I just adjusted my timing up a bit, now when I hit the brakes my tach fluctuates large I put my timing back and it still does it. Any thoughts? Makes my rig sound like crap. It only happens once it's warmed up to. 87 4Runner

My 89 dose the same think when it is idling (after it is warmed up) runs fine when I'm on the gas. It has been doing it for some time now and I think it is probably the O2 sensor but I haven't real looked in to it. May be a good place to start looking.

Hello Gecko,
If the O2 sensor is not the problem then you might want to try the throttle position sensor and/or the air flow meter. If you would like to just swap them out to test them I have all of these things including the O2 sensor. Let me know if this is something you would like to try.
Later,
Tye

Ok I have it figured now. When I put my header on I got rid of my O2 sensor which on these trucks is only for when the rig is idling. Hence with no sensor it just automatically goes to a grand in revs when the brake is on cause it can't re adjust with no sensor. So I piddled around last night got the timing back to normal. It's all good now and runs like a charm I just need to find out how to make it so I can adjust my timing now without that fluctuation and without the O2 sensor. Thanks for your thoughts fellow Yota lovers.

Check the throttle positioning sensor