Hi Paul,

In reading over you information I’m not to sure that you have a transition problem at that high of an RPM.

This is not a stumble “at idle” or incorrect speed the engine is trying to recover from.
When an air balance is off, around the throttle, the ECU knows nothing but what the O2 sensor says has happened in the recent past. It only has that throttle switch.

It never hurts to do maintenance around in there even though it looks so simple.
We are bringing crankcase oils and oddly timed fuel vapors to an incinerating device!
We shouldn’t expect electric motor performances.

If a sticky IAC valve is loosing capability to respond this allows the engine to slip or succumb to say, a transmission loading into drive or reverse.

The transition term is related to the IAC’s own vane being positioned almost closed and the throttle plate making air to go around and through the IAC.

The IACs throat size is fully capable of racing the engine as does so each time the engine is started.
That’s how we know it working properly and is adjusted to its range to give the ECU time to get engaged for more power!


The crack you see at the bottom of the throttle is only one side, there is one top side too!
The top is controlling bleed vacuum to operate the fuel canister valve and/or air-vapors in from the fuel evaporation canister. All at idle.
You are right they don’t affect much, but it’s behind the AMM!
It can and does affect mixtures and we are not allowed to have any extras or we get more and more improper operations.

You are stumbling above idle speed because the breathing of the engine is starting to come up “ on the cam.” This is the beginning of the torque / horsepower band for pulling or doing real work!
IPD is not doing that much magic!
They are just moving the band up or down in the RPMS range. Shifting draw bar torque into brake horsepower and vice versa.

The stumble has little to do with the cam.
It’s a set thing and doesn’t cause stumble to come and go.
I could be wrong but if you can go back to stock Zero, it may not be so radical and might even change a to new RPM range but going away, doubtful.
But you can surprise me, as I’ll can still learn things all the time! (:-)

I would say you having some air volume disruption going on, in which, it goes too lean as it’s becoming a more uncontrollable vacuum leak!
It can start at the AMM hose and ends with the intake valves. Out flow from there is another story on power, but it’s not causing backfiring.
These engines have aged intake gaskets and I would be suspicious that there is a gain of air in order to get speeds up and be too lean.

It explains the backfires that is erratic mixtures of combustion, at speed, that varies consistent flow, causing Variable RPM speeds despite a held steady throttle plate.
You said you were accelerating the engine?
So is this when you hold it steady or are you driving and changing the loads a lot?
Please clarify.
A weak responding FPR can effect powering up, as it’s supposed to provide more enrichment during acceleration more exclusively and then other symptoms.

Keep us posted!

Phil