Determining Main Jet Size

[(Ian)]

I hear the default is .7mm, is this true, or do certain engines come with different sizes? I've got a Grubee Skyhawk 48cc, although I'm pretty sure the main jet is the same size as a 70cc.

How do I tell what size it is, and how should I go about choosing a smaller jet?

 

[GearNut]

The stock Chinese jets are not marked. You need to take it to a machinist or friendly auto repair shop and they can size it for you with a wire gauge.

Choosing a jet is trial and error. Observation of how the engine runs, and the color of the spark plug ceramic is the norm for tuning these Chinese engines.

 

[AussieSteve]

Ian, mine's a 66cc and had an 0.79mm main jet originally. Seems to depend on source as to whether you get an 0.7 or an 0.79. (Might be other sizes too.)

If you can't get it measured, you could buy an 0.74 or 0.75 and see if it makes it richer or leaner than it is now, then work from there with the 'known' size. (Pretty much what I did. I bought an 0.76 and the mixture was leaner, so I knew that the stock one was larger than that. ie 0.79)

Is it 4-stroking, or is the plug just a little dark and/or oily?

 

[(Ian)]

When I go WOT it doesn't quite four-stroke but it feels pretty close. It definitely bogs down and doesn't accelerate right. Plug is oily/ dark. When I just go 0-3/4 it is a bit less oily and is more brown/tan, though still dark. I've had to set my needle to the richest setting to get the most out of it 0-3/4. Seemed to lose a lot of juice the next slot leaner, but WOT was smoother.

 

[AussieSteve]

I can't argue with your diagnosis - sounds like the main jet is too big to me.
Forgot to mention earlier, Sick Bike Parts have a range of main jets to suit, here. (Dellorto 5mm)

 

[Pablo]

I hear the default is .7mm, is this true, or do certain engines come with different sizes? I've got a Grubee Skyhawk 48cc, although I'm pretty sure the main jet is the same size as a 70cc.

How do I tell what size it is, and how should I go about choosing a smaller jet?

Every stock jet I have measured is right at 0.70mm or very close. Measured with a SmartScope (more accurate than a pin gauge, but pins/wire are fine). Anyway, sounds like you are on the right track. If you go to the downloads section of our page and click on tuned pipe installation instructions there are some carb tuning tips.

http://www.sickbikeparts.com/Manuals/Tuned Pipe Installation Instructions.pdf

 

[AussieSteve]

Paul, the ones from ZBox here in Oz are 0.79mm.
Tony Cicchio at Rock Solid Engines first told me that, then when I got an 0.76mm the engine definitely ran slightly leaner, indicating that the original was >0.76.
(Mine actually runs best with an 0.72mm jet.)

 

[Pablo]

Good to know!! Now I know - you actually went leaner. THANKS!

 

[AussieSteve]

Yeah, even an 0.74mm causes four-stroking in my engine.

Incidentally, Tony's surname is Riccio, not Cicchio as I wrote earlier.
I just found Tony's email re the jets. (In the end, by trial and error, I ended up at 0.72mm, his original recommendation.)
An excerpt:-

Hi Steve, Express post Satchel is $11.50, and yes the prices you have stated are correct, the jet size for the carb I would recommend a .72 or .73 the jet which comes standard from manufacture in the carb is .79.......Cheers Tony Riccio

 

[Fabian]

Well i like to be different, or at least my engines (all 5 of them) like to be different.

After spending a lot of time messing around with the jetting of my last 66cc engine (as all the previous engines failed due to lean jetting) supplied with the standard carburettor (approx No 70 main jet), which happens to be the same carburettor used on the 48cc engines.
The solution was to rejet and my best result was with a Dellorto No 82 (0.82mm) jet, and a No 84 (0.84mm) jet having the smoothest power delivery, not to mention a noticably cooler running engine.

Fabian