Larger carb jets, little help?

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justcruising

New Member
Mar 9, 2011
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ontario canada
Hi, i've had my 80cc(66cc) china engine kit for quite some time, and without upgrading anything, i feel as if my motor can do more just from hearing the revs. I go about 70% throttle, and it runs fine, but when i give it WOT it barely nudges and has a loud ugly bogging noise..just doesn't sound like a proper running 2 stroker.

So im thinking besides an expansion chamber to help me out(which im hoping i'll get soon) im thinking a larger carb jet would help me out with this bogging issue. I've recently looked at a post (http://motorbicycling.com/f30/chinese-bicycle-motor-jet-size-1662.html) Where someone had a similiar issue, and somebody posted about the stock jet not being big enough. Luckily for me he mentioned a dellorto jet being a great replacement from a place nearby from me (MAIN JETS FOR SHA & PHBG - SIZE 50 TO 130 - ( 51,111,113,117,123,124,129 - NOT AVAIL) - CODE 1486) that i can get my hands on.

So my questions are : will this really work? Is it a good idea? And if so, how big of a jet would be ideal? Any criticism/opinions are greatly appreciated, as i love my build and would like to see it improve as i plan to do a road trip on it this summer.
 
wayne z

wayne z

Active Member
Dec 5, 2010
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louisiana
Try adding a little choke while it's bogging. If it runs better, you may need to raise the needle some. I've never seen one that needed a larger jet either.
 
Venice Motor Bikes

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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Los Angeles, CA.
I concur with Cam... You probably need a smaller jet.
Also, don't worry about jetting the carb until you have run about 5 gallons of gas through the engine, because it will keep running better & better the more you ride it! ;)
 
J

justcruising

New Member
Mar 9, 2011
12
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ontario canada
Thanks for words of advice guys, and after watching a video on knowing what 'four stroking' is, my bike is definitely doing so. I dont think i'll be putting on a different jet size just yet, i just want this four stroking issue fixed. Might just buy an air filter instead and see what that does.
 
A

Alson333

New Member
Nov 9, 2011
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Kankakee Illinois
Yes, good idea about the new air filter, I got a SBP high performance air filter, alot more air flow, might fix your problem. Mine was fore stroking and I looked at the plug and the gap opened up to .35 put it back to .31 fore stroking went away. Then I put an xpansion chamber on and it 4 stroked again went to a .66 jet was 70 jet 4 stroking went away, still go 37 mph top end with a NT speed carb and 40t gear. Cops took that bike now with high flow exaust and high flow air filter I got a 72 jet get 36 mph, but have a 36t gear.shft.
 
dragray

dragray

New Member
Mar 10, 2012
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Indiana
well, both of my nt carbs needed bigger jets.
one got a .72, while the other got a .74 (the stock jets in both carbs were .70)
NORMALLY a bog at w.o.t. indicates that it's too lean, which would dictate a bigger main jet. moving the needle will not do anything because the needle is only in play from idle until about 3/4 throttle. anything after 3/4 throttle relys soley on the main jet.

No one can tell you what size jet you need.....your engine will tell you. No 2 engines or carbs are the same. what works for joe schmoe, may not work for you. The air, air temp. humidity where you live will have an effect on the way your engine runs too.
so if joe schmoe in california is running a .70 jet and he says his engine runs great...that's because it's HIS engine and HIS environment. The same size jet may not work for someone in wisconsin.
you need to go through a tuning session with YOUR carb and dial it in according to what jet it likes best. read the plug alot and just listen to the way the engine runs. You'll know when you've hit the right jet size just by the way the thing runs.
it's a long process, but it's well worth the effort.
it's all trial and error and it can be kind of expensive because you'll need a full array of jet sizes to try.
 
2door

2door

Moderator
Staff member
Sep 15, 2008
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Littleton, Colorado
Solder, flux and a set or orifice drills.

Dragray's advice is right on. Your local will have a lot to do with proper carburetor tuning. Higher altitudes require a smaller jet size, lower altitudes need more fuel.
It's also true that the main jet size determines the fuel flow after about 3/4 throttle. At WOT the needle is lifted out of the jet orifice and the size of that orifice then determines how much fuel is delivered to the engine.

Read your spark plug. It will tell you which way to go. Too dark and you need to decrease the fuel to air ratio. Too light and you need more fuel to air.

Aftermarket or modified air filters don't change the engine's requirements.

Tom
 
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