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It's leaning out at high fuel demand. LOTS of things can cause this.

Have you put a fuel pressure gauge on it? Hard to tell without that. 
Did this come on all at once or over time? Right after someone worked 
on it or all by itself?

In order of likelihood:

-Vacuum hoses routed wrong putting vacuum to regulator all the time 
when running but NOT when cold (or bad thermal vacuum switch);
-CO out of adjustment - use a dwell meter or exhaust analyzer;
O2 sensor not working/not hooked up (you can make it run without this 
though)(if the engine is hunting after warmup, it's not using the O2 
sensor for some reason, weak, set too lean, etc.);
-Warmup regulator bad, setting too lean all the time (you can tune 
around this at idle with the CO adjust, but won't make up for it at 
high demand);
-Unmetered air leak, most common is idle motor tube, second is the 
rubber gasketing between the fuel injection body and the throttle;
-Fuel Injectors aren't as good as you think they are;
-Fuel filter plugged;
-Fuel Distributor problem;
-Bad vacuum switch putting vacuum at the wrong place/wrong time.

PS - the microswitch does not control the frequency valve, it turns 
on the idle motor ECU and turns on vacuum advance using that little 
can next to the ignition distributor. There's a clue there, normally 
(warmed up) once you crack the throttle off that switch vacuum 
advance comes on and the idle jumps to 1400 or so immediatly. That 
tells me you are not getting vacuum advance to the distributor which 
points at mis-routed vacuum hoses or a bad vacuum switch (the one 
screwed into the y-pipe under the intake.)

--- In dmcnews@xxxxxxxxxxxxxxx, mike clemens <rmclemns@...> wrote:
> Josh's car has once again got me stumped.  Here's what is 
happening:  Start the car up and it idles right on specs, try to give 
it gas and it wants to hesitate and almost quit.  Back off the gas, 
it goes back to idle.  As the car warms up, you can get more and more 
rpm but never full throttle.  After the car is warmed up to where the 
fans come on, the idle hunts up and down, between 650 and 850 rpm.  
The car starts backfiring if you try to go above 5000 rpm.  If you 
move the throttle off the microswitch that controls the frequency 
valve, idle stabilizes at 950-100 rpm.
> The plugs are new, the idle speed motor is new, the injectors have 
been cleaned by JH and are all working.  All vacuum lines have either 
been checked or are new, no leaks.
> Any ideas of what might be wrong?
> Mike   TPS   1630


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