Carby question

[shakeyCZ]

Hi All I have just checked my carby and found it tis a 28mm VM Mikuni, are these any good for a CZ 250 as I can't find any jet setting for this size carby? 34mm or 36mm seams to be the only carby i can find setting for, can I just get any Mikuni carby and use it? any ideas or help ASAP would be great in this time of crisis!!

I have checked all the specs on Mikuni website..  Intake side ID is 43.5mm the engine side is ID 34mm my throttle slide ID is 36mm from these specs is says it should be a 28mm carby but Im lost as to what size it is

needle is -6DH3
Needle jet has p-0 and  188 stamped on it
main jet is - 240
pilot jet is 35

and the throttle slide is 3.0
and my float valve has33 stamped on it


hmmm

[Freakshow]

thats sounds more like a 34mm carby.  if thats what the diameter at the inside middle way is.

The specs you give sound more like a 250cc jetting but closer to a non reed model.

P series tubes are 250 but that son the leaner side,and a #270 would be more normal and your on a #240 so id say its jetted for about a 175-250 or a piston port.

You can use whatever carby size fits in the intake manifold really, you just have to jet accourdingly, however there is a rider on that  - there is a limit to size though in your air ratio, so on a 250 unless you on methonal id stick to a 34/36mm carb reed or 32mm non reed.

iM NOT FAMILIAR WITH cZ STUFF BUT 28mm sounds way to small.   Even for a Piston port id be moving up to a 32mm, with mains around #200

whats stamped on the front near the choke on the carbies you have ?

[090]

the engine side is ID 34mm

That makes it a 34.
Jetting specs are in the ball park.

[shakeyCZ]

This is my carby, and it dosen't have reeds

[tony27]

Try here for some starting points

http://www.braigasen.com/Mikuni_jetting_chart_two_stroke.htm

[shakeyCZ]

also got one more question my motor CZ 250 has 105psi compression what should it be, as I don't understand 10.5:1 ratio, should i put a new piston and rings in it?

[Freakshow]

Are the old piston and rings in spec ? end gap on rings and bore clearance ?

[Freakshow]

use the specs for a 34mm carby. you also need to find the choke as its seems to be missing.

all the specs you have are right about on a non reed motor and hshould if anyting be on the rich side.  you need to ride it and do some plug chops to see where you go from there.

[shakeyCZ]

Are the old piston and rings in spec ? end gap on rings and bore clearance ?

It starts okay but it just seams to back fire when warm out the carby, I checked the plug it was black, so I changed some carby setting to lean it off abit and still the same, its very hard to get it up and going i realy have to ride the clutch but she wont go well just dies feels like your towing a truck!!, So I did a compression test to see if it was down, I do have a new piston.

 so how do I do the end gap on rings and bore? sorry to sound dumb, Would the float valve cause the carby/cyclinder to flood if not sealing correctly ?

Ps yes the choke was out for a inspection

[GMC]

Before you get too caught up chasing your tail you should check the timing as it might account for some of the issues you describe

[Gippslander]

My 72 CZ250 has what looks to be the same carby as yours -- side entry main jet and all -- and it runs fine with a 270 main jet.

Agree with GMC, make sure your timing's right as a first step, then feel the compression, if there is noticable resistance when kickstarting you have enough compression to get running and see what it's like, if it's down a little on compression it will still run ok but not up to max hp.

So:
-- got some compression resistance on kickover?
-- timing?
-- carby all clean and all assembled correct with correct float level and choke fitted (like the above poster said -- it's missing)
-- jetting as it has been when it last ran (270 main should be ok)
-- clean plug?
-- exhaust not blocked?
-- air filter clean?.
-- no airleaks, intake side and in the motor/crank seals?
= should run ok 

[Freakshow]

prity put the ring only in the bore , AT LEAST TO THE TOP OF THE FIRST PORT and see if it has a gap, big gap replace small gap panic.

put the piston in DO THE SAME, if its sloppy replace if it get stuck panic.

[tony27]

http://czmadness.net/techdata/Clymer%20Vol%20II/Clymer%20Vol2.html
This should help explain how to check ring condition & a few other things
Heres a few different guys set ups from the states, I'm picking most are running the carb where the main jet screws into the bottom of the needle jet rather than in the float bowl like yours

Stock cylinder / 34mm mikuni / Copping ignition / PFR pipe/
Golden Spectro @ 40:1 / 76 Union 100 octane /

34mm Mikuni
260 main jet
45 pilot jet
6FJ6 needle, middle - Sudco p/n 003-217
Q-5 Needle jet
2.5 slide
Air jet installed

Non reed motor / PFR pipe / BUS ignition / 34mm
Stock cylinder / 100 oct @ 40:1 w/Castrol A747

Q5 needle jet
6DH2 needle
310 Main
40 Pilot
No air jet
2.5 slide

Gives a broad powerband with the 2 step tapered needle and jet

1972 CZ250 / standard cylinder / 34mm mikuni / BUS ignition/
PFR pipe / Castrol A747 @40:1 / Union 76 100 octane

34mm Mikuni
260 main jet
35 pilot jet
Q-2 needle
6DH4 needle jet
2.5 slide
No air jet
Air screw 1.25 turns out

34 mm Mikuni
250 main jet
40 pilot 1 1/2 turns
6FJ6 Needle second bottom
3.0 slide
P8 Needle jet
No air jet

This is with ACK inlet manifold non standard pipe and small
internal rotor motoplat At sea level running 98 ron fuel and 32:1
oil ratio. This combination starts and runs cleanly throughout the range.
All bikes vary but this could be a good starting point.


Just occured to me, what is the condition of the oring at the bottom of the needle jet?
If it's damaged or missing the main jet will have no effect

[shakeyCZ]

Tony27, i got a new o-ring the other week as thats what i thought as well but no change its kind of intermitent some times it will leak fuel out the overflow and then times it won't.

So the bike dose run just no power idles great till it warms up then it turns bad, timing is good as i did check that with my light, havent checked muffler yet but i don't think its blocked, dosent seam to be any air leaks, so I will just try and get a new float valve as it looks realy worn, well off i go to try these things. i will post some plug photos they are not good quailty but you can see the colour

and this one was before i changed the needle height. centre setting but when warm didnt run right backfire threw carby and no power



after needle change... richer side 2 up

[shakeyCZ]

Got a question, can I use a flat slide carby? I can get a mikuni 34mm flat slide it use's the same jets and parts as a round slide, any input would be great.

and one more question would a slightly ported CZ 250 engine require a larger jet?