Aprilia Mojito 50cc....No Spark

Possible that the reeds have eaten the rings, no compression and no negative pressure in the carb....

 

Not checked them yet......

 

Just about to send for a compression tester......time I had one of my own........

 

Pointless on a 2 stroke, you need to do a leakdown test.

Blank the exhaust port, simple enough to make a plate, do the same for the inlet but fit a pushbike tyre valve into it. I cut one from an old inner tube and glued it into the plate I made leaving enough spare innertube to act as a gasket too.

Put 4 or 5 psi into the engine and have a brew. If the 4 or 5 psi is still there your crank seals and gaskets are good.

From what I have read my money is on a dead crank seal, no primary compression pulling fuel.

Also, crank it with a power drill on the flywheel nut, if there is even the faintest chance of it running it will then. Saves the starter (wimpy little motor) or your leg.

 

I wouldn't do a leak down test til after I had checked the comp and reeds.
But it's a fair point. Often when a bike has been stood, the crankcase fills with oil leaking down through the pump, then the first attempt at starting blows the seals.
That's why I do the whole thing of turning the engine over with no plug in it.

 

So if the reed valves are u/s, or stuck, will that give the impression of low compression ? and prevent the fuel being drawn through ?......I don't really understand how they work, although I did once check them on one of my Honda Sky's......

 

Honestly, as long as they have not broken entirely they will work enough for the engine to run.

Reed valve 101.

Piston drops in the cylinder this pressurises the crankcase (called primary compression) the same way as the charge gets compressed on the top of the piston. The reed valves are forced shut by this pressure so the only place the charge can go is up on top of the piston when the ports clear as the piston drops.
Once the piston starts on its way back up then the crank case pressure switches as the piston moving up creates the vacume this pulls the fuel through the carb, opening the reeds allowing the fuel in.

They could have stuck closed I suppose, unlikely as everything is coated in a fine layer of oil (2 stroke after all everything gets covered in oil). Suppose it can happen.

If they had broken or not sealing you would have fuel blowing back out of the carb as there would be nothing keeping the charge in the cases.

 

Thanks for that.......so if the Reeds have become weak, and lost their "springyness" and remain open when they should be closed, wouldn't that affect the vacuum needed to draw fuel up from the tank ?......

 

One thing I did notice, which may, or may not be significant , when I drained the tank and replenished it with new petrol, I over filled it and petrol was leaking from the top of the sender unit.....if air is getting in there , would that affect the vacuum pressure needed to draw the fuel through ?.....

 

No and no.

The reeds should be flat. Over time they can develop a bit of a memory and not sit flat. Losing a little of that primary compression. They will still work enough for your current needs.

 

Normally reeds have a tiny gap, like sometimes totally new ones do.
Buy yeah, a petrol leak isn't going to affect your vacuum pump.
But thinking about it, even with no reeds it should suck some fuel into the float... Hmmm maybe it has blown a crank seal... Or the vacuum tap is fucked

 

Not checked Reeds yet, or crank seals.....anyone know how to tell what engine is in this old girl ?......it's a 2002 Aprilia Mojito 50cc two stroke ?

 

Not sure if it's minarelli or Aprilia. Can check for certain tonight.

 

Thanks for that, from what I can make out, they chaned the engine from a Morini, to a Piaggio, in 2003.....so if this is the original engine, and I think it is, it should be a Morini.....there must be a way of telling for sure.....

 

Easy. Where is the oil pump? On the piagio you pull a rubber bung from the outer case under where the variator lives. Around 30mm across .

 

Get a pic of it

 

Here it is.....no rubber bung near variator......

 

Nope. Not the piagio lump.

 

Thanks.....I thought not, must be the Morini engine then.....it's probably the original......