Carb upgrade ( PZ30 to Mikuni VM3* )

[Syanur]

Hi,

I have a Shineray XY250GY-2 SuperMoto. It comes with PZ30 carb which is very common in chinese bikes. I want to put a mikuni VM carburator. But I couldn't decide VM30, 32 or 34mm.

I think PZ30 has 100 main and 20 pilot(slow) jets (or close).

For Mikunies;

VM30: 250 main, 40 pilot
VM32: 200 main, 35 pilot
VM34: 260 main, 35 pilot jets...

Take a look;
http://www.motorcyclecarbs.com/About...etors_W143.cfm

I am confused with these numbers. I know carb. setting is not so simple but main jets are the biggest factor in performance. So in here, don't the main jets seem a little bit inconsistent ??

And another thing, when I checked the link above, VM32 has largest volume(acrd. to dimensions). It should be better for good fuel/air mixture IMO. Isn't it??

I can get a VM32 but not sure for jets...Do you have any suggestion for a jet combination? (There are jet kits for VM carbs.)

[Zorge]

Merhaba.

Did you find out what kind of carbs and jets are on the engines with same displacement and similar construction (similar compression ratio, DOHC, 4V ...)? I would use someone else's successful formula to achive engine's optimum performance.

By the way, since you are the sole owner of the XY 250 GY-2 in this forum, could you make for us its detailed walkaround?

[Syanur]

Selam,

Sorry for late reply. I am still searching...

It was a good idea. I searched the NX250 carb. Which I find is; #142 main and #38 pilot jet. And this a carb. rebuild kit for NX250 ; #138/38 main/pilot. The NX is known as a inconvenient bike to modify.

I also searched Yamaha TTR250, which has a similar DOHC+4Valve, ~25hp engine , and it uses #142/48 or #147/50 main/pilot jets. And for re-jet, users tried 138~152 main jet numbers. And I found a comparison for Mikuni-Yamaha jets. As we can see numbers are not same but very close for equivalent dyno power.

I forgot to look for KLR/KLX - 250.

Actually, a 30 or 32mm carb. is normal for a 250cc but Mıkuni's numbers are a bit weird. And I checked the link on my first mssg again, in the fist table;
"Dim A=40 mm(Intake Manif.)" for VM30,32,34 carbs. Does that mean it is a 40mm carb.? But I don't think Mikuni is fool enough to give a name VM30 for carb. which is actually 40mm. I think there is a big conflict on Mikuni numbering. I hope I will solve it...

By the way, I am new in forum and I thougt XY-2 is well known here... I will make a good walkaround vid. when I get a good cam. from someone

[Zorge]

40 mm on a 1/4 of a liter, ~25 HP, engine - looks like overkill to me.

However, you see that the 40 mm refers to the diameter of a intake manfold. My experience with all kinds of Chinese two-wheelers says that carburetor diameter and manfold diameter does not have to be the same.

[Nima Naderi]

hi

the first problem is you cant fit a 40mm mikuni carb between the frame where it must be located in front of the intake manifold !!!!

some of VM series carb are for 2 stroke engines be aware cause the jet numbers are confusing and go for mikuni TM 33 or more

[Zorge]

I realised that numbers on the jets can mean (at least) three things:
1. diameter of the jet orifice.
2. flow of fuel during a defined time and conditions
3. arbitrary number determined by carburetor/jet manufacturer

Why does have to be simple, when it can be complicated?

[Syanur]

hi

the first problem is you cant fit a 40mm mikuni carb between the frame where it must be located in front of the intake manifold !!!!

some of VM series carb are for 2 stroke engines be aware cause the jet numbers are confusing and go for mikuni TM 33 or more

Thanks Nima. Actually we also not sure it(VM34mm) is a 40mm carb or not. But I know people do VM34 fit to this bike. I talked with them.

I checked the TM series. Commonly, they are claimed to give better perf. than VM's(also higher prices). And I saw TM33 (and TM36,TM40) has accelarator pump.
http://www.sudco.com/mikuni3.html and http://www.motorcyclecarbs.com/Mikun..._W138C1395.cfm

Other TM's don't use accl. pump like VM series.

TM33 uses #127,5/37,5 main/pilot jets but TM34(no pump) has #280/50 jetts as standart. So I think, accl. pump lets to use much smaller jetts bec. of different working characteristic. @Zorge this may be the reason of number confusing...I was thinking #280 or 260 is too much for a 250cc.

Now can we say, with the right jetting, TM33 is able to give better performance (also better fuel efficiency) than TM34 because of accl. pump ? I am comparing because TM33 is 50~70$ more expensive. I started to think It will be a good choice

[Zorge]

Komşu, if you saw same bike as yours fitted with carb with accelerator pump, I think that yоur search is over. Buy it!

[Syanur]

Komşu, if you saw same bike as yours fitted with carb with accelerator pump, I think that yоur search is over. Buy it!

:D The guys were from sweeden I talked(e-mailed) They had fitted MV34 to their bike(which has no pump.) They had raced with the motocross model of this bike 2~3 years ago. And they made certain changes; cam shafts, Big bore kits, Exhaust modify etc....

I will measure the space for the carb. on my bike. If TM33 is OK, I will go with it. Otherwise I will get a MV34 which is smaller and cheaper.

Yes, the "search" seems almost over, but searching on forums never ends I like to talk with experienced people ;)

[Zorge]

I like to talk with big breasted, preferably unexperienced, girls- it's more... informative. But it's only me...

By the way, the carburetor with acceleration pump is a wonderful thing. Perhaps classic design carburetor can give better overall performance, but the acceleration pump gives smoother ride without hiccups.