Some good advice so far, here's another 2 cents (4, actually). Keep in mind that the 99T in US trim was rated at 135hp/160lb-ft torque. So, consider what you're comparing it to, what is "poor performance"? When new, the 99T was good for 9.5-10 second 0-60 time at best. A lot of 99Ts got "blow up" by folks trying to crank up the boost - the engine and fuel system have some limits beyond which expensive stuff starts breaking....
1) Base engine hardware: Since you have to pull the valve cover off to check the valve clearances anyway, check to make sure all the timing marks line up. Then check/set the valve clearance. Do a cylinder leakdown test preferrably, but at least a compression test. 20% leakdown likely means exhaust valve seat(s) worn.
2) Ignition timing: Check to make sure that both vacuum advance and centrifugal advance are working correctly. The distributors tend to sieze up over time which gives you "fixed advance" - you can set it perfectly and the engine will still run poorly because the timing's not varying when it should.
3) Fuel: Use a dwell tach hooked to the lamda service connector by the fuse panel to make sure that at a warm idle the lambda duty cycle is fluctuating between 30-70% or 40-60%. You should hear the idle "hunt" ever so slightly in tune with the fluctuating pulsewidth. Adjust the mixture using this measure and you don't need a CO meter. Then, make sure (as was pointed out) that the pulsewidth goes up to 80% and stays there when the throttle microswitch is closed (don't need to open the throttle, just reach over and click the switch). With the engine off you can make sure that the switch is being activated when the throttle actually gets near open.
4) Boost: The US cars were set to run 8 psi max on 87 octane fuel. (about the mark between red and yellow on the gauge - 3600 rpm steady wide open throttle in 3rd gear - ride the brake until it stabilizes). If your cooling system is real good and you run 93 octane fuel you can go 10 psi, but I would not go above that on a stock 99T motor unless you love doing valve jobs.
As was already suggested, you can get some additional power by running richer, but you would need to use an air/fuel ratio sensor to "tune" that along with the trick warm-up regulator, etc. 11.5:1 Air:Fuel ratio is plenty rich, more than that you are just spraying money in there and turning it to smoke.
If you decide to pull of the catalyst (which I don't recommend unless it's clogged) and find that it's physically intact, I'd be interested in having it. I have a used test pipe I'll trade if you want. But, assuming the catalyst is good, that is going to be a small performance change (probably take a stopwatch to tell the difference) compared to getting the rest of the system sorted.
posted by 198.208.15...
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