I am rebuilding an old Romanian moped, which I purchase for my kids some 2 months ago for 125 $. The machine is 20+ old, and for sure, it has seen better days. So far I had to re-build the crankshaft threads, and the flywheel key channel. I also had to replace the crankshaft main bearings, including making custom cages as the original bearings were not available anymore.
As for the electrical part, I have set the timing arbitrarily at 2 mm before TDC, as I have no documentation available for this engine. After cleaning the carburetor, I give it a kick, in fact several kicks, and the engine finally started. The idling speed is way too high, and whenever I want to adjust it the engine dies. I am pretty confident there are no air leaks on the suction side, as I have replaced all gaskets, seals, etc. However, I do not have its original 14 mm carburetor, which has been replaced by a more common 17 mm carburetor.
In order to isolate the problem, please provide some timing advise:
1. What would be the best ignition advance to be set for an unknown engine?. I had a look on the mopedriders.org manuals, and the advance for various mopeds is between 1.3 and 3 mm before TDC. It seems that a higher compression requires a lower advance (e.g. for Tomos A35 the compression ratio is 9.1, and the advance is 1.3 mm, while Mobylette AV 7 S engine has a compression ratio of 6.5 and the advance is 3 mm).
2. How the engine behaves if the advance is set higher or lower than normal (e.g. 3 mm instead of 2 mm, or 1 mm instead of 2 mm respectively) ?.
3. I know this engine has many miles on it, and it has low compression capabilities. However, I did not have a compression gauge to test it. With this respect would it be better to set a low advance, say 1 mm before TDC, to make it easier to start? My idea is to initiate the spark when the fuel-air mixture pressure is as high as possible.
The second issue is piston facing. I have been told that piston ring locating pins shall face downward, towards the exhaust port, and I have installed the piston accordingly. Reading Franco Morini M1 Service Manual, I found that the piston shall be installed so that the piston ring locating pins shall face upward and away from the exhaust port (Service Manual M1 Reed Valve Engine, page 30). Should this type of facing be associated with a reed valve engine only? (My engine is piston ported).