High Rpm / full throttle Miss misfire stumble

[John Brandt]

OK, I really would appreciate it if some of the heavies could chime in on this

Last summer I did a major surgery on my ’84 3.2 Carrera engine to fix broken Headstuds. In addition to the new steel studs, I re-ringed the Mahle pistons, and went to 1 5/8" racing headers, since my car is pretty much only goes to the track. I put on a 2 in / 1 out muffler stolen from my 71 E.



My problem is that at very high RPM, as in 6000, 6200 the engine starts to misfire in a way that sounds sort of like the rev limiter but with an extra bit of machine gun staccato; but it's too early to actually be the rev limiter. The first time I heard it I thought my valves and pistons were getting acquainted. Obviously that’s not the case as it has happened dozens of times, and it still is fast enough to keep pace with other well-driven Carreras. I just don’t have the last 500-800 RPMs

many times the engine will rev out to 6700 as it's supposed to but not at full throttle

I have arleady done a few things that I thought were likely suspects and it has not eliminated the issue. THisn included :Modifying the muffler to sport specs as described in UP Fixin' (very happy with it), new cap and rotor, new fuelfilter, and new Magnacor 8.5mm wires


so,,,,What now?

Here's my current list of suspects:


Cams timed too aggressively / wrong…as I noted above, I set them up at .540 and .548 , which is the high range of overlap; I thought this would help me but ??? also I can say that the car is not exactly eager to start the first time of the day, but the battery checks out within spec When I put on my booster, it starts right up though and at all times it idles well and runs superbly until the misfire.


It is running too lean after the change in exhaust, and what I’m hearing is detonation.


It does not like its new set of Bosch Platinum +4 plugs even though they are exactly like the old ones.


One of the flywheel sensors is bad due to being dropped on the floor from a foot high


Headers are not square on one exhaust port, and not allowing proper flow. I had to do a LOT of massaging to get the headers to fit on the studs. I felt that I had mainly rotated the flange but it could be a scant amount off center. (Not happy with the Pelican “356six”who sold these to me…he knew they were a problem)





Some other electrical problem. The issue seems to be more severe as it gets hotter.


Some other thing that is not even on my radar???????




Guys, please help me out here. I know this is a toughie but I only ask for help when I really need it

[RoninLB]

wild guess is a fuel delivery problem. I guess I'd check simple fuel pump flow of about a liter per 30sec.

disclamer.. i know very little about your FI.

[Steve@Rennsport]

John:

You've made a LOT of changes here that affect fuel delivery,.....are you using stock software?

I would check the fuel pump for pressure & volume after you replace the fuel filter (if its old).

Replace those gawd-awful +4 plugs with either Bosch Copper or Bosch Silver ones. Those are quite troublesome plugs and I'll not use them.

Then, you need to put the car on a dyno and check the AFR's at WOT.

[John Brandt]

Steve,

Thanks for the response. My car has stock electronics, the fuel filter is New.
I will check fuel volume tonight. I do not have a fuel pressure tester, but I'll see if I can get one today.

Let me reiterate that the car starts, Idles well, and runs fantastically right up to the rat-a-tat at 6000. I did however notice a little bit of Idle hunting after a long drive yesterday which is new

Also occured to me that I oiled my k&n cone right around that time last year; Is it possible i over oiled and got gunk in the airflow sensor? Whats the preferred way of cleaning it?

The previous set of +4 worked fine, but I suppose I can get a set of supers to try

As far as the dyno goes, I don't have access to one...is there not a basline mixture adjustment that I can experiment with?

[John Brandt]

fuelpressure was a dead end...the problem is corrected after an adjustment of the full throttle switch