Found out that the NT carburettors use the same main jet for the 48cc engine and the 69cc engine, on the shipments the shop was receiving.
The retail outlet was having a shocking return rate with the larger engines; mostly failing from big end connecting rod bearing damage, yet the return rate on the 48cc engines was quite low.
The solution was so simple - replace the main jet with a Dellorto No 84.
Although a No 84 is not needed, i've fitted my bike with a shift kit and the engine runs at 100% duty cycle; the 84 jet has the motor running somewhat cooler.
The engine ran best with a No 82 but a No 83, gave softer and smoother operation.
With a No 84 and wide open throttle at 2,000 rpm, the carburettor is supplying too much fuel for the NGK BP8HS to maintain enough heat in the plug tip to prevent fuel fouling, but an NGK BP5HS plug may very well hold enough heat to cleanly burn the mixture under those low rpm conditions.
It all came down to the 69cc engines running lean, with detonation pounding out the big end connecting rod bearing.
The only modification to my engine is a ported intake tube and SickBikeParts high performance air filter for better flow, everything else is stock standard.
There are so many reported stories that these China Girl engines run with much improved performance on the 69cc engines using drastically reduced jetting, in the range of No 72 or even smaller.
I found exactly the opposite to be true.
Some time ago i ordered a Dellorto SHA Clone and Billet Intake Tube from Rock Solid Engines and a Billet Cylinder Head (but the Billet Cylinder Head is another story in itself). It came fitted with a No 72 main jet and also had the standard No 79 main jet included in the package.
For the life of me, i was never able to get the engine to run properly with the No 72 main jet fitted and strangely at the time, the engine ran better with the No 79.
The results from my jetting experiments correlate with those of my SHA clone experience: the engine was under jetted with a No 79, worse still with a No 72 which had the engine running terribly hot, to the point where smoke would be rising from the cylinder head at slow bicycle speeds under wide open throttle, not to mention a pathetic level of power.
All the obvious things were checked: carburettor and intake air leaks, plug leak, gasket leaks, but they came up perfect when spraying fluid around the component parts of the engine likely to cause leaks.
I have no idea at all how others are making more power without pounding out the big end connecting rod bearing through low jetting numbers, as my experience has shown the opposite to be true.
As things so stand, the 4th engine with a replacement crankshaft that's been partially damaged from 350 Km (220 miles) of detonation is now passing 3,000 Km (1,860 miles) using a No 84 main jet in the Nt carburettor - not bad for 100% duty cycle!
Ok, 100% duty cycle might be stretching the truth just a little, but i'd be prepared to stand behind a figure of 90%.
My choice of sparkplug heat range comes down to the load factors experienced by the engine.
With the bike & trailer; shift kit combo and heavy haulage, the engine is always working her guts out; never any respite on a journey, of which some tend to be quite long (60 - 70 mile round trips) with a few painfully steep, 5 mile an hour hill climbs.
Fabian
Fabian, as I said elsewhere, all these engines appear to tune differently. Mine 4-strokes
constantly with the standard 0.79mm jet and won't rev out, fouls plugs and smokes way too heavily. (At least I know that I have no air leaks.)
Like you, I have a 66cc, port-matched and billet head. It runs at a nice temperature with no pinging whatsoever. In fact, unlike many stories I've heard, mine hardly even vibrates.The plug stays a nice clean light tan colour, not grey or white indicating a lean mix.
It runs slightly warmer than with the stock head, but that's the billet head, not the mixture.
From the day I bought mine, a little over 500km ago, it's never missed a beat or let me down in any way, except running out of petrol once. (11km walk home.)
The only thing I'm doing differently to you is that I add octane booster to suit the head
to avoid detonation.
If I run larger than an 0.74mm jet, the engine 4-strokes and actually slows at WOT due to a rich mixture.
I did plug chops at mid-throttle and WOT to fine-tune.
I've been tuning 2-strokes on and off for 30 years and know how to recognise a lean mixture or over-hot engine. Not the case here - even in mid-Summer my engine runs at a good temperature.
The biggest other difference between our situations is that you're about 90m higher. (I'd guess about 5-10m here in Nowra. I'm barely above the coastal wetlands.)
EnFlaMed, (Andre in the Adelaide hills), went up to a 1.5mm jet to get his engine running well. Whatever it is, something is up to chance a bit and my guess is still an air leak somewhere that the big main jet compensates for, unless it
is very slight differences in altitude making that much difference.
Interesting to hear the various stories on this subject.
By the way, which 'retail outlet' were you referring to?