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
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..
1968 Corvette Convertible, matching numbers, ps, 58k original miles
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.
Another thing I found, not sure how frequent this one is..
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.
bad spark plug (internal crack you can't see)
bad ignition wire
(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.
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
Is it doing it all the time or just while shifting?
I have a button on the shifter and when the button is pressed, the 2 step is activated and the rev limit goes down to 4000 RPM and when I release it, it goes back up to 7000. When you launch off the transbrake (which I actually haven't done yet), you push that button and floor the pedal. The RPM will stay at 4000 RPM and when the green light drops, I activate the transbrake and release the button, the car launches and the limit goes right up to 7000 RPM
Last edited by GrandSportC3; 07-06-2006 at 09:02 PM.
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?
I have not tried it without the 2-step.. I'll do that at the next track visit. The misfiring happens between 6000 and 7000 RPM several times.. It happens in low and high... In low, it revs up so quickly that it's hardly noticeable..
A couple of things it might be if it occured after installing the new limiter..
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.
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 :
Originally Posted by 427Hotrod
Have you tried a pass without the 2 step/rev limiter hooked up?