car misses

[TURBOKID]

I need help, I have got a miss in my car now that I can not find and want to put it out there to see if anyone else has come across this before.

the car runs normal until 7650 rpms and then it starts to break up, as if it is on a rev. limit. the car has stock coil module with the dis4 box, stock cam and crank sensors, felpro 160 lbs. I have leak the motor and it is ok about 3% on all but the two center cly. and they are 8%. I have changed valve springs, plug wires, coil and module, change to the new dis4 high output box, change the harness for the dis4, changed my rpm box for the shifter that has a 7800 chip in it and I have to change to a 7600rpm chip after about two runs because then the car will not pull to 7800 in first gear. the first runs off the trailer the car will go to 7800 in first.

On the log it show good voltage while this is happing about 14.3, nothing funny on the corr.% or the A/F, boost, nothing looks funny but the rpm line. I have change or put new ends on all the power and ground wires and solder all the new ends on. Have good grounds straight from the battery, motor and frame.

The high end rev limit is set at 8500rpms and I moved it up to 9000rpms this weekend at a race to see if it changed anything but it didn't. I even by passed the msd box and put just the stock stuff on the car but it would not run with out that extra juice.
Could there be something in the felpro box bad that could do this? the settings in it are higher than when it starts to miss.

Has anyone had this msd setup and changed to a dist. set up? What all do you have to change and do to the harness to get it to work. I need this damn car fixed so I can win the points down here.

Thanks for any help anyone can give
Richie

 

[bobc455]

I'm not exactly 100% clear on your ignition, but it sounds like you have the two rev limiters- one from the FAST (now set to 9000) and one other, I think from an MSD box.

Because it runs correctly at first but develops problems after a couple of runs, I am thinking that it is temperature related.

Also by coincidence, the value of a electrical resistor generally changes slightly with temperature.

Since the "chip" for the MSD is essentially a resistor, I wonder if your chip isn't warming up slightly, changing resistance value, and limiting your revs.

I like to think of an MSD rev limit as a safety limit- if my valvetrain can take 6500 RPM, then I put in a 6400 chip even if I shift at 5800. Have you tried taking out the rev limit from the MSD? Or maybe put in an 8000 chip for a test?

Also, I don't know about the rest of your engine setup, but do you have enough spring pressure for the boost level that you're running? Since you are running a boosted motor to 7800, it sounds like a fairly hot setup, and I wonder if your boost level combined with your RPM level might be keeping the valves open longer than your cam intends to.

Just a couple of guesses...

-Bob Cunningham

 

[TURBOKID]

both of my rev. limits are set with the dis4 box high and low. the rpm box is only for shifting the shifter. the car has run with this same combo for about 4 years, and I'm only running 27lbs of boost thats all I can get. It has a new set of springs which are the same that I have been running for the past years.

 

[Z9's]

I had a similiar problem with my car,sbc 383 YS Trim vortech.On the dyno it would seem to hit the rev limiter .Turned out it was blowing out the spark, MSD7al3 Pro power coil,NGK R5724-8 plugs gapped at .030. I closed the gaps to .025 and the problem was gone.This was with 23lbs of boost. Just giving ya something else to look at. Good Luck