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my 66 ford mustang had the valves adjusted and now the car looks to be a lot slower, and it chokes up when it gets above 4800 rpm. it used to go to 6500 rpm. could the valves be too tight? any ideas could be useful.

6500 rpm's? must be solids on the 271, the stock 225 would not rev that high, why did you have them adjusted, if they're hydraulic, so no modification needed. it's easy to get them to tight and you must be cautious not to :

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I could have them checked again, they may have set them too tight.

Were thay adjusted HOT?
If not, they should have been.

It may be that you are valves aren't adjusted right, period.take it to a cartified mechanic and have them change them.you'll be much happier.
And on a side note, have you changed your fuel filter recently??? Dirty fuel filters just have about the same problems.

Sounds like they were adjusted too tight, or adjusted when the motor was cold

Sounds like the timing is off, the valves may not have the correct clearance, but most probably your timing is retarded. attempt using a timing light and advance your timing two degrees by loosening, then turning your distributer. Re tighten then take it out for a turn. make sure to listen for detonation, you don't want to advance your timing too much. Hope it assists.

i am not sure what engine you have but most ford v8 engines stock had no modifications for the valves you either bought longer or shorter push rods,what was your complaint when they decided to change the valves? if it was spark knock witch can sound like valves out of modification,which could be a common prob with this engine caused by the cheep gas we're running these days I could nearly bet they retarded the timing and as for setting it pull the specification from the book and set it"advance it two deg" how do you know how far they retarded it duh. if they retarded it at all,and yes if they're indeed adjustable and were set cold or wrong this may be your problem

The correct setting for the valves are 3/4 turn from zero lash. One by one loosen rocker arm nut till you hear a clicking, tighten nut gradually till clicking stops, now tighten nut 1/4 turn at a time till it's tightened 3/4 turn. I had a 1965 289 4barrel 225 HP. the valves could float at 3,300 RPM,so I played with them, finding 3/4 turns was right. three months after I figured out the setting, Ford came out with shop bulletin saying 3/4 turn. By the way I payed $2,658.00 turn key for my 1965 mustang. your 1966 should be worth around eight to ten k if in cherry condition. and is a numbered car, being all parts are original. Did I have the only 289 made with hydraulic lifter that could turn 6,800 RPM before the valves could float. It was a four speed, had duel exhaust, 3.0 rear end gears. I ran 775 tires on the rear with fractional leaf spring between main and secondary leaf, springs were clamped. I ran twelve degrees advance on the crank. With no other changes it could do 1/4 mile in 13 seconds flat. And yes I out ran 271 HP left and right. My car was fitted Kansas on a Wednesday and was the second run on the line.