I have seen something similar a long time ago, caused by slight over advance of ignition timing. Try nudging it back a bit, after marking where it is now.

40MPH(64kph) is too slow for 4th gear? You mean drive everywhere around town in no higher than 3rd? My S is auto, so cant compare, but it has no trouble in top at lowere speed that that.
jp 83 Euro S AT 54k

 

Ken, have you adjusted your timing lately?

I just did the TB and adjusted timing. I'll have to check it again (car's in storage for the winter, so next time I have it accessible). But, when I set it the left cam was set right on the mark where it was before. The right cam was previously advanced 1 tooth. I set it up retarded by about 1/4 tooth (you can actually see my settings in my TB write-up). Assuming nothing has changed, does that rule out this theory in my case? That is, it was advanced before and didn't do it. Now it's slightly retarded but does it.

 

Howdy,
I'm pretty sure know what youyou're talking about here.
imho 123 quattro nailed it. Before you go chasing parts around, change the gear oil in the diff. I bought redline for the 84 at first then went to royal purple(not looking to start the debate on brands) 80-90w and it reduced it dramatically.
point being: he thicker oil reduced the gear back lash ever so slightly.
I also had a Jeep that alert me that the drive shaft was loose this way. Oing wheling is hard on the driveline and it needed fairly frequent tightening.
I vote backlash first
loose driveline second
worn suspension third
or all of the above
cheers
bil

 

Many thank,s to all who posted. , really appreciated. There,s a number of interesting suggestions here, with each post being relevant, I will endeavor to try them. Currently I,m awaiting my aircon compressor to come back from an overhaul ( warranty work) so my car is at the moment sat in my garage with aircon hose,s sealed off etc untill the arrival of the "again" refurbished compressor.
The advance/retard sounds interesting ?

Ken
80 928 S
5 Speed

UK

 

How old are your motor mounts ?

 

as far as I know they,re original Terry, but who,s to say they have,nt been swapped out previously. I,ve checked them out before ,and the rod inserts are still proud of the sphere,s.., ie they are,nt fully compressed.

Ken
80 928 S
5 Speed

UK

 

Mine look good too.

 

Today I loosened the timing belt just to within the window on the loose side. It was on the tight side before. Before I changed the belt last fall, it was on the loose side of the window. I don't want to speak too soon because I just took the car for a short jaunt, but the jerking at low speed-low rpm was gone. I had taken the car for one other quick jaunt around the block last week and did notice it then. So, it seems to have made a difference. Ken, have you figure out your issue yet? If not, you might want to give it a try.

 

My one 79 used to do that also but only on cool nights or days I just richend up the mixture by 1/4 turn to the right and it went away.

 

fuel filters?

 

Hi Doug, since this post my car has been hibernating, but in the next two to three weeks I,ll be getting her out, pity it was,nt now as the weather is very warm.
That,s an interesting deduction of your before and after belt experience.
The car is about due a belt change and I might get it done end of April, I will certainly bare in mind your observations .., Thanks .

Ken
80 928 S
5 Speed

UK

 

Check the fit between the throttle shaft and housing. If worn air will leak by and intermittantly change the AFR hence the rpm. Also ill fitting vac lines will also leak air.