Why when i accelerate on my 85 mustang lx it catches speed

In general the best and safest method is to use the timing marks on the crankshaft pulley and the camshaft pulleys. If no work has been done on the engine a simple way is to listen to the engine as it is running. An engine that is out of time when at idle and being revved will run unevenly, have rough acceleration and possibly a lack of power. If it is really far out of timing it will make lots of nasty noises as the valves hit the pistons.
If your setting up the engine after major work, like a timing belt change or a head gasket replacement, You MUST use the timing marks on the crank and camshaft pulleys to avoid doing serious damage.

21 engine coolant temperature sensor, 25 Mass air flow, 42 fuel mixture rich, 53 throttle position sensor, 54 air charge temperature, 85 EVAP purge circuit .

1962 Ford Taunus 17 M 60 hp: The maximum speed is 135 km/h (85 mph)
1976 Ford Mustang 100 hp: The maximum speed is 173 km/h (107.5 mph)
1978 Ford Fiesta 66 hp: The maximum speed is 145 km/h (90 mph)
1978 Ford Capri 107 hp: The maximum speed is 186 km/h (115.58 mph)

Please note that these speeds are approximate and can vary based on factors such as the car's condition and modifications

The cheapest would be a cold air kit (probably less than $200.00). It is easy to install, but won't quite get you there. The next is a better exhaust system (probably under $200.00). The other easiest modification is a performance chip or engine tuner, but they can be upwards of $300.00. A high performance coil, spark plugs & wires can help as well. The bad news is that each of these will make a minimal improvement by themself. Together they make a grand improvement. Everything is expensive.

Start by checking for vacuum leaks, compression then a scan for fault codes. One of those should turn up a problem

in the throttle body, yes

needs a new clutch thats at all not a difficult job for a skilled man but this job is not within the realms of a DIY job

Sounds like a faulty cam sensor. ford is known for this to happen. This does not show up because it is a mechanically working peice. get it checked and replaced it necessary.

make sure the air filter is intact and clean, the engine needs to breathe
do you have fuel pressure?
how about spark, do you have a good spark ?
change plugs and wires

under the dash on the left of the steering wheel way up in the dash. U have to get down on your knee's to see it.