while my 25 t engine is running i walk back to listen to the exhaust. there is a spitting noise that is most irregular. If i have someone cut out one of the cylinders (doesn't matter which one) the spitting noise becomes regular. It doesn't matter if the engine is warm or cold and accelerating the engine only accelerates the spitting.
Also, have a tapping noise about in the first cylinder that doesn't change by cutting out the cylinder or if the engine is warm or cold.
Take a compression test. Find out how your valves (and the rest of your engine) are doing. Wether the valves are OK or not another possible reason for sputtering is an ignition system not working properly. Coils re-habbed & HCCT'd?
did you check the valve springs?? I had one broken and the engine did about the same as you describe.
Can be one of 3 things. 1. Valves problem sticking or burnt
2. Ignition problems, coils, timer, spark plugs or coil box.
3. Fuel problems, needle out of adjustment, improper float lever, leaking fuel float valve, clogged passageway.
A very unlikely problem, but possible, could be broken rings or severely scored cylinder.