72 cc

[keewayboi]

How to get the most of 72 cc 139 qmb 10"

[scootscoot72]

I have a 72cc kit on stock head with decompression tube, a 24mm Nibbi carb, intake adapter, intake pipe and filter, naraku coil, 6.5 g dr pulley sliders with stock variator, blue clutch springs on stock clutch, and 49/17 upgear kit. I've hit 54 mph max but it likes to cruise at about 48 give or take.
I use non ethanol 90 octane with a little octane boost to raise it just a tad. I used royal purple break in oil and swapped to Rotella T 15w40, and also use royal purple for gear oil.
So far I've enjoyed this set up but I'm pretty sure i could get a tad for out of it with the large valve head and cam installed as well.

[GrumpyUnk]

What is a "decompression tube'?
Better you than me at those speeds. Me chicken. I admit to that. But I do not feel indestructible, so there's that...
tom

[br4inl3ss]

Jun 3, 2022 8:38:38 GMT -5 GrumpyUnk said:

What is a "decompression tube'?
Better you than me at those speeds. Me chicken. I admit to that. But I do not feel indestructible, so there's that...
tom

thats a decompression tube. its to lower the pressure in the crankcase.
people been doing this on atv and dirtbike since 80's and i guess one woke up one morning and decided the idea was good and call it thier invention and while there created a huge lie that its necessary both stock and small bore increase.
even my bro made one for his 250x honda trike ( overbore 270cc ) back in 1993.

ncystore.com/ncy-oil-decompression-tube-17mm-kit-qmb-gy6

useless stock and 72cc, almost necessary with bigger bbk unless you lower the compression like hell. for a short time its ok tho. but if for 2+ months, hello hole in the crankcase goodbye oil. had one ordered for my 81cc but going with a 72 so i cancelled it before dealer receive it since it have absolutly no use.

[GrumpyUnk]

"hello hole in the block..." how is that going to happen? excessive pressure from blowby past the rings?

Essentially it's a long tube that allows the in/out flow from the crankcase through the dipstick hole, into a length of tubing(longer==bigger volume to handle) so the air/gas can puff back and forth and I assume use gravity to keep any oil droplets captured and plopped onto the i.d. of the tubing so they can flow back to the crankcase.
The larger diameter piston motion causes suction/pressure within as it moves. Add in some blowby, and push the oil out via the factory breather.
I guess the rings start to float at higher rpms and don't offer as much scraping to push the oil back down off the cylinder walls, or allows a lot more gas to slip by the rings into the sump.
Ok. got it.
tom

[br4inl3ss]

Jun 5, 2022 14:52:28 GMT -5 GrumpyUnk said:

"hello hole in the block..." how is that going to happen? excessive pressure from blowby past the rings?

Essentially it's a long tube that allows the in/out flow from the crankcase through the dipstick hole, into a length of tubing(longer==bigger volume to handle) so the air/gas can puff back and forth and I assume use gravity to keep any oil droplets captured and plopped onto the i.d. of the tubing so they can flow back to the crankcase.
The larger diameter piston motion causes suction/pressure within as it moves. Add in some blowby, and push the oil out via the factory breather.
I guess the rings start to float at higher rpms and don't offer as much scraping to push the oil back down off the cylinder walls, or allows a lot more gas to slip by the rings into the sump.
Ok. got it.
tom

basically the higher pressure ( air ) cause the oil to blowby. and when its too mich, they can put a hole in the crankcase where its more fragile. crankcase are really fragile on the flat ( far from corner or other mold )
its basically the same thing as oil catch can on turbo modified car, without the can itself and some stock car are so badly made they require one anyway ( hi subaru hi bmw )

[GrumpyUnk]

So, Saran Wrap is actually a usable substitute for crankcase walls, eh? Well, maybe aluminum foil. Saran might not stick well enough, and the foil can be bent so it will hold on, right?
SHeesh. Who knew the walls were that flimsy.
tom