No power going up from 3rd to 4th

Afternoon all,

Got a problem on the 109 ( well not counting the 21 mot fails )

When i'm driving it, accelerates fine in 1st,2nd and 3rd, but go up into 4th and it looses all real acceleration and only manages maybe 35mph?

I've just rebuild the head but it had this problem before the head rebuild and i've no idea what it could be?

thanks
mat

Lack of power would seem the obvious.
If its the same as before the head rebuild, that would seem to exclude valves
Can you do a compression check to exclude the rings?
air filter?

If engine checks out and exhibits no other symptoms then I guess that leaves drag, e.g binding brakes, handbrake or stiff wheel bearings. Does anything get hot (unexpectedly) do wheels turn freely.