In reply to a recent question about loss of power in a 1500HL with twin HS4s
I've had and sorted the same problem in my 1973 1500 RWD. It seemed to be
two fold, firstly I changed from the Waxtat jets to the later (and about
half the price) to straight through units. The Waxtats are great when new
but start to fail in funny and unpredictable ways when they get on a bit.
Secondly, I changed the grade of oil in the piston dampers. I was running
too light an oil and the pistons were comming up too fast and screwing up
the demand curves etc. Give it a go if you're still getting the power loss