I hava a 1992 lexus sc400 that will not start,it only backfire from the tail pipe
The backfiring may indicate there is a valve timing problem.
Check the timing belt, sounds like it’s broken or slipped.
The 92 uses a non interference V8 engine therefore there will be no mechanical damage, you’ll just need to replace the timing belt, tensioner and maybe the water pump.
If this were a 95 - 97, your engine would make a very large but elegant paperweight.
I was surprise, and you are lucky, that it is a “non-interference” engine according to www.gates.com (the timing belt, and other belts, maker).
If it’s hasn’t had a tune recently, try new spark plugs first. Then spark plug wires, fuel filter, air filter, etc. If the replacement timing belt isn’t in the maintenance records, or receipts, it’s probably a timing belt which is years past its scheduled change.