upper rpm break up

[hotrod68]

Having some issues with my daughters dart, we took the car to a test and tune and in drive towards the end of the 1/4 it acts like its hitting the rev limit, msd 6al2 so I turned the limit way up thinking maybe the dial was off but still the same. I tried a different pump thinking maybe it was starving but no luck. Its a mild 440, rpm heads 10.5 comp. and a hughes hy. mid sized cam. On the last pass on the return road I could smell something hot (electrical smell) and the msd box was pretty hot, thinking maybe the box is the problem? runs great in 1st and 2nd all the way to 6300 shift but in 3rd holding high rpms at the end is when this happens. Any ideas??

 

[Dana67Dart]

Floating valves?

 

[Rmoore]

Could have a plug wire or wires bad. Even a bad plug. Msd, I would think would be bad all the time. Anything electric tho can cause some goofy stuff.

 

[my68barracuda]

What do the spark plugs look like at the end of the run?
I had a miss at high load and high rpm and swapping the HEI ignition over to a MDS box 100% fixed it. I would not be surprised if the MSD box was warm, but I don’t think it should be hot at the end of a 1/4 mile run.
Maybe go over the MSD wiring and make sure all the connections are solid. Look close for heat affected wiring, that would lead you to the issue.

 

[JoeSBP]

What distributor? Might need phase it.

 

[mbaird]

It may be running out of fuel.
I chased this problem for a long time on my 73 340 4 Speed Dart Sport . Then one day I was changing rear gears and notice my fuel line was kinked where it came out of the tank.
Start with the simple stuff. Look over your fuel lines and try a new pump/ filter.

 

[AJ/FormS]

runs great in 1st and 2nd all the way to 6300 shift but in 3rd holding high rpms at the end is when this happens. Any ideas??

What's "holding high rpms?" More than 6300? More than 6800?

Since you hit 6300 in first and second, you should still be able to hit 6300 in third;and Ima guessing you're not triggering the MSD with points, so I doubt this is an ignition system issue.
Since it occurs under "severe" load, IMO, it has to be fuel or plugs. I'm leaning towards overheating plugs or possibly detonation.
Install new colder plugs,upjet both sides 2 numbers,and back off the power-timing 4 degrees; then try again.
If that cures it, go back to tuning.
If it doesn't cure it; IDK.

 

[66jim]

More fuel is needed in high gear than lower gear, this shows up in upper RPM. Start with fuel restrictions first. Popping, missing is electrical miss, surging is fuel.

 

[hotrod68]

What's "holding high rpms?" More than 6300? More than 6800?

Since you hit 6300 in first and second, you should still be able to hit 6300 in third;and Ima guessing you're not triggering the MSD with points, so I doubt this is an ignition system issue.
Since it occurs under "severe" load, IMO, it has to be fuel or plugs. I'm leaning towards overheating plugs or possibly detonation.
Install new colder plugs,upjet both sides 2 numbers,and back off the power-timing 4 degrees; then try again.
If that cures it, go back to tuning.
If it doesn't cure it; IDK.

This happens nearing the stripe, couple hundred rpm over the shift points. Its msd dist locked out at 34, I"ll try your suggestions Thank You!

 

[AJ/FormS]

This happens nearing the stripe, couple hundred rpm over the shift points. Its msd dist locked out at 34, I"ll try your suggestions Thank You!

So couple of hundred over 6300; well that could be valvesprings losing control; simple test is rev to same stripe-rpm in lower gear; springs are not usually load sensitive. So if it revs to stripe-rpm in second, it ain't the springs.
My 10.95Scr,tight-Q,aluminum headed, 367SBM, on the street, is happy at 34*. I found, by seat of the pants, no difference in performance anywhere between 32* to 36*, so 32/34* is what it has always run.
So then
I changed my mind about timing and possible detonation. 32* should be fine.
But overheated plugs would still be my #1 "idea".
And they may be that way from lean-running. And my experience with that is,that they never fully recover. So you could try fattening up the jets first , but I think, that's think, the plugs will have to be colder.
But before you do any changes, do the second-gear pull to stripe-rpm first, cuz you may be looking at a new rear gear.
It kindof depends on your cam.

 

[JoeSBP]

Spark scatter, check the phasing on the MSD distributor, and I also suggest dropping down to 32°. Can you post a pic of the plugs?