91 Tracker stumbles above 2800 RPM

[Annabelle]

First let me say how very useful all the FixKick info has been for my project.
I have a '91 Tracker with 8 valve engine. Have just re-assembled freshly machined engine with .020 over pistons and a total of .020 off cylinder head and block for a bit more compression. My machinist calculated that lowering the head by .020 would retard cam (and also ignition) timing by @3 degrees so the only other thing that is not stock is a 4 degree offset key that advances the crank drive. Timing is set at stock 8 degrees BTDC which should be @ 1 degree advanced. Engine runs great at idle, accelerates fine, runs smooth at steady throttle setting below 2,800 RPM but stumbles noticeably above that (only with steady throttle setting) -accelerates right a way to higher RPM but still lurches at steady throttle setting at any RPM above 2,800 RPM
Any ideas on where to look to solve this.
Thanks,
Steve (aka Annabelle)

[fixkick]

what does wide open throttle do?
any good TDC gauge will show you TDC error. no need to guess.
the engine does tilt to driver (LHD) side. but the cam is fully above the crank
as seen here, head moves down, to the crank in a flat plane.
http://www.fixkick.com/t-belt/donebelt1.jpg
so milling this head will not change came timing, (physically) but sure, with super high compression you need premium fuel. so it dont knock(ping).

spark gapped to 0.028" not using out of box. dead wrong 0.045" spark is weak on this car and does not like higher compression, but it should work.
ECU flashes CEL code 12= good,?
tps calibrate with feeler gages per book if not it will hesitate, then power resumes, for sure WOT full power,
the fuel pressure Low.


if not idea what this means.
not stock is a 4 degree offset key that advances the crank drive??? huh? you should photo parts not stock, for sure.

[Annabelle]

Checked all the items you suggested and all were good. Disconnected O2 sensor and problem disappeard but then came back (but not so pronounced) in about 20 miles. Figured the ECU took a while to adapt to disconnected O2 senser. Installed new senser and problem gone, thanks for the help
Steve

[fixkick]

Checked all the items you suggested and all were good. Disconnected O2 sensor and problem disappeard but then came back (but not so pronounced) in about 20 miles. Figured the ECU took a while to adapt to disconnected O2 senser. Installed new senser and problem gone, thanks for the help
Steve

that is one good test 02 pulls put the 02 voltage to 0.45v.  ( 0 to 1v is range)
0v = lean, AFR, ECU goes rich max.
1v = rich , AFR ECU goes lean

stuck at 0 or 1volt both cause ECU to cause TRIM to get stuck  full lean or full rich. on the rich side it misfire, and spark plugs go black tipped
the lean side , hesitations happen at throttle TIP IN, with toe, and then correct. (mostly)

02 bad or mainfold gasket leak down low can make 02 stick at 0v
some O2 the heater inside (rare) most burn out but some leak heater current to the sensor side (CELL+) and cause AFR to go SUPER LEAN. that is one nasty 02 if true.
02 are engineered to fail at 0v, this is their dream and is mostly true. but not always.

try new O2. cross fingers just that.
on the other had removing an O2 can hide other problems. (masking real) low fuel pressure can be hidden by removing (connector) O2. as 2 wrongs make a right (nearly)