Well, have to say the devil bike is running sweet now.  Had a 40 mile ride today without incident.  Power delivery surges at 3500rpm and is fairly linear above that.

SinceI am more comfortable making it sing and going further afield, I have discovered two things.

1. The speedo is distinctly under reading.  I activated two radar controlled speed warning signs today.  One was a 30 mph one and the other a 40 mph one.  In both cases, the speedo was reading the correct speed. So maybe she is performimg better than was first thought.

2. I have noticed that in top gear pulling 5000rpm/50mph into wind ,if I roll off thethrottle about 10%, she will gradually pick up a little more speed than keeping the throttle pinned.  I am suspecting the air box restricting top end.  It is certainly very small and has a very convulated inlet arrangement that the parts book calls a "silencer".

Is airbox restriction a thing on this model and will removing the silencer improve matters?


 

Are you talking about #23 in the diagram at this link? www.partzilla.com/catalog/yamaha/motorcycle/1978/dt175e/intake

In the USA, that part only came on the 1978 model...  the 1979 model here had the same airbox but no baffle. 

It was also common to drill holes in various places in the 78-79 airbox to allow more air in...  The most common place I've seen holes drilled on 78-79 bikes I've bought is a few small holes around the outside of the filter lid... NOT in the center where the filter is located.   Not sure that the holes help, but it must have been in a magazine or something because I've seen quite a few modified that way. 

It could also be a little lean on the main?  The classic sign of a lean main is speeding up when throttle is rolled back slightly.  But that's often a temporary "surge" more than a gradual increase.  Still thought I'd better mention it. 

Thanks for that Mark. I have ordered a 135 main. At present I have a 130 in, which is as supplied from the factory for 2X2 UK model. Available sizes in the parts book are 125, 130 and 135.
Its nice to have someone with a lot of knowledge about carb tuning for racing. Not something I have ever had much to do with.  I never heard of roll back tuning before.

Since I have the std jet fitted, all I could think of was not enough airflow through the box to keep the engine fed at WOT.
Never thought about a lean main. I thought it was going rich as the main came fully into its own and not getting enough oxygen to burn it, esp after the holy piston affair.

Still cannot understand why the Europe 79 175 has a 130 main as std and the US one has a 140 with ostensibly the same carb.