Low Clouds

I’ve had my blue 1976A almost as long as my red 1976A – since the 1980’s – and I use it as my parts bike or ‘mule’ for testing oil pumps, carbs etc, or as a place to hang good spare parts on till I need them. As such it is a bit of a ‘bitsa’ in some respects, but the engine has always been solid. At the moment it is using stock BS40 carbs, stock airbox. points based ignition, stock exhaust and the bottom end and transmission is original. It has standard bores and pistons – the barrels were changed in the 1990’s and some very mild clean up done on the ports at that time. Sadly, it is currently leaking coolant from the water pump weep hole, and smoke and oil billow out of the right side exhaust pipe indicating that I’ve blown out one of the crankshaft seals. Bottom line is that the engine now is very tired, and so I have decided to do a refresh of the engine this winter.

As this is really a bike just for playing with, after several discussions with a friend of mine in the USA, I’ve decided that I might just as well do some mild ‘tuning’ and have a bit of fun with it ! The plan then is to use a lift plate under the barrels, do some clean-up of the ports, skim the head and see what sort of a power boost I can get. As these are piston ported two stroke engines, putting a lift plate under the barrels to raise them (and milling that amount of lift off the top of the block) changes the intake/exhaust timing of the engine analogous (sort of) to changing the camshaft in a four stroke engine. The porting just improves how the engine breathes and skimming the head changes the compression ratio. The net effect – on paper at least – is a boost in horsepower, which can be further improved in a two stroke engine by modifying the exhaust system.

Before doing any work, I thought it would be useful to get a ‘before’ shot of what the engine was producing so I’d have a baseline or ‘before’ view of engine health to compare a fresh engine against. This meant doing a run on a dynamometer to map the horsepower and torque of the engine against rpm. Luckily, a dyno was available at the shop where I have my crankshafts rebuilt in Okotoks just south of Calgary. All I did in the way of preparation was to check the timing and carburettor balance, change the plugs and give it some fresh gas before the run. Here is how it looked on the dyno:

That’s Joe at RPM Services Ltd. seated in the booth amidst the cloud of oil. The sheet you see in the photo is to try and catch some of the gearbox oil being blown out the right side pipe due to the failed seal.

Given the condition of the engine, I wasn’t expecting much in the way of positive results and so was quite pleasantly surprised ! The best run of the five runs showed 55 HP at about 6000 rpm and maximum torque at about 5400 rpm of about 47 ft/lbs. Interestingly, the tachometer read about 240 rpm higher than the dyno recorded at 7000 rpm so it was a good correlation. Ambient air was about 16C. The engine actually improved slightly with each run, and the low cloud of unburned gearbox oil has dampened the mosquito population in the area considerably. :D

After the rebuild (and more discussions no doubt with my friend in the USA) I’ll re-run the dyno test and see what if any improvement I get ! Stay tuned.

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