Re-jetting Your Carb, As Easy As 1,2,3 ( quarkdude )

T

TWalker

Guest
Thanks but I did get it figured out, I was gouing about it the wrong way. The procedure for this motorcycle is the same as the bikes.

But this bike is even more rare/unusual than our china engines, which is why I was considering soldering. This is a 34 year old bike out of east germany, very unusual an MZ 250.

But now I know the proper procedure all thanks to the MB forums.
 


rule413

New Member
Joined
Sep 18, 2008
Messages
26
what would work for an elevation of 70 feet??? should i leave it stock or could i benefit from some adjustments? i cut the baffle out of my stock pipe and installed a amsoil air filter. thank you in advance.
 
F

fetor56

Guest
what would work for an elevation of 70 feet??? should i leave it stock or could i benefit from some adjustments? i cut the baffle out of my stock pipe and installed a amsoil air filter. thank you in advance.
70 feet in the grand scheme of things is nothing...leave it stock.
 
Q

quarkdude

Guest
Re-Jetting by soldering

I'm glad this helped you out. I went thru the same thing when I built my bike and was fustrated that I couldn't get a different jet. But soldering it and re-drilling it was actually pretty easy and only takes about an hour the first time.
quarkdude

This answers aot of my questions. My bike has been running too rich and I was looking around to see if there was a place you could just buy these carb jets for higher altitude. Haven't had any luck yet so I'll end up making this modification myself.

Thanks quarkdude!
 
Q

quarkdude

Guest
Thanks Ravo for the pics

Thanks for the great pictures of the jet and the carb apart. I never got a chance to take pictures of the mod and then I sold my bike to fund a screamin deal on a 1973 Yamaha RD350 cafe racer that I purchased. Still very much into Motor bicycles.
quarkdude

Bumping for a few pics because I drilled today.


That brass thing is your jet. Remove the tip if youre going to fill it with solder, or you can remove the entire brass tube part if youre just going to drill (like I did)


yes, you drill the smaller side. Don't use much force and blow on it real hard to clear out any brass junk after youre done.
 

jack stewart

New Member
Joined
Jun 5, 2012
Messages
1
Your instructions seem to work for me!

I followed your instructions and soldered the existing jet port and drilled a smaller hole to reflect my current elevation, 1900 ft.

No more 4 stroking!!!!

Thanks for the tip and keep up the excellent work!

I love this cite!

JS

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
throttle cable.

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
getting.

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.
..................Thanks, quarkdude
 

GreenMantis

Member
Joined
Dec 8, 2012
Messages
317
I want to rejet one to burn r/c glow fuel (nitromethane) if I want to. It will have to flow 8 times as much fuel as gasoline, more or less. That's a lot of jetting, I'm not sure if the jets can be drilled out that much. Any thoughts?

I just noticed someone said the stock jet size is .o30, if my math is right, then I would need to open it up to .24, at least. Does that sound right?
 
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