Well if you get the ignition timing badly wrong it wont want to do much of anything!! If its retarded it'll be sluggish, if its advanced it'll detonate.
Do you mean under power or in neutral?
The main limiting factors on max revs are the strength of the rotating assembly and valvetrain stability. Cam specs and breathing capability will determine how much power is available at high RPM.
Newer engines usually have a rev limiter built into the mapping in the computer that controls the spark which cuts the spark at a given number of revs .. older points motors will theoretically rev faster and faster til they explode or melt down but things like valve bounce , points not having time to close properly before they open again . carbs not being able to manage the high flow rate ect will hold the revs back a little but it will still go bang eventually
The car runs good timing ok, seems to loose power and die just for a second on top revs,as soon as I let my foot of the gas its fine again,going to fit a rev counter and see what revs its dying at,It might be doing 5k revs,but cant be sure.Seems like it has a rev limited,but its a 351c engine IN 70 mustang so no limited fitted.mark
Agree with Briz it sounds very much like you are using more fuel from your float bowls than the pump is able to supply, could be due to blocked filter, failing/not large enough fuel pump, old fuel lines degrading and restricting the flow, a blocked pick up in the tank, choke stuck open a bit, really dirty air filter etc similarly it could also be too large of a main jet in the carb doing the opposite and flooding the motor, hence when you back off the loud pedal it runs as normal.
Just start with the easy stuff and work your way through the (non exhaustive) list. I'd be checking filters first then jet sizes
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