RE: MVVS 1.20 with long pipe
AJF,
I was referring to the twin...
A 1.20 single was not produced in my time...
The smallest big single is now the 1.60 (26 cc) and previously it was the 1.50.
Can you please post a digiphoto of your engine?
The new carburettors MVVS uses are much better than the older ones.
Some customers of mine substituted new #3216 and #3219 carburettors, on their OS and clone engines, because they were running too rich in the mid-range.
They needed shim stock wrapped around the spigot, to make the 13 mm MVVS fit the 0.5" carburettor hole in the crankcase.
As for the normal R/C carburettor for the larger singles; it requires fuel to be fed under some pressure.
Using a fuel pump is recommended (preferable to exhaust pressure) and this crankcase pressure fitting is used to supply pulsed pressure to the pump. This pressure tap is also needed for the optional Walbro carburettor, on these engines.