Jettning an Athena 50cc cylinder?

[Dea_944tTopic starterBerichten: 87]

Hi

I’m having some problems jettning the carburator correctly for my Athena 50cc cylinder.

The set-up is:
– Athena (Eurocylindri) 50cc
– Polini high compression head without decompression valve
– Modified carter (max intake size, extra channels for the cylinder, match ported)
– Original crank with increased intake timing
– 13/13 Dellorto
– Polini high flow filter
– Gianelli original power exhaust
– Standard gearing (lot’s of hills here)
– Running alkylat fuel with 3% syhthetic oil

– GPS tested max speed is 53 km/h and up-hill 50 km/h …. should probably get a variator

Normally I increase the jet size until the engine 4 strokes at full throttle, tested at actual drivning, then I back-off 3-5 in jet size.

I started with a jet size of 65 which felt OK in the mid and high rev ranges but it was impossible to get good idle (it dies) and it never 4 strokes.
The sparkplug color looks OK, slightly dark.

I increased to jet size 70, to test, which ended up in no idle at all, and poor low rev running but still no 4 strokes at full throttle ??

I assume that the correct jet is somewhere 60-65 in size but I can’t understand why it doesn’t 4 stroke att full throttle?

Any ideas?

/Dea

[ritzeBijdragerBerichten: 3421]

i think you have got a problem with the seals in your engine…
what you have to do is this.

-remove the carburator
-put a hose on the manifold/inlet pipe
-then you turn the crank until the piston is sitting top dead centre
-suck or blow into the hose,

if you can blow trough the hose, your inlet valve is broken
the crankshaft has to close of the inlet pipe to make sure that de fuel/air mixure is getting compressed en flows into the cilinder.

when you mill your intake and you mill it to far it won`t seal the cases, and then you are not able to tune it.

also this is one of the fastest 50cc around and should be able to make speeds of about 75km/h with a 64/68 jet

i made this one, and the inlet is really maxed out this one

De budget beun tjoener: geen geld is te duur of het werkt nog niet
Link naar Mijn collectie / Projecten.

[Dea_944tTopic starterBerichten: 87]

Hi,

I followed Malossis recommendation and only increased the height of the intake window so sealing shouldn’t be a problem.

The low top speed is due to that I kept the original rear pulley, I testeed with a slightly smaller rear pulley and it reached 65 km/h without any problems, but since I don’t have a variator I want to keep the original rear pulley.

/Dea

[ritzeBijdragerBerichten: 3421]

put a short film op youtube then we can hear it

De budget beun tjoener: geen geld is te duur of het werkt nog niet
Link naar Mijn collectie / Projecten.

[Dea_944tTopic starterBerichten: 87]

Problem found (and hopefully solved….)

The carburator started leaking fuel so I replaced it with a 12/10 carb with a 62 jet instead and suddenly my idle was back and the engine ran better.
I lost a little torque and power but max speed was just ~3 km/h lower so obviously there was something wrong with the 13/13 carb.

Closer inspection of the 13/13 carb showed that there threads for the fuel inlet fastening screw were destroyed so the seal was not working as supposed.

I have a new 13/13 carb coming in a few days so I’ll make a new test.

Btw, is your carter modified for the DR Evo 65cc? Just bought one of those as I want more low-end torque.

/Dea

[Gebruiker]

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