Backfire when i down shift it does a back fire also if i put my rpms to high it also backfires, it is straight piped 2.5 wide

I would start with fuel filter.

Please seek your local mechanic.

Check the timing chain. Some times the chain will shift with worn teeth and get out of synch. This will cause the engine to lose power and back fire. Sometimes with a flame through the carb. I did that one once. Easy fix if it is the problem.

Engine back fires due to:
- Wrong heat range of the spark plugs.
- Spark plugs gaps too wide.
- High tension cords defective.
- Rotor, distributor cap or distributor assembly defective. (Only on distributor type ignition system).
- Ignition coils.
- Fuel pump pressure too high. (regulating valve stuck closed).

Engine back fires due to:
- Wrong heat range of the spark plugs.
- Spark plugs gaps too wide.
- High tension cords defective.
- Rotor, distributor cap or distributor assembly defective. (Only on distributor type ignition system).
- Ignition coils.
- Fuel pump pressure too high. (regulating valve stuck closed).

* Very hard to brake:
-Check for lose brake booster hose.
*Back fire:
-Check for clog air filter.

mai

would be worth having the cat converter check for blocking the exhaust system

first thing is check fuel pressure. or blow thru the fuel filter to see if its clogged. If the filter is OK you still need to check pressure.

replace the plug wires, these are famous for cross firing cylinders and backfiring through the carb