One of my main aims for this year for my ’32 Scott racer was to set up the carburation properly for petrol again, having decided to move away from Methanol. Methanol worked really well, and the power characteristics really suited the three speed box. The bike was fast and really responsive to ride with the single overbored Amal type 89, but I worked out that the venturi area was less than optimum and resolved to improve that.
With the single carb, the shape and position of the torque curve was pretty much exactly what I’d expected based on the calculations I’d made using Gordon Jennings formulas relating to port time/area relationships. Basically, the you need a certain area of port available to enable adequate gas flow at a certain revs and that area increases as the revs increase as the gas has less time to pass through it. Nothing I have done over this winter should change that torque curve shape too much, as I’ve not changed port timings. Changes in exhaust pulse characteristics relating to the increase in temperature anticipated with the change to petrol will make a difference, and quite possibly not a beneficial one but we’ll see. I do expect that the new fuel system, using a pair of 1″ Amal type 76 carbs on a vee type manifold, should keep that curve from tailing off quite when it does at the moment. I spent hours setting up the float heights and chamber positions so I hope it works. The extra breathing coupled with an attempt to tune the inlet lengths to work better with the slightly extended inlet timing duration I have on this engine should in theory pay off. With the single carb, you could see an inch or so ‘stand off’ of mixture blown out (and sucked back) at low revs. The longer tracts will help eliminate that. I have also spent a significant time working on the head profile to try to allow a more direct route for the flame front to move to the extremities of the combustion chamber but there’s a fair bit of finger in the air stuff… with a bit of borrowed knowledge and the rest; ideas formed through slow but incremental observation of what has already happened.
So, with a completely new fuel system, I need testing to get it right. The most straightforward way for me to do this is by getting onto the dyno that I’ve been using for my other tests so far. This time though, not to do a run but to do a full set up.
So this week, I’ve booked a morning at Alan Jeffery’s dyno in Plymouth, run by Steve from GT motorcycles who seems to spend most of his time tuning NSR 500 Honda GP bikes for people all over the place. A good man to have on-side.
I’ve tried to make sure that I’ll be ready to make the most of it. Roger sent me his block of main jets, so I’ve got changes to make. I’ve fitted the exhaust temperature sensors to enable me to use that information to help establish whether we are too lean. I’ve got a couple of plugs from a different heat range and on top of that… I also picked up some AVGAS as I’ve been convinced of the advantages of this over even the higher octane rated unleaded from the forecourt.
I’ve also ordered a radiator hose connector which allows the fitting of an 1/8″ temperature sensor and at some point I’d like to fit one of these to go with my Scitsu rev counter which (although it probably needs servicing) stopped working altogether when I switched to methanol.
Hopefully, I’ll get a better result that the first dyno run I did last year. That gave a maximum torque figure of around 38ft/lbs at near to 4000 rpm and a maximum horsepower of 33 at 5000rpm.
I really want to get out to do more racing this year and I really want it to be competitive.