Robin writes
> My '84 1.6 NA diesel Westy, which has just had a junkyard engine
> transplant, and a head gasket renewal, has the following problem:
>
> Cold start - fine. Engine noisy, but that's normal. Accelerate through
> the gears - ok. At around 90 km/h (55 mph) in fourth, pedal to the
> metal, speed starts dropping down to 70 km/h (45 mph). It will maintain
> this speed with difficulty. Lots of smoke from exhaust.

Black smoke => It gets (too much) Diesel, so not filter clogging.

>
> Note: this doesn't always happen. Occasionally, I can drive as fast as
> 105-110 km/h (to test, of course, not permanently) and it will sustain
> this. Without smoke.

My guess is that this is injection pump related. Likely drifting timing,
but can also be somethig else sticking in the injection pump.

When it acts up and will hardly idle, try the cold start advance
handle and see if there is a difference in either position.

Check that injection pump, it's pulley, the timing belt and the
crank drive pulley and woodruff key are all tight/in place etc.
This was disturbed during the head gasket replacement.

> Another note: both batteries (this Westy has factory aux battery) are
> pretty flat, as the van hasn't been driven for a while. Could the load
> on the alternator caused by these two half-charged batteries be so high
> that this would hamper performance of this (50hp only) engine to such an
> extent?

No. Alt takes at most about 1 hp = 1/50th of your power.