More Low Clouds

Very nearly two years ago I started a project to rebuild the engine of my 1976A test mule by doing some dyno testing. This was to establish a baseline of how the engine was performing before actually making any changes – I wrote about it here.  The work kept being put to one side as life got in the way, but eventually the job was done and this morning I re-ran the dyno testing on the fresh engine and we have a result ! First though, a bit of a refresh.

This bike is a basically a test rig, and over the years I have hung various bits and pieces on it – it was for a while a very convenient place to store good spares for my other 1976A, it has had a couple of different engines installed for testing, as well as several carburettor banks, again for testing. This time I wanted to try something different, and after some discussion with a friend of mine in the USA decided to do a two-step tuning process:

  • Step one would involve seeing what difference a lift plate by itself made to the engine performance
  • I wanted to then see what difference various exhaust systems actually made on the new fresh engine

Important note: I am not any sort of tuning expert. There are many people out there who will gladly step forward and make that claim – I do not.  Secondly, the few people I do consider to be ‘expert’ also are trying to make a living preparing racing machines, and so do not generally share what it is exactly they do to an engine to make it perform ! As a result getting any genuinely useful information is not easy. For this reason, whatever level of performance I have been able to achieve I know can probably be much improved – all it really takes it money.

The Suzuki GT750 engine can be made to produce in excess of 100 HP, but that level is not really suitable or safe for a road bike of this vintage still using a stock frame. For usability, a target of 70 to 80 HP is commonly seen.  I am not going to go into the theory behind the ‘how’ part – there are lots of books out there that describe this, but to get that sort of power with a 2 stroke engine normally means having to do two things: change the porting to make the engine breath better, and then use a tuned exhaust pipe to hold the fuel charge in the cylinder long enough for it to generate power when combusted.

One of the problems with the GT750 engine is that to open up the exhaust ports risks cutting through into the cooling water jackets on the later cylinder blocks. For this reason a ‘lift plate’ is commonly inserted under the block to raise it and the ports up, and then the amount of rise is milled off the top of the block.  I used a 2.5mm lift plate and after adding in the gasket thickness  had a bit over 3mm milled off the top of the block, such that the piston outside edge was even with the top of the block when at TDC.

Note: I did not do any actual changes to the size or shape of the ports themselves, as at this point what I wanted was to determine what if any change was attributable to just using the lift plate. I did clean up the castings, smooth out rough spots and open up the intakes to match the inlet rubbers.

This is a photo of the lift plate fitted in position with the two gaskets (just click for a larger image):

I’ll continue this in Part 2 !

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