typing (I'm slow) 76 replied and this info from MSD sounds like it could be your problem afterall. Good luck with it
bad spark plug (internal crack you can't see)
bad ignition wire
bad coil
(new doesn't mean they can't still be bad!)
float level in carb too low
also, for what's it's worth, over the winter I replaced (as part of a big engine bay clean-up project) the spark plugs, ignition wires, distributor cap, rotor, rebuilt the carb, and rebuilt the distributor, etc, etc.
Not long after getting her running again I discovered a high rpm miss.
The spark plugs I was running were the same kind I had always run - AC R45's.
What i ended up doing to fix the miss issue was swap them out to a non-resistor plug - the NGK B4. As soon as I did that the miss went away.
My firend who is EXTREMELY knowledgable on motors suggested that as the first thing to try and it works.
His thereoy was that the new wires were higher supression wires than the older ones and along with the standard resisitpr spark plugs it created just enough of a voltage drop to cause a miss at higher rpm's. Remember, back when these older cars were new the spark plugs were not resisitr plugs like most of them are now.
I'm not saying this is your problem, but if you try everything else and it doesn't fix the issue it's worth a try to throw in some non-resisitor plugs and see what happens.
EDIT:
opps while I was typing (I'm slow) 76 replied and this info from MSD sounds like it could be your problem afterall. Good luck with it
Last edited by BarryK; 07-06-2006 at 08:24 PM.
#7
Team Owner
Thread Starter
Originally Posted by 76custompaint
And of course it can always be valve float, if it starts happening sooner and sooner in RPM's that would probably be it.
76custompaint
76custompaint
My valvetrain is good to 9000 RPM...
#8
Team Owner
Thread Starter
Originally Posted by 76custompaint
Another thing I found, not sure how frequent this one is..
Found at : http://www.lcengineering.com/newslet...30837e50ef61a3
Found at : http://www.lcengineering.com/newslet...30837e50ef61a3
I think that it has to do with my 2-step setup.. The problem started happening since I installed the 2-step...
#9
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Location: Thief River Falls Minnesota
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Originally Posted by GrandSportC3
no valve float on my solid roller setup!!
My valvetrain is good to 9000 RPM...
My valvetrain is good to 9000 RPM...
Originally Posted by GrandSportC3
I think that it has to do with my 2-step setup.. The problem started happening since I installed the 2-step...
#10
Team Owner
Thread Starter
Originally Posted by 76custompaint
Solid rollers are the way to go.
What's a 2-step setup?
What's a 2-step setup?
#12
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Thread Starter
Originally Posted by 76custompaint
Is it doing it all the time or just while shifting?
Last edited by GrandSportC3; 07-06-2006 at 09:02 PM.
#13
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Have you tried a pass without the 2 step/rev limiter hooked up?
Did you ever upgrade the fuel system?
Is it RPM related..or speed related? Meaning does it do it at the same RPM in the first couple of gears or only on the big end in high?
JIM
Did you ever upgrade the fuel system?
Is it RPM related..or speed related? Meaning does it do it at the same RPM in the first couple of gears or only on the big end in high?
JIM
#14
Team Owner
Thread Starter
Originally Posted by 427Hotrod
Have you tried a pass without the 2 step/rev limiter hooked up?
Did you ever upgrade the fuel system?
Is it RPM related..or speed related? Meaning does it do it at the same RPM in the first couple of gears or only on the big end in high?
JIM
Did you ever upgrade the fuel system?
Is it RPM related..or speed related? Meaning does it do it at the same RPM in the first couple of gears or only on the big end in high?
JIM
#15
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Location: Thief River Falls Minnesota
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A couple of things it might be if it occured after installing the new limiter..
So if you ran the switch for your limiter or some of the other wires of your new rev limiter with other wires this would be a good thing to check. Also did you check the distributor cap for arcing?
Found it here :
That would really help isolate the problem.
I have been chasing a high(er) RPM "false rev limiter" on two cars with MSD 6AL's for the last few months. One is a 69 Charger with a 528 Hemi and the other a 69 Super Bee with a 440 in it. After a lot of work and chasing shadows for way too long, I think that I, with the help of MSD, have found a solution. It seems as though many of the newer MSD pieces are very sensitive to RFI and EMI interference. Here are a couple of things to check and do:
1. When you installed your MSD, did you cut the approproate wires to length or did you coil them up and tuck them out of the way? If they are coiled upon themselves, this can be a source if interference, especially under load.
2. When you intalled your MSD, did you run any of the signal or trigger wires along any 12 volt power wires, near a plug wire, the alternator, or the coil? This too can cause interference (although most everyone avoids these issues).
If the installation seems "text book" and you avoided any installation pitfalls, the next step is to go with a sheilding kit. I know, I have run MSD's for twenty years as well, and I never needed a shielding kit either. It seems as if one or two things have changed (although MSD won't admit to either) - the 6's are more vulnerable to interference than in the past (possibly due to a change in shielding of the electrical wires) or the shielding on the 8.5's is not as efficient as it was in the past. These are my inferences (for what they are worth). I believe that MSD now makes a shielding kit (from what they told me on the phone yesterday) or you can go to Radio Shack and buy grounded electrical wire shielding. That should help.
Worth a try.
1. When you installed your MSD, did you cut the approproate wires to length or did you coil them up and tuck them out of the way? If they are coiled upon themselves, this can be a source if interference, especially under load.
2. When you intalled your MSD, did you run any of the signal or trigger wires along any 12 volt power wires, near a plug wire, the alternator, or the coil? This too can cause interference (although most everyone avoids these issues).
If the installation seems "text book" and you avoided any installation pitfalls, the next step is to go with a sheilding kit. I know, I have run MSD's for twenty years as well, and I never needed a shielding kit either. It seems as if one or two things have changed (although MSD won't admit to either) - the 6's are more vulnerable to interference than in the past (possibly due to a change in shielding of the electrical wires) or the shielding on the 8.5's is not as efficient as it was in the past. These are my inferences (for what they are worth). I believe that MSD now makes a shielding kit (from what they told me on the phone yesterday) or you can go to Radio Shack and buy grounded electrical wire shielding. That should help.
Worth a try.
Found it here :
Originally Posted by 427Hotrod
Have you tried a pass without the 2 step/rev limiter hooked up?
#16
Team Owner
Thread Starter
Originally Posted by 76custompaint
That would really help isolate the problem.