Jet sizing

[theswede]

So, the new cylinder and head are installed and I got the scoot started easily yesterday. However, it will only idle and dies as soon as I give it any significant throttle. I assume this is because I need to go up in jet size, and I was hoping for some advice on this.
The kit I installed is this: cgi.ebay.com/ebaymotors/80cc-kit-Linhai-Vento-Baja-Roketa-GY6-50cc-QJ139QMB-_W0QQcmdZViewItemQQhashZitem4cf55b0f6aQQitemZ330533900138QQptZMotorcyclesQ5fPartsQ5fAccessories, it has significantly larger valves and ports than the 139QMB standard head.
My old head was a standard model, and I ran a #92 jet in my 20mm carb with that, which worked fine. The intake port on that head was 17mm.
On the new head the intake port is 22mm and I am using the same carb, but with a new 22mm intake manifold.

I am thinking a #96 or possibly #98 jet might be a good start, but I was hoping for hints. Is there a good way to calculate these things?

/Otto

[Rebel]

I've never seen a calculation for jet sizing. Leapfrogging a size to get in the area looks like a good idea to me.

[jmkjr72]

well if you knew flow numbers you might be able to take a stab at it
if teh new head flows 20 % better then you would need to increase the jet size by 20% unless there was some thing restricting it

i deal with 2 strokes so normaly when jetting i go so large the engine 4 strokes and work my way back so thats not an option for you

[theswede]

I made a stab at trying to calculate the jet size...

Increasing the intake from 17 to 22mm equals a 167% (!) increase in intake area.
Assuming that the jet area needs to be increased as much as the intake area was increased, I need to install a #120 jet.
This seems a bit big to me, so I'll try a #100 and see where that gets me. I'd rather have a too rich mixture than a too lean for starters.

[Rocketdog]

I'm thinking you might have a leak in your intake. Check and make sure. 72cc isn't that great an increase. I have run a stock carb with an 88 jet and it did just fine.

RD

[theswede]

An update: After a lot of swapping back and forth I ended up with a #96 jet.
I do however have a slight hesitation at about mid-trottle, but below or above that the engine runs fine, and idles well.
It may be I need a different low-speed jet.
Anyway, the scooter is back on the road!
;D

[moore716]

well I will take a stab at this. sorry if this seems wrong but I am a newb here. i dont know if you are working on a 4 or a smoker but here goes. I was taught to jet this way by a guy who is an engine builder for MX bikes.
The same should be true for scoots. find a nice flat area that you can ride in circles in. AKA parking lot. I marked the throttle with tape at 1/4 1/2 3/4 and wot settings. Now ride in the circle at a steady throttle and see where you have hesitation. keep track of where you are on the throttle settings because that will help decide which jets are in question. 1/4 throttle will be the air screw and 3/4 will be the main jet. Yes these do overlap slightly but it will give you an idea. Now pay big attn to the sound the scoot makes at these settings. If you get a boowah sound then you are running lean and this is bad. You can engage the choke to verify this. If the bike makes a crackling sound and smokes a lot you are running rich which may not be to bad depending on how rich you are.
Also keep in mind that the float being adjusted improperly will give a false bad jetting feel. If you start to see ooze drizzling from the silender than you are way to rich and have been for some time. Oil soaked packing in the silencer will also hinder performance and make the bike sound like crap.

[moore716]

I might add that the needle and main jet do overlap. If you are good at 3/4 on up throttle I would suspect that the needle needs adjusting

[Rebel]

moore716

theswede has a 4 stroke, he has since this post started installing a fuel injection system. But good information you posted.

[noday]

wish I had seen this thread before. now things make more sense re: Otto's first failed top end with the EFI..... and his ongoing problems

a FYI digression... the 50cc GY6, for all intents was designed as a 72cc (80cc) engine.

18mm is the optimal intake diameter for a 4T, 72cc, 1 cylinder, peak torque @ 6500rpm, engine.

22mm is overbore

while 4mm does not seen like much, intake air speed is inversely proportional to the square of the intake radius.

18/2=9 9*9=81 22/2=11 11*11=121

1/81 (0.0123) v 1/121 (0.00826)

a secondary effect of lower intake air flow speeds is poor fuel air mixes.

lower flow rates cause uneven distribution of fuel droplets ( and larger droplets) in the intake charge and uneven combustion.


bigger is not always better.

[moore716]

I am still trying to learn what is what concerning engines on these things. I am used to 125 or 250f stuff LOL. Oh well I tried.

[Rebel]

No problem at all moore. Glad you are here and jumping in.

[noday]

add my apology moore716

welcome to this forum

theswede 's problem has bothered me for a while.

thank you for the post