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07-24-2007 #1quarkdude Guest
Re-jetting Your Carb, As Easy As 1,2,3 ( quarkdude )
Re-jetting Your Carb, As Easy As 1,2,3
Step 1: Remove carb from engine or if there is room, loosen the clamp
that holds the carb to the intake and rotate the carb to get
access to the float bowl on the bottom of the carb. Rotating
the carb saves you from having to unscrew and disconnect the
Step 2: This is a good time to close the fuel petcock so that when the
float bowl is removed, the fuel won't be gushing out. Also
it helps to put a rag underneath the carb so any spillage is
captured. Remove the 2 screws holding the float bowl on.
A little bit of fuel in the float bowl may be present. Remove
the float bowl, there is a gasket on top of the float bowl, be
sure to save it for reassembly. There will be a white plastic
round float inside that can be set aside. Now the brass jet
that we want to modify is exposed. It has a hex head on it
to put a small box end wrench or equivelant on to remove it.
Take the jet out. The one I have screws into anothe brass
piece, so I seperated the two pieces and took the jet itself
to the soldering iron.
Step 3: Here you can take a pair of needle nose vise grips and clamp
the jet in the jaws lightly to hold it while it is being soldered
( gets pretty hot ). Use at least a 60 watt soldering iron to
have enough heat to heat up the brass jet. Use 99% pure
silver solder to melt into the middle of the jet to close up the
hole. Use lots of flux to help transfer the heat of the soldering
iron to the brass so that the solder flows nicely into the middle
of the jet and seals it off completely. Check underneath the jet
to see if the solder has filled it all the way thru. If you get too
much solder, just lift the needle nose vise grips and lightly
bang it to the work surface while the solder it molten to
remove any excess solder. Now let the brass jet cool for at
least 10 to 15 minutes before touching it - Very very hot before
that. Now that the jet is cool, it is time to drill it to the size that
will work with your setup. I find that starting a little small on
the drill size helps to narrow down the mystery of what size
the jet really needs to be for your altitude and perfomance
needs. Start small, reassemble the float and remount or
tighten the carb and try a test ride to see how the bike feels.
Usually if the jet is too lean (which in this case will probably
be the case because we started small ) the bike will either not
go as fast as it did before the mod or it will bog a little. For
long term this would not be what you want, but for testing and
tuning around the block it should be fine. Keep taking the float
off the carb and removing the jet and drilling it up one size
then ride the bike around the block for a test and tune
evaluation. When the bike feels like it has a crisp acceleration
after releasing the cluch and doesn't bog on the top end, then
that is roughly the jetting that will work with your setup. The
idle screw on the side of the carb may need to be adjusted.
Sometimes an adjustment of the needle clip might need to be
done. I'll look for a link from the forum that addresses the
needle clip adjustment, I saw it a couple of days ago. I did
some research on the web as to what drill size to use for a
50cc pocket bike motor and it was roughly a 0.025 inch drill bit
pretty small. That is actually where I started and stepped the
drill bits up 0.025 then 0.026, 0.027 ( Test and tune between
each one) and then finally 0.028 and this is what worked the
best. The exhaust temperature out of the head at the top of
exhust pipe can be measured using an infrared thermometer.
The hotter the exhaust is getting, the leaner the jetting is
I hope this helps make the carb jetting not such a big mystery. It is actually pretty easy, I tried to be very detailed as not to leave out any assumed information. I might try to turn this into a video and put it on youtube.com as suggested by our moderator. I will put some pictures here in the not too distant future to help clarify some of the steps. Lets gets those engines running extra smooth with a little more top end.
07-25-2007 #3andyinchville1 Guest
Good article but just curious as a carb newbie....Does repositioning our needle valve using the little E clip in our carb effectively do the same thing?....What are the benefits of re jetting VS changing the height of the pin in our carb or is re jetting best used when the carb needs to be adjusted beyond the available changes allowed by the pin adjustments?
Also a newbie who doesn't even own a chuck key, I'm not drillin' nuttin.
BUT, this has to do with "altitude" and "gas mixes particular to your region, more than anything else?? :?
What I see is how MBer's are starting to organize by region, and so one person could do this at say the PacNW or Arizona chapters, do a buncha carbs at one time, and optimize all to the local standard.
If a hundreth of an inch in SoCal improves 5 bikes, and a diff measurement suits Denver, such a post as Quarks is "tested science".
If gambling ever comes to the site, bet on the guy with the Quarkmod.
07-25-2007 #5quarkdude Guest
Hello andyinchville1 and bamabikeguy,
You both are correct. The stock jet was at 0.030 inches before I did the mod. I tried moving the needle valve and it just made the thing run worse. So bamabikeguy, you are correct for California - you guys are close to the altitude that China is at, so the stock jet setting and factory needle valve are pretty close to where you need to be. Sometimes moving the needle valve up or down one notch from the stock position may help to get the engine to run just that little bit better - my guess is from sea level to say about 1800 feet above sea level, a needle valve adjustment can help. I do like the idea of using the re-jetting instructions for higher altitude mods - not for sea level up to around 1800-2000 feet above sea level. So both of you make a good point, to do this mod if you live above sea level and in the mountainous areas.
Thanks for the feedback, that's what makes this better and better.
01-14-2008 #6Mogyver69 Guest
Hi Guys, I wanted to respond to how this 2 stroke carb truly works. First off quarkdude great article.
So let me start by responding to what everyone is saying about moving the c-clip up or down on the pin. This adjustment is good for low and mid range performance. What it does is by moving the c-clip down one setting your lifting the needle up allowing more fuel to travel pass the main jet and into the engine. During full throttle the needle lifts up allowing full fuel flow. During half throttle the needle is dropped back down restricting fuel flow. By adjusting this setting down 1 setting you are allowing more fuel flow to travel by the main jet. But during full excelleration you will not gain anything unless you actually change the jet size. Example: If you have a hose with running water and you place your finger inside the hole you restrict the hole slowing down flow but if you remove your finger you get full flow. The only way to get more flow is to make the hole larger. Adjusting the c-clip will not give you anymore flow than what was aready available at full throttle. If your running lean at idle adjust the c-clip and or if your running lean at mid range. If you need more top end speed due to a lean condition you can do one of two things. Eather a bye a larger jet or b quarkdudes suggestion. Find out what size your jet is by placing a drill bit inside the hole. Use the opposite side of the drill bit. The side that generally would go into the drill. Once you determine what size it is. Go up 1 or 2 sizes at a time. My general rule of thumb is richer is always better then lean. So once you find the ideal setting jet it up one more. Your engine will last a hole lot longer. Hope this helped. Ive read many articles about sanding on the needle and enlarging the hole where the needle travels. Although these things may work you should start with these suggestions first I believe you will find no other adjustments will be needed.
01-16-2008 #7Senior Member
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Where can I get drill bits that small? Finding 'em is my problem. I actually need to go to a smaller diamter because of my high altitude.
01-16-2008 #8quarkdude Guest
Finding small drill bits for Jetting
I found a website with a bunch of these kinds of drill bits and small chucks to hold them here: http://www.ehobbytools.com/en-us/dept_8.html
Prices are really good as well. I hope this helps.
01-16-2008 #9Mogyver69 Guest
01-17-2008 #10quarkdude Guest
Reading Drill sizes for jetting your carb
I checked the site again to make sure I didn't post something we couldn't reference: http://www.ehobbytools.com/en-us/dept_8.html
This site uses the drill# which also has a decimal equivalent. Most of the kits on that site have the drill sizes we need so it is a good reference. A good kit for us would be one that has Drill#65 to Drill#78 or so. The decimal equivalent of a Drill#65 is 0.0350 and the Drill#78 is 0.0160. Most of the carb jets I have redone on these Chinese 80cc carbs start out around 0.030 and you go up or down from there as you know. I would guess that if you were to drill your jet ( I know you already filed it ) it would be good to step it up 1000ths of an inch at a time. Using Drill#68(0.310) do a test ride and see if the engine runs smooth thru the whole RPM range, if it hesitates on the low end go up one thousands to Drill#67(0.320) and keep going up one at a time until the engine runs smooth thru the whoe rpm range. Then when it feels good riding it - the spark plug will tell you when it is perfect by looking for a light brown to grey color on the electrode. Black and sooty sometimes wet=too rich. Hope this helps
Last edited by quarkdude; 01-17-2008 at 07:38 AM.