66cc standard China Girl HT (haha) motor
Mountain bike frame
Everything stock except:
Sick Bike Parts air filter
Sick Bike Parts fuel valve
Inline fuel filter
NGK BP6HS spark plug
I built my fine bike in January of this year (2013). After working out the usual bugs one will likely encounter, she has been running great! Over 1000 miles on the odometer (about a 1/3 of those coasting-no-engine miles).
After a 60-mile run around the north end of the island (Taiwan), I started noticing she wasn't pulling the same steep parts of hills I ride ALL the time. Had to pedal more. A lot more on very steep sections. We're talking up to 30% slope (grade)! For comparison the steepest grade on the US Interstate Highway System is only 8%.
Here's some data from the little video cam I use showing the elevations:
This has stumped me because the bike starts very easily. It sounds great. It still runs at the original top speed on medium grades. Doesn't leak any oil. Chocolate plug. Idles fine, even after chugging up a big ol' hill and suddenly disengaging the clutch! Doesn't smoke. By nearly all accounts this is perfectly broken-in power plant running on a 32:1 oil/fuel ratio. Head torqued down properly. No air leaks. Vented gas cap. Fuel flows freely to the carb. No four-stroking. I weight 130 lbs.
Why have I lost a bit of low end torque?
Even after installing a brand new plug and a spare new NT carb today, the performance on the hills I know so well was considerably less than a month ago consistent with recent results. It runs OK, no problem, on the lesser grades.
But later today I decided to remove the nut on the muffler cap and pull the innards out. Seems there are many variations. My cap has one hole for the fastener bolt and one 7 inch or so tube that extends from the exhaust hole. Whatever else is inside is welded into the casing...so I can't really see it. The inside is carbonized dust...not oily. Looks good.
So for a quick 300 yard test uphill, I left the cap off and fired it up. Had to immediately back off the idle screw on the carb, but then it was running fine. And it had this really nice snappy sound to it. Obvious a bit louder with the cap off.
Then I took off up the hill. Wow...definitely a true increase in performance. But I've read here from old-timers...that it's not good to run without the muffler cap...2 strokers need back pressure. That's true. But it seems like there's some baffles still in the muffler which are still providing some back pressure.
In any case...I plan to buy a SBP's expansion chamber, even though it will cost $60 bucks to ship to Taiwan! (And it's probably made across the pond from me in China.) I know it will give me all the power I need to climb based on today's experiment.
In the meantime does any one have an ideas about what is causing a loss of power? The original performance was perfectly acceptable to me. I don't mind pedaling a bit up those steep bits. But now pedaling much more than before.