Random stumble at ~4-4.5k RPM after rebuild *Help Needed*

[jburroughs12]

Hello everyone,
I rebuilt my engine and converted from C4 to C2 last year (242whp & 227tq woo hoo!). I've only been able to get out on track twice since then and have noticed two things.

1) I need to relearn how to drive the car like a 911. Before I was so down on power & C4 that getting the car to rotate on throttle was very difficult. Much easier now and I need to get comfortable driving it like an old 911.
2) This is where I need some help. I get a stumble between 4-4,500 RPM. It doesn't happen all the time, it's not consistent with partial or full throttle application. Once I get the car above 4,500 RPM it pulls strong all the way to redline.
I have taken apart my intake to see if there is anything bent in there or any vacuum leaks (due to my bent flywheel during rebuild I may or may not have blown my intake off 3 or 4 times ) but everything looks good. I did not replace the knock sensors during the rebuild, but I think that may be the next step to look at. They're >20 years old and had a bunch of corrosion on them which I cleaned off, but could be an odd resonant frequency that's causing them to pull timing?

Any insight or thoughts y'all can provide would be more than welcome. I don't have access to a hammer to pull codes/clear codes, unfortunately. The car is 95% of the way there, but I'm gobsmacked with this random stumble.

Someone on Rennlist suggested I take a look at my AFM wipers to see if there's a worn spot, but to my eyes they look good.

[TorqueHead]

Are you able to rotate the wiper manually? If so. you could try to measure the resistance with a multimeter while you slowly rotate the wiper. I expect it should show a smooth change, no spikes, drops or dead spots.

I would use Deoxit on the wiper while you are in there. Also is that a crack in the insulation of the lower of the two white wires just where they turn 90 degrees to duck under the wiper board?
Might be worth testing the wire to see if there are any issues with it.

[jburroughs12]

Originally Posted by TorqueHead

Are you able to rotate the wiper manually? If so. you could try to measure the resistance with a multimeter while you slowly rotate the wiper. I expect it should show a smooth change, no spikes, drops or dead spots.

I would use Deoxit on the wiper while you are in there. Also is that a crack in the insulation of the lower of the two white wires just where they turn 90 degrees to duck under the wiper board?
Might be worth testing the wire to see if there are any issues with it.

I put a multimeter on each of the two wires & the center hard mount that goes to the wiper pivot. There seemed to be a smooth sweep from closed to open, slight variances, but no significant jumps up or down.

What looks like a crack is just a bit of the clear coating coming off, the wire is fine.

[ridgebk995]

I had a similar problem after a rebuild, turned out the cams were not indexed properly, once that was done, pulls like a freight train.

[Steve W]

You may have reinstalled your distributor one tooth off. Take off the distributor caps, and rotate the crank pulley noting where the rotors point to on the TDC marks on the distributor, as the crank pulley approaches the TDC mark on the case. They should point simultaneously within a couple of degrees to the mark.