My 87 savage will start fine, idle and rev at low rpm. but when I hit throttle it will backfire, pop, ****, etc?1 any ideas?

On some float-type carburetors, you can adjust the air-fuel mixture and engine speed at idle. Check for an idle speed screw designed to keep the throttle plate from closing completely, and an idle mixture screw that limits the flow of fuel at idle. If your carburetor contains these screws, proceed below.

1. With the engine off, remove the air filter and air cartridge.
2. Locate the idle mixture screw and turn it clockwise until the needle lightly touches the seat. Then, turn the screw counterclockwise 1-1/2 turns.
3. If your carburetor has a main jet adjustment screw at the base of the float bowl, turn the screw clockwise until you feel it just touch the seat inside the emulsion tube. Then, turn the screw counterclockwise 1 to 1-1/2 turns. Replace the air cleaner assembly and start the engine for final carburetor adjustments.
4. Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, turn the idle mixture screw slowly clockwise until the engine begins to slow. Turn the screw in the opposite direction until the engine again begins to slow . Finally, turn the screw back to the midpoint.
5. Using a tachometer to gauge engine speed , set the idle speed screw to bring the engine to 1750 RPM for aluminum-cylinder engine or 1200 RPM for engine with a cast-iron cylinder sleeve.
6. With the engine running at idle , hold the throttle lever against the idle speed screw to bring the engine speed to "true idle." Then, repeat the idle mixture screw adjustments from Step 4 to fine-tune the mixture .

Carburetor needs adjusted. May need rebuilt or just cleaned. Could be one of the jets is plugged.

AT 5000 RPM? What is the redline on the XT 600? I had a XT 500 Do not remember my year exactly, I was thinking the redline was close to 5000 RPM and if that is the case you could be possibly hitting a REV limiter or floating the valves.
Have you adjusted or checked the Valve clearance lately along with the Timing chain tensioner to be sure all are within adjustment specs?
The example you gave where it does it in 2nd gear, 3rd gear, I am sure it would do it in any gear as it sounds exactly as if teh engine is running up against a rev limiter or mechanical limits of teh valve train.

There is not an actual rev limiter. The top end is controlled by the carb. You actually want it to sound a bit rich at full throttle so that it has enough fuel when cutting under a load. What causes me some alarm is the revving up part. If it revs too fast that is a lean condition and can quickly ruin the piston and cylinder much like running straight gas. A torn intake boot can cause this. The high speed spec for the TS400 is 9,600 rpm + OR - 500 RPM. Your saw may not have a needle to adjust the high side as some have only the low speed adjuster. Also when you disassemble and clean the carb always use a new kit with the gaskets and diaphragm. Any Stihl dealer can furnish the kits and they aren't terribly expensive. Often between $10-20.

The carb screws are fuel screws, not air screws, gently turn them both in ( clockwise ) onto there seats, now back off the L screw 1 1/2 turns, back the H screw off 1 turn, you will have to fine tune from there, start the saw, increase the idle screw so it will idle if needed, start by screwing the L screw in, the revs will start to increase due to the weakening mixture, it will eventually start to die as it is too weak, so at this point start unscrewing the screw 1/4 turn, rev the saw, it should pick up smoothly, if not increase the L screw another bit turn until it revs smoothly, now set the idle screw so it runs 3000 rpm approx, the H screw is the govenor, the more you screw it in the higher the revs, the max rpm should be set with a taco to 12500rpm, do not exceed this or it may seize, before any adjustment, replace the filter in the fuel tank, use fresh fuel, and clean the air filter.
Good luck.

Assuming your valves have been checked, and the air filter is clean...
You carb has an accelerator pump, and these can get corroded. Make sure your carb is thoroughly cleaned.
There is an air idle adjustment screw on the bottom, that will affect starting, and idle.
The last issue could be a slow jet that may be too rich or lean. This will affect from idle to 1/4 throttle. The best way to read it is run it there for a while, and remove the plug to see the color. Too white is lean, and black sooty is too rich.

Try checking the switch for your parking brake. It is the top wire in the clutch handle boot. This wire connects to your CDI box, and restricts the RPM's while the parking brake is engauged. To find out for sure, trace the wire down to the plug under the headlight assembly and disconnect it. Now try starting you machine and give it some throttle!

First there is an throttle idle screw that adjust the amount of throttle at idle try turning it ccw until it idles down. If this adj is at its lowest level then adjust the L screw until you get the idle you want. Then try to rev up it should rev up without hesitation if not hold down on throttle untill you get max rpm it it tries to cut out etc. at high rpm adj. the H screw until you get max rpm. then adj your L screw again for proper idle. You may need to do this back and forth for awhile until you get max. performance.

could be. I had a 91 kx 250 that was acting crazy and it turned out to be the stator and cdi box. you can check the stator easly with a omes meter but the cdi box is nearly impossible to check. you pretty much have to buy a new one and try it. also check to see if your power valve is working.

rx7`s should rev to 3000 when started cold then it should settle to 750 to 1000 after about 30 to 60 seconds.
mine suffers a similar problem. so far I have been told various answers, mostly involving the bac valve, lambda sensor, air flow meter, etc. try starting of by adjusting the idle adjust screw (on top of the throttle body under plastic cover) this may be all it is. I have not had time to do mine. Its been fine otherwise and idles fine after a few minutes. mine is non turbo 87.